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ENGINEERING CHEMISTRY
Course Code : 18CHE12/22 Credits : 4.5
CIE Marks : 50+25
Exam Hours : 03+03 SEE Marks : 50+25
COURSE OUTCOMES: On completion of the course student will be able to:
CO1 Recall and explain the principles of chemistry related to electrochemistry, metals, natural
resources, polymers and engineering materials.
CO2 Apply the knowledge of chemistry in solving societal problems related to public health,
safety, environmental issues and developing new materials.
CO3 Identify, analyze and interpret engineering problems in chemistry perspective to achieve
solutions.
CO4 Select the solutions to engineering problems for their suitability and sustainability.
CO5 Perform the various types of titrations for quantitative estimation of industrially important
materials and gain the hands on experience in handling the different type of instruments for chemical analysis
Mapping of Course Outcomes to Program Outcomes:
PO1 PO2 PO3 PO4 PO5 PO6 PO7 PO8 PO9 PO10 PO11 PO12
CO1 3 3 - - - - - - - 2 - 3
CO2 3 3 3 2 2 3 3 2 2 2 - 3
CO3 3 3 3 3 2 3 3 2 2 2 - 3
CO4 3 3 2 2 2 2 3 2 2 2 - 3
Module Contents of the Module
Hours COs
I Electrochemistry-Introduction to galvanic cells, Derivation of Nernst
equation for single electrode potential. Emf of the cell, electrochemical
conventions and problems. Reference electrodes - Construction, working
and applications of Calomel and Ag-AgCl electrodes. Measurement of
electrode potential using calomel electrode. Electrolyte Concentration cells:
Numerical problems on electrolyte concentration cells. Construction and
working of glass electrode, determination of pH using glass electrode.
Battery Technology – Introduction, classification-primary, secondary and
reserve batteries. Construction, working and applications Lead acid battery.
Zn-Air battery and Lithium ion battery (LiCoO2).
Fuel Cells: Introduction, Construction, working and applications of
Methanol-oxygen fuel cell. Super Capacitors – Principle, explanation and
construction.
9
CO
1,C
O2,C
O3&
CO
4,C
O5
List of Experiments
1. Estimation of iron content in the given solution by potetiometry.
2. Determination of pKa value of a weak acid using pH meter
6
II Corrosion and Metal Finishing
Corrosion – Introduction, Electrochemical theory of corrosion. Factors
affecting rate of corrosion, anodic and cathodic area, Nature of metal, Nature
of corrosion product and pH. Types of corrosion –differential metal,
differential aeration corrosion (pitting and waterline) and stress corrosion.
Corrosion control techniques: – protective coatings – metal coatings (Anodic
and Cathodic metal coatings taking Galvanization and Tinning as example).
Inorganic coatings - Anodizing of aluminum. Cathodic protection by
sacrificial anodic method and Impressed voltage method.
Metal Finishing-Introduction and technological importance. Polarization,
decomposition potential and over voltage with respect to metal finishing.
Factors influencing the nature of electro deposit- current density, concentration
of metal ions, pH, temperature, additives( organic additives and complexing
agents).Throwing power of plating bath and its determination by Haring -
Blum cell. Electro plating of Gold (Alkaline cyanide bath). Electroless
plating –Introduction, distinction between electro plating and electroless
plating. Electroless plating of copper and its applications in making PCB.
9
CO
1,C
O2,C
O3&
CO
4,C
O5
List of Experiments
1. Determination of percentage of iron in haematite ore.
2. Estimation of copper in given solution by Iodometry. 3. Determination of % CaO in Cement solution using std EDTA solution.
9
III Chemical Energy Sources and Photovoltaic Cells
Chemical Energy Sources: Introduction, classification, importance of
hydrocarbons. Calorific value – Gross and Net calorific value. Determination
of calorific value of fuel using Bomb calorimeter-Numerical problems.
Cracking – Introduction, Fluidized catalytic cracking. Reformation of petrol.
Octane and Cetane numbers. Mechanism of knocking in petrol and diesel
engines. Anti knocking agents, unleaded petrol, power alcohol and biodiesel.
Photovoltaic cells
Introduction, importance, conversion and utilization of solar energy.
Construction and Working of photo voltaic cells. Advantages and
disadvantages of PV cells. Production of solar grade silicon (union carbide
process). Purification of silicon by Zone refining.
8
CO
1,C
O2,C
O3&
CO
4,C
O5
List of Experiments
Determination of viscocity coefficient of given organic liquid. 3
IV Water Technology: - Introduction. Boiler feed water. boiler troubles - Scale
and sludge formation, Priming and foaming, Boiler corrosion due to
dissolved O2, CO2, MgCl2 and prevention. Determination of COD-
Numerical problems. Softening of water by ion exchange process.
Desalination of sea water by electro dialysis. Sewage treatment: Primary and
Secondary treatment (activated sludge method).
Instrumental Methods of Analysis:
Principle, theory, instrumentation and applications of conductometry,
colorimetry and flame photometry,
9
CO
1,C
O2,C
O3&
CO
4,C
O5
List of Experiments
1. Determination of total hardness of water sample by preparing std. EDTA solution
2. Determination of chemical oxygen demand (COD ) of the given
18
industrial waste sample 3. Determination of total alkalinity of a given sample of water using
standard Hydrochloric acid.
4. Estimation of HCl and CH3COOH in a mixture using std. NaOH by conductometry.
5. Estimation of sodium in the given sample by flame photometry. 6. Estimation of copper in the given test sample by colorimetry.
Polymers- Introduction, types of polymerization- addition and condensation.
Free radical mechanism taking vinyl chloride as an example. Glass transition
temperature, Factors influencing Tg-Flexibility, intermolecular forces,
molecular mass, branching, cross linking, significance of Tg. Synthesis,
properties and applications of Polyurethane, Teflon and Kevlar fibre.
Polymer composites –Introduction, properties and applications.
Biodegradable polymers – meaning, poly lactic acid – synthesis and
applications.
Nanomaterials: Introduction, Classification based on dimension (0D, 1D, 2D
and 3D), properties (size dependent – Catalytic, Thermal and Optical).
Synthesis - Bottom up approach. Precipitation technique and Chemical
vapour deposition with one example. General applications of nano materials
9
CO
1,C
O2,C
O3&
CO
4
Text Books
1. Chemistry for Engineering Students, B. S. Jaiprakash, R. Venugopal, Shivakumaraiah
and PushpaIyengar, 2015 Edition,SubhashPublications, Bangalore
2. Engineering Chemistry by R. V. Gadag and A. NityanandaShetty, , 3rd Edition, 2014 I
K International Publishing House Pvt. Ltd., New Delhi.
3. Engineering Chemistry by V R Kulkarni and K.Ramakrishna Reddy, 1st Edition, 2016,
New Age International Publishers.
4. A Text Book of Engineering Chemistry, Jain and Jain, 3rd Edition, 2014 Dhanpatrai
Publications
Reference Books
1. Engineering Chemistry by O. G. Palanna, Tata McGraw Hill Education Pvt. Ltd.
2. Corrosion Engineering by M. G. Fontana, Tata McGraw Hill Education Pvt. Ltd. New
Delhi.
3. Engineering Chemistry, Wiley India second Edition 2014.
4. Nanochemistry A Chemical Approach to Nanomaterials by G. A. Ozin and A. C.
Arsenault.
ELECTROCHEMISTRY
Electrochemical cells
An electrochemical cell is a device which converts chemical energy into electrical energy or
electrical into chemical energy. Thus, there can be two types of electrochemical cells.
*Galvanic cell or Voltaic cell and * Electrolytic cell
Galvanic Cell
Galvanic cells are devices which converts chemical energy into electrical energy. The Daniel
cell is an example of a galvanic cell. It consists of a zinc rod dipped in a 1M solution of zinc
sulphate. This forms one half-cell. A copper rod dipped in a 1M solution of copper sulphate
constitutes the other half cell.
The two electrodes are connected internally by a salt bridge. A salt bridge consists of a jelly
containing KCl or a solution of KCl placed in a U-tube fitted with porous plugs at either end.
The salt bridge allows the flow of ions but prevents the mixing of the solutions that would allow
direct reaction of the cell reactants. The two electrodes are connected externally by a wire
through a voltmeter. Oxidation of Zn to Zn2+ and reduction of Cu2+ to Cu occur at the zinc and
copper electrodes are represented by equations 1 &2
Zn(s) Zn2+(aq) + 2e ----------------------------- (1)
Cu2+ +2e Cu(s) --------------------------------------- (2)
The net cell reaction is obtained by adding equations 1 and 2
Zn(s)+ Cu2+(aq) Zn2+(aq)+Cu(s)
The electrons released at the zinc half-cell build up an electrical potential difference between the
electrodes. This is indicated by the voltmeter.
Nernst Equation
Single electrode potential (E), change in free energy( G) and the concentration Mn+ are related
by Nernst equation. According to thermodynamics, the decrease in free energy (-
G)represents the maximum amount of work that can be obtained from a chemical reaction.
- G=Wmax -------------- (1)
The work performed by an electrochemical cell depends on the number coulombs of electricity
that flow and the energy available per coulomb.
Work = number of coulombs x energy available
coulomb
The number of coulombs is the product of number of moles (n) of electrons involved in the cell
reaction and the faraday(F).
Number of coulombs = nF
Energy available per coulomb is the cell potential E, since volt is equal to energy per coulomb.
The potential is maximum when the work derived from the cell is maximum.
Thus,
Wmax= nxFxE -------------- (2)
-∆ G= nFE -----------------(3)
Under standard conditions
ΔGO= - nFE0 ------------ (4)
Where E0 is a constant called the standard electrode potential.
For the reaction,
Mn+ + ne M
the equilibrium constant Kc is related to change in free energy by the Vant Hoff equation,
G= G0 + RT ln Kc
K c= [𝑀] Therefore,
[𝑀𝑛+ ]
G= G0 + RT ln [𝑀] [𝑀𝑛 + ]
=∆G0+RTln[M]-RTln[Mn+]
Substituting from Eqs.3&4 for G and G0,
- nFE = -nFE0+ RTln[M]-RTln[Mn+]
Dividing throughout by -nF,
−nFE =
−nFE0
+ RTln M
- RTln [Mn +]
−nF −nF −nF −nF
Under standard conditions M=1.Hence the above equation becomes
E = E0 + 𝐑𝐓 ln [Mn+] --------------------- (5)
𝐧𝐅
Where E = electrode potential; E0 = standard electrode potential; Substituting the values
of R = Universal Gas Constant [8.314J/K/mol]; T = temperature in Kelvin [298K]; F =
Faraday Constant [96500C/mol]; n=number of electrons involved.
E = E0+ 𝟎.𝟎𝟓𝟗𝟏 log[Mn+] 𝐧
This is the Nernst Equation for the Single Electrode Potential.
Nernst Equation for Cell Potential
Ecell = E0+
0.0591 log
[species at cathode ]
n [species at anode ]
Nernst Equation for Daniel Cell:
Zn (s) + Cu2+ (aq) Zn2+ (aq) + Cu(s)
E = E0+
0.0591 log
[Cu 2+] cell
2 [Zn 2+]
Electromotive force of the Cell
The potential difference which causes a current to flow from an electrode at higher
potential to that at lower potential is called the electromotive force(emf).The emf is represented
as Ecell. It is expressed in volts.
The measured EMF of a cell is Ecell=Ecathode-Eanode
Ecell=Eright-Eleft
Where Ecathode(Eright) and Eanode(Eleft) are the reduction electrode potentials of the cathode and
anode respectively.
Standard emf of a cell is defined as the emf of a galvanic cell when the reactants and products
of the cell reaction are at unit concentration or unit activity , at 298K at 1 atmospheric pressure.
As represented by thermodynamic relation, ∆G= -nFE
Where ∆G is the free energy change accompanying the cell reaction, n is the number of electron
transfer the cell reaction and F is faraday.
The cell reaction is spontaneous or feasible when ∆G is negative. ∆G can be negative only if the
cell, E is positive, because, the other two factors, n and F are always positive. Thus the emf of a
galvanic cell is always positive. The positive emf value indicates the spontaneity of the cell
reaction in the given direction.
Cell Notations and Conventions
Convention- 1 The half-cell at which oxidation occurs (the anode compartment) is written on the
left and the half-cell at which reduction occurs (the cathode compartment)is written on the right
For example , in the Daniell cell represented by Zn/Zn2+( 1 𝑀 ) //Cu2+/Cu, the reaction proceeds
spontaneously from left to right. The cell potential is given as+1.10 V.
Convention-2 Suppose that Daniell cell is written in the order
Cu/Cu2+(1M)//𝑍 𝑛 2 + (1𝑀)/Zn
Its cell potential is written as -1.10V and the cell reaction is non-spontaneous. This convention is
summarized as follows
Direction of flow of electrons Nature of cell reaction Ecell
Left to Right Spontaneous +ve
Right to Left Non-spontaneous -ve
EMF Problems
1. The electrode potentials of zinc and copper are -0.76V and +0.34 V respectively, calculate
the cell potential.
Ecell =ER-EL
Ecell =E -E
Ag /Ag
Ag /Ag
Cu /Cu
Cu /Cu
Ecell =E -E
2+ 2+ Cu /Cu Zn /Zn
=0.34-(-0.76)
Ecell =1.1 V
2. If the emf of the given cell is 0.46V,calculate the electrode potential of Ag ,the electrode
potential of Cu is 0.34V.
Ecell =ER-EL
+ 2+ Ag /Ag Cu /Cu
0.46 V= E + - 0.34 V
E + =0.80V
3. Calculate the standard electrode potential of Cu2+/Cu if its electrode potential at 250C is
0.296V and [Cu2+]is 0.015M.
E = E0+ 0.0591 log[Mn+] n
E=0.296V,n=2,[Mn+]=0.015M
E0 2+ =0.296V
0.296= E0+ 0.0591 log[0.015] 2
E0 2+ =0.296+0.0540
=0.35V
4. A galvanic cell is obtained by the combination of Fe rod immersed in ferrous sulphate solution
of concentration 0.25M and Cu rod immersed in copper sulphate solution of concentration
0.45M.Give the cell representation, cell reactions and calculate EMF of cell at 300C.Given that
standard potentials of Cu and Fe electrodes are 0.34 and -0.41V respectively.
Representation of cell is Fe/Fe2+//Cu2+/Cu
Cell reactions are Anode:Fe→Fe2++2e-
Cathode:Cu2++2e-→Cu
Overall reaction: Fe+ Cu2+→ Fe2++Cu
E = E0+
0.0591 log
[Cu 2+] cell
2 [Fe 2+]
E = (0.34-(-0.41)+ 2.303 x8.314 x3030 .0591
log [0.45]
=0.76V
2x96500 [0.25]
REFERENCE ELECTRODES
Reference electrodes are the electrodes with reference to those, electrode potential of any
electrode can be measured. Standard hydrogen electrode is the primary reference electrodes as
the electrode potential value of other electrodes are assigned with respect to it.
Secondary reference electrodes
Secondary reference electrodes are those whose potential is known with respect to SHE. They
are commonly used to determine electrode potentials of other electrodes. Examples – glass
electrode, Ag/AgCl electrode, Calomel Electrode.
Calomel Electrode
It is a metal-metal salt ion electrode.
Construction-
1) The calomel electrode consists of a solid Mercurous chloride in contact with mercury placed
at the bottom of the long glass tube.
2) Mercurous chloride [Hg2Cl2] and the mercury paste is placed over mercury; the remaining
part of the tube is filled with KCl solution.
3) A platinum wire is dipped in mercury; it is used for external electrical contact.
4) The calomel electrode may act as anode or cathode depending on the nature of the other
electrode.
5) The electrode is represented as-
Hg| Hg2Cl2(s)| Cl-
If it acts as anode-
2Hg + 2Cl- Hg2Cl2 + 2e-
E= E0 – 0.0591 log [Cl-]
If it acts as cathode-
Hg2Cl2 + 2e-
2Hg + 2Cl-
The net reversible electrode reaction is
Hg2Cl2(s)+ 2e- 2Hg (l)+ 2Cl-
The potential developed from calomel electrode depends on the concentration of KCl solution.
E = E0–2.303 RTlog[Cl-]2
nF
E = E0 – 2.303 RTlog[Cl-] F
At 298K,
The electrode potential is decided by the concentration of chloride ions and the electrode is
reversible with chloride ions.
For 0.1M KCl 0.334 V
For 1MKCl 0.280 V
For saturated KCl 0.241 V ;
Applications
Used as a reference electrode for the determination of pH.
It is used in potentiometric titrations.
Determination of single electrode potential:
Measurement of single electrode potential using calomel electrode
Measurement of single electrode potential is not possible, only difference in the potentials
between the two electrodes can be measured using potentiometer.
To measure the electrode potential of given electrode it is coupled with a reference electrode
like calomel electrode. The EMF of the cell so formed is measured. Knowing the potential
offered by calomel electrode the potential of given electrode is calculated as follows.
EMF = E Cathode – E Anode
The anode and cathode of the cell formed is identified by connecting the electrodes to
appropriate terminals of potentiometer.
Zn /Zn
Zn /Zn
Cu /Cu
Example:
To determine the single electrode potential of Zinc electrode, it is coupled with the reference
electrode like calomel electrode and the cell may be represented as ZnZnSO4 Cl-
Hg2Cl2Hg. Let the EMF of the cell so formed is 1 V
1 V = Ecal –E 2+
E 2+ = 0.24V – (-0.1V)
= - 0.76V
To determine the single electrode potential of Copper electrode, it is coupled calomel electrode
and the cell may be represented as Hg Hg2Cl2 Cl- CuSO4Cu. Let the EMF of the cell so
formed is 0. 1V
0.1V = E Cu2+
/Cu –Ecal
E 2+ = 0.24V +0.1V
= 0.34V
Ag / AgCl electrode
Ag / AgCl electrode is also a metal-metal salt ion electrode. Silver and its sparingly soluble salt
silver chloride are in contact with a solution of Chloride ions.
Construction
It is prepared by coating thin layer AgCl electronically on to a silver wire.
A small sheet or wire of platinum is first coated with silver by electrolysis of sodium
argentocyanide solution.
The silver is then partly converted into silver chloride by making it an anode in KCl
solution and passing a current of low density for 30 minutes.
The electrode when placed in a solution of KCl with 1 to 2 drops of 1 M silver nitrate
develops potential.
Potential of 0.1N KCl is 0.29V
1N KCl is 0.223V
Saturated KCl is 0.199V
Cell representation –
Ag / AgCl / Cl-
When it acts as Anode:- Ag(s) + Cl- AgCl + e-
When it acts as Cathode:- AgCl(s) + e- Ag(s) + Cl-
The half cell reaction is
AgCl(s) + e- Ag(s) + Cl-
Applying Nernst equation,
E = E0 – 2.303RT log[Cl-] nF
E= E0 – 0.0591 log[Cl-]
Applications:-
It is used along with glass electrode to find pH of a solution.
It is non-toxic compared to calomel electrode and inexpensive.
It is stable and can detect voltages to 1mV.
Used to determine the uniformity of potential distribution in ships hulls & underground pipe
lines.
ION- SELECTIVE ELECTRODES - These electrodes have the ability to respond to specific
ions and develop a potential in the mixture and ignoring the other ion. The potential developed at
the electrode is a function of the concentration of that ion in the solution.
GLASS ELECTRODE
Principal - When two solutions of different pH are separated by a thin glass membrane, a
potential is developed and this potential is proportional to the pH value.
Glass Electrode
Construction:
It consist of a long glass tube with a thin glass membrane bulb at the bottom which is made
up of a special type of corning glass(Na2SiO3).
The corning glass has low melting point and high electrical conductivity.
The bulb is filled with 0.1M HCl and is inserted with aAg|AgCl electrode in it. It is also used
for external contact.
1M HCl is taken in a beaker and the electrode is immersed inside it. They cell is represented
as:
Due to the difference in the concentration of ions and the pH a potential is developed at the
membrane- [Ej] junction potential or boundary potential. At first we calculate the junction
potential and then finally the total cell potential.
At equilibrium-
Na+ (glass) + H+(aq) H+ (glass) + Na+ (aq)
By Nernst equation-
Ej =
2.303RT log
C1
nF C2
Ej = 2.303RT
logC1 -2.303 RT
log C2
nF nF
At 298 K (250C),
E=0.0591 log[C ]- 0.0591 log [C2]
n 1 n
If [C2] is kept constant, -0.0591 log [C2] =K n
Ej=0.0591 log C + K n 1
Thus total potential EG = Ej + Ereference+Easy(Since Ag/AgCl/Cl- electrode is present within the
glass electrode)
EG = 0.0591 log[C
] + K + Ereference+EasyEasy -Asymmetric potential arises due to the difference
n 1
in responses of the inner and outer surface of the glass bulb to changes in H+ ion activity.
[K + Ereference+Easy] = E0
G
EG = E0G + 0.0591 log[C ]
n 1
EG = E0
G + 0.0591 log [H+] pH = -log [H+] n
as n = 1 for H+
0 (𝐄𝟎 −𝐄𝐆) EG = E G – 0.0591 pH [pH = 𝐆 ]
𝟎.𝟎𝟓𝟗𝟏
Determination of pH using glass calomel electrodes:
To determine the pH of a given solution, the glass electrode is dipped in a given solution whose
pH needs to be determined.
Hg|Hg2Cl2|Cl-/ solution of unknown pH / Glass /Ag|AgCl|Cl-
The glass electrode is coupled with calomel electrode and dipped in the solution whose pH is
determined. Calomel acts as anode and glass acts as cathode.
By general expression-
Ecell = Ecathode - Eanode
Ecell = EG - Ecalomel
Ecell = E0G – 0.0591 pH -
Ecalomel Therefore-
pH = 𝐄𝟎
𝐆−𝐄𝐜𝐞𝐥𝐥−𝐄𝐜𝐚𝐥𝐨𝐦𝐞𝐥
𝟎.𝟎𝟓𝟗𝟏
(Since, Esat. calomel=0.24V)
Advantages of glass electrodes:
1) Glass electrode can be used in the presence of oxidising and reducing agents.
2) Electrode does not get poisoned.
3) It is simple to operate and is used in industries, agricultural labs, etc.
4) Accurate results are obtained between pH range 1-9. By using special glass electrode pH 1-
14 can be measured.
Disadvantage
1) Because of high resistance of glass, simple potentiometers cannot be used. It requires
sensitive electronic potentiometers for emf measurement.
2) The electrode can be used up to a pH 13 but becomes sensitive to Na+ ions above pH 9
resulting in an alkaline error.
Applications of ion selective electrodes
- It is used to find the concentrations of electrolyte solution at anodes and cathodes.
- To measure the level of CO2 in blood sample.
Electrolyte Concentration Cells
It is an electronic cell in which both anode and cathode are made up of the same metal in contact
with same electrolytes but varying concentrations.
Construction-
Two copper electrodes are dipped in copper sulphate solution of different concentrations. The
Cu electrode which is in contact with dilute solution acts as anode and that with concentrated
solution acts as cathode, and the two solutions are connected through a salt bridge.
The flow of electrons takes place from dilute solution to concentrated solution, such a cell is
called concentration cell with transference.
At Anode:-
Cu(s) Cu+2(aq) [M1] + 2e-
At Cathode:-
Cu2+(aq) [M2] + 2e- Cu(s)
Net cell reaction:-
Cu2+(aq)[M2] Cu2+(aq)[M1]
Concentration cell is represented as-
Cu(s) | Cu2+[M1] || Cu2+[M2] | Cu(s)
Applying Nernst equation-
Ecell = Ecathode - Eanode
Eanode = E0+ 0.0591 log[M1] n
Ecathode = E0+ 0.0591 log[M2] n
Ecell = [E0+ 0.0591 log[M2] – (E0+ 0.0591 log[M1])] n n
By the above equation
If [M1] = [M2] , emf of the cell is zero (Ecell = 0)
If [M2] > [M1], emf is positive
If [M2] is large, Ecell is large.
[M1]
Problem:1.The concentration cell tin/tin ion (0.024M)//tin ion(0.064M)/tin develops a potential
of 0.0126V at 250C.Calculate the valency of tin.
Ecell= 𝟎.𝟎
log [𝐌𝟐]
𝐧 [𝐌𝟏]
n= 0.0591
log [𝐌𝟐]
Ecell [𝐌𝟏]
n= 0.0591
log 0.064
0.0126 0.024
=1.998=2
Problem:2.Two copper rods placed in copper sulphate solutions of equal concentrations are
connected to form a concentration cell
a) What is cell potential?
b) Calculate the cell potential if one of the solutions is diluted to1/5th of its original concentration
a) Ecell= 𝟎.𝟎𝟓𝟗𝟏
log [𝐌𝟐]
𝐧 [𝐌𝟏]
Given M1=M2,Hence cell potential=0
b)Assume M2=1M then M1=1/5M
Ecell= 𝟎.𝟎
log [𝐌𝟐]
𝐧 [𝐌𝟏]
Ecell= 0.0591
log 1
2 1/5
[𝐌𝟏] 𝐧 Ecell=
𝟎.𝟎𝟓𝟗𝟏 log
[𝐌𝟐]
= 0.0591
log 5
2
=0.02955X0.699=0.0206V
Problem:3 A concentration cell is constructed by dipping copper rods in 0.001M and 0.1M
CuSO4 solutions.Calculate the emf of the cell at 298K.
Solution:
The cell is represented as,Cu(s)/Cu2+(0.001M)//Cu2+(0.1M)/Cu(s)
At the anode, Cu→Cu2++2e-
At the cathode, Cu2++2e-→Cu
Ecell =
𝟎.𝟎𝟓𝟗𝟏log
𝑴𝟐
𝒏 𝑴𝟏
=0.𝑂591
log 0.1
2 0.001
=0.𝑂591
log100 2
= 0.0591V
BATTERIES AND FUEL CELLS
BATTERY TECHNOLOGY
INTRODUCTION: A battery is a compact device consisting of two or more galvanic cells
connected in series or parallel or both. It stores chemical energy in the form of active materials
and on demand converts it into electrical energy through redox reactions. Thus, a battery acts as
portable source of electrical energy. Battery technology has acquired importance in view of
development in microelectronics and increased demand for portable gadgets. The trend of
miniaturization has been challenging to the battery manufacturers.
The size of a battery ranges from a fraction of a cubic centimeter to several cubic decimeters.
Batteries are used in calculators, digital watches, pace makers, hearing aids, portable computers,
electronically controlled cameras, car engines, stand-by power supplies, emergency lighting and
electroplating, industrial traction, telecommunication and military and space applications.
Battery technology has made possible replacement of petrol driven automobiles by electrically
powered ones. Electronic gadgets have become more reliable with the use of rechargeable
batteries. A battery is designed and manufactured for a specific performance such as, to power a
torch, to start a car engine, to supply emergency power to a hospital or to generate a very precise
voltage to maintain heartbeats.
COMPONENTS OF A BATTERY :
The basic electrochemical unit in a battery is the galvanic cell. The major components of a
battery are described below:
1. Anode or negative electrode : It releases electrons to the external circuit by undergoing
oxidation during electrochemical reaction.
2. Cathode or positive electrode : It accepts electrons from the external circuit and reduction
of an active species occurs.
3. Electrolyte : It is the active mass in the anode and cathode compartments. A solution of an
acid, alkali or salt having high ionic conductivity is commonly used as electrolyte. Solid
electrolytes with appreciable ionic conductivity at the operating temperature of the cell are
also used.
4. Separator : It separates the anode and the cathode in a battery to prevent internal short-
circuiting. It is permeable to the electrolyte and maintains the desired ionic conductivity.
The main function of the separator is to transport ions from the anode compartment to the
cathode compartment and vice versa. Fibrous forms of regenerated cellulose, vinyl polymers
and polyolefins, cellophane and nafion membranes are used as separators.
CLASSIFICATION OF BATTERIES
Batteries are classified into:
1. Primary Batteries
2. Secondary Batteries
3. Reserve Batteries
Primary Batteries
A battery which cannot be recharged and discarded when the battery has delivered all its
electrical energy is called Primary Battery.
They are non- rechargeable because the cell reactions are irreversible.
Example: Zn-MnO2 battery, Li-MnO2 battery
Secondary Batteries
A battery which after discharging can be recharged again by passing the electric current
through it in the opposite direction to that of discharge is known as Secondary Battery.
They are rechargeable because the cell reactions are reversible.
The secondary battery is also called as storage battery as it is the storage device for
electrical energy.
Example: Lead storage battery, Nickel Cadmium Battery, etc.
Reserve Batteries
One of the components is stored separately and incorporated into the battery when required.
Usually, the electrolyte is stored separately. Such batteries are called Reserve Batteries.
They are used to deliver high power for relatively short period of time in missiles, trajectory
and other weapon systems.
Example: Mg-AgCl and Mg-CuCl batteries both are activated by adding water.
Zinc-Air Battery
Zinc- air battery consists of a porous carbon plate as cathode. The cathode is activated for better
reduction of oxygen and treated with water repellants. The anode consists of rectangular flat
plates of zinc placed on either side of the cathode. The electrolyte is 20% NaOH. The outer
container is made of glass or ebonite.
Components of battery:
Anode-powdered zinc;
Cathode-Carbon;
Electrolyte- NaOH,
Separator- polyethylene.
Zinc-air battery is a primary battery. Its advantage is that air does not contribute to the mass of
the battery, hence offers high energy density. The cell is represented as,
𝒁𝒏 𝑵𝒂𝑶𝑯 𝟓 𝑴 𝒂𝒊𝒓, 𝑪
When air is passed through the cell, zinc is oxidized to ZnO at the anode, during discharge. The
oxygen of the air reacts with water at the cathode. The half-cell reactions are
𝟐𝒁𝒏 + 𝟒𝑶𝑯− → 𝟐𝒁𝒏𝑶 + 𝟐𝑯𝟐𝑶 + 𝟒𝒆 𝒂𝒏𝒐𝒅𝒆
𝑶𝟐 + 𝟐𝑯𝟐𝑶 + 𝟒𝒆 → 𝟒𝑶𝑯− 𝒄𝒂𝒕𝒉𝒐𝒅𝒆
The overall reaction is 𝟐𝒁𝒏 + 𝑶𝟐 → 𝟐 𝒁𝒏𝑶
Zn-air battery has an energy density of about 100 W h kg-1. This is three times that of the
classical lead acid battery and twice that of Ni-Cd battery.
Advantages:
1. High capacity 2.Low cost 3. Long shelf life
Disadvantages:
1.Limited power out put 2.Short activated life
Applications:
It finds applications in hearing aids, electronic pager ,railroad signaling, medical devices,
remote communications and military radio receivers.
Lead -Acid battery Secondary battery (or) Storage battery
The lead storage battery consists of two electrodes made of flat grids of lead as shown in Fig.
The anode grid is packed with a paste consisting of spongy lead and additives such as graphite
powder (0.25%) lignin sulphonate (0.2%) and barium sulphate (0.35%).
𝟒
𝟒
𝟒
Components :
Anode - Spongy lead,
Cathode-Lead grid (Pb02 and Pb)
Electrolyte -sulphuric acid
Separator - polyethylene.
The graphite improves the conductivity and the cathode lead grid is packed with a paste
consisting of equal amounts of PbO2 and Pb. Several such pairs of anode-cathode grids are
immersed alternately in 5M(37%) sulphuric acid (specific gravity 1.25) which acts as the
electrolyte. Separators made of micro porous polyethylene is used in between each set of
electrodes to insulate each plate from its neighboring counter electrode. The separators,
however, allow acid transport into and out of the plates. The thickness of the electrodes ranges
from 0.1 to 0.5 cm in car batteries. The battery is encased in a plastic or glass container.
The cell is represented as 𝑷𝒃, 𝑷𝒃𝑺𝑶𝟒 𝒔 𝑯𝟐𝑺𝑶𝟒 𝟓𝑴 𝑷𝒃𝑺 , 𝑷𝒃𝑶𝟐 𝒔 , 𝑷𝒃
𝑷𝒃 𝒔 + 𝑺𝑶𝟐− 𝒂𝒒 𝑷𝒃𝑺𝑶𝟒 𝒔 + 𝟐𝒆 𝒂𝒏𝒐𝒅𝒆
𝑷𝒃𝑶𝟐 𝒔 + 𝟒𝑯+ 𝒂𝒒 + 𝑺𝑶𝟐− 𝒂𝒒 + 𝟐𝒆 𝑷𝒃𝑺𝑶𝟒 𝒔 + 𝟐𝑯𝟐𝑶 𝒍 𝒄𝒂𝒕𝒉𝒐𝒅𝒆
The overall reaction is
𝑷𝒃 𝒔 + 𝑷𝒃𝑶𝟐 𝒔 + 𝟒𝑯+ 𝒂𝒒 + 𝟐𝑺𝑶𝟐− 𝒂𝒒 𝟐𝑷𝒃𝑺𝑶𝟒 𝒔 + 𝟐𝑯𝟐𝑶 𝒍
Thus at 250C and at a concentration of 7.4% sulphuric acid the potential developed by a pair of
electrode is 2V and a battery consisting of six such electrode pairs gives about 12V. Such a
battery is used in cars. Note that the cell potentials are found to be additive when connected in
series.
The life of the battery is limited because some PbSO4 plated over the electrode falls to the
bottom and hence will be no longer available for charging. This happens particularly when the
cell gets completely discharged.
Applications:
It is used in automobiles ,hospitals, laboratories, emergency power supplies, telephone
exchangers.
Limitations:
1. Potential decreases with decrease in concentration of H2SO4
2. Cell potential reduced at low temperature
3. Excessive discharge &quick charging shortens the life of battery.
Lithium Ion Battery:
Lithium-ion battery is a rechargeable battery best suited to mobile devices that require small
size, light weight and high performance. It has high energy and high voltage (3.6V).
Components:
Representation: Li/Li+, C/LiPF6/Li-CoO2
1. Anode: Lithium carbide.
2. Cathode: Lithium cobalt oxide(LiCoO2 ),where M is Cobalt( Co )
3. The electrolyte consists of Li salt in an organic solvent (LiPF6, LiBF4) in ethylene carbonate.
In anode, lithium atoms occupy the position between the graphite lattice layers.
4. Microporous polyethylene or polypropylene is used as separator.
The container is made up of stainless steel or aluminium alloy
In Li-ion battery, lithium ion move between the electrodes, using an intercalated electrode
material. Metal atoms and ions can enter layered solids reversibly. Such a process is referred as
intercalation.
During discharging of a battery,
Anode:
LiC6Li++e-+6C
Cathode:
Li++e-+CoO2LiCoO2
Net reaction:
LiC6 +CoO2LiCoO2+6C
During discharging of battery, Lithium atoms present in graphite layer (one Li atom is present
for every 6C atoms) are oxidized, liberating electrons and lithium ions. Electrons flow through
external circuit to cathode and lithium ions flow through the organic electrolyte towards cathode.
At cathode, lithium ions are reduced to lithium atoms and are inserted in to the layered structure
of metal oxide.
During charging of battery, lithium atoms present in layered structure of metal oxide are
oxidized,liberating electrons and lithium ions. Electrons flow through external circuit and
lithium ions flow through the organic electrolyte towards graphite carbon electrode. At graphite
electrode, lithium ions are reduced to lithium atoms and are inserted in to the layered structure
of graphite.
Applications:
Used in: Cameras, Calculator, Portable radio, TV, Laptops.
Fuel cells
Introduction The principle of the fuel cell was discovered in 1839 by Sir William Grove, who
has been acknowledged as the “Father of the Fuel Cell ”.
A fuel cell is a galvanic cell in which the chemical energy contained in a readily available
fuel oxidant system is converted directly into electrical energy by means of electrochemical
processes in which the fuel is oxidized at the anode.
Like any other electrochemical cell, the fuel cell has two electrodes and an electrolyte. However
, the fuel and the oxidizing agents are continuously and separately supplied to the two electrodes
of the cell, at which they undergo reactions. These cells are capable of supplying current as long
as they are supplied with the reactants.
A fuel cell essentially consists of the following arrangement:
Fuel/electrode/electrolyte/electrode/oxidant
At the anode, fuel under goes oxidation:
Fuel Oxidation product+ ne-
At the cathode, the oxidant gets reduced:
Oxidant+ ne- Reduction products
The electrons liberated from the oxidation process at the anode can perform useful work when
they pass through the external circuit to the cathode.
Advantages
1) Their power efficiency is high
2) They are eco-friendly since the products of the overall reactions are not toxic
3) They can produce direct currents for long periods at a low cost.
4) They are used as auxiliary power generators in space vehicles. Fuel cells are used in car
engines, domestic lighting and heating.
Limitations
1. Cost of power is high as a result of the cost of the electrodes
2. Fuels in the form of gases and oxygen need to be stored in tanks under high pressure.
3. Power output is moderate
4. To have an appreciable voltage, a battery of fuel cells must be available.
Methanol-Oxygen Fuel cell
Methanol is an efficient electro active organic fuels at low temperatures. The advantages of
methanol are
(i) It has low carbon content
(ii) It has a readily oxidizable OH group
(iii) It has high solubility in aqueous electrolytes
Methanol mixed with sulphuric acid (3.7M) is circulated through the anode chamber. Pure
oxygen is passed through the cathode chamber and sulphuric acid(electrolyte) is placed in the
central compartment. Both the electrodes are made of platinum. A membrane is inserted close to
the cathode to minimize diffusion of methanol into the cathode thereby reducing the
concentration of methanol near the cathode. In the absence of a membrane ,methanol diffuses
through the electrolyte into the cathode and undergoes oxidation.
The half-cell reactions are
CH3OH (l) +H2O (l) → CO2 (g) + 6H+(aq)+6e anode
3/2O2 (g) +6H+ (aq) + 6e→3 H2O(l) cathode
The overall reaction is
CH3OH (l)+ 3/2O2(g)→ CO2(g)+ 2H2O(l)
The advantage of acid electrolyte is that the CO2 , a product of the reaction, can be easily
removed. The cell potential is 1.21V at 250C.The methanol-oxygen fuel cell is used in military
applications and in large scale power production.
Super capacitors
Super capacitors, ultra capacitors or electrochemical double-layer capacitor are devices
that can be used as energy storage systems, that have high energy and power densities.
Super capacitor devices consist of two electrodes, a separator and an electrolyte. An ion-
permeable separator is placed between the electrodes in order to prevent electrical contact, but
still allows ions from the electrolyte to pass through. The electrodes are made with high effective
surface materials, such as porous carbon, graphene, carbon nano tubes .
The applied potential on the positive electrode attracts the negative ions in the
electrolyte, while the potential on the negative electrode attracts the positive ions. This results in
the formation of an electrical double-layer at each electrode/electrolyte interface that allows the
plates to store charge. These double layers, coupled with an increase in surface area and a
decrease in the distance between electrodes, allow super capacitors to achieve higher energy
densities than conventional capacitors.
In conventional capacitor, energy is stored by the removal of charge carries, typically
electrons, from one metal and depositing them on another. Unlike in a battery, the positive and
negative charges in a super capacitors are produced entirely by static electricity; no chemical
reactions are involved. The dielectric separator between the electrodes prevents the charge from
moving between the two electrodes.
Advantages:
1. Long life
2. Low cost per cycle
3. Good reversibility
4. Very high rates
5. High power output
Disadvantages:
1. High self-discharge
2. Cells have low voltages
3. Low energy density
4. Power is available only for a very short duration
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Applications:
1. They can be used in PC cards, flash photography devices in digital cameras, flash lights,
portable media players.
2. As an intermediate energy storage to power a variety of portable electrical and electronic
devices such as MP3 players, AM/FM radios, cell phones, and emergency kits
3. They have applications as energy-storage devices for smaller applications like home solar
systems where extremely fast charging is a valuable feature.
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Module 2
Corrosion and Metal Finishing
Definition:
Corrosion is defined as the destruction of metals or alloys by the surrounding environment
through chemical or electrochemical changes.
Ex:-Rusting of iron & green scales formation on copper vessels.
Depending upon the corrosion medium, corrosion is classified into
1. Dry corrosion&2.Wet corrosion
1. Dry corrosion
Dry corrosion involves the direct attack of metals by dry gases mainly through chemical reactions.
Dry corrosion is less common.
Ex:-Attack of dry air or oxygen on a metal to form an oxide layer.
2. Wet corrosion
It occurs due to electrochemical reaction of metals with air (O2) in aqueous medium
Ex:-Rusting of iron
Wet corrosion is explained on the basis of electrochemical theory.
Electrochemical theory of corrosion
When metal like iron is exposed to atmosphere, the following electrochemical change takes place
1. Formation of galvanic cells: Anodic and cathodic areas are formed resulting in minute galvanic cells.
2. Anodic reaction: At the anodic area, oxidation takes place resulting in the corrosion of iron. At anode
(oxidation reaction): M →Mn+
+ ne-
When iron undergoes corrosion: Fe → Fe+2
+ 2e-
3. Cathodic reaction: The electrons flow from the anodic to cathodic area and cause reduction. There are
different ways in which the reduction can take place.
At cathodic region
1. Liberation of hydrogen takes place in the absence of oxygen
2. Absorption of oxygen takes place in the presence of oxygen
2
1. Liberation of hydrogen
If the medium is acidic, in the absence of dissolved oxygen: 2H+ + 2e
-→ H2
In neutral and alkaline medium in the absence of dissolved oxygen :2H2O+2e-→ 2OH
- + H2
2. Absorption of oxygen takes place in the presence of oxygen
In acidic medium in the presence of oxygen:4H+ + O2 + 4e
-→2H2O
In neutral or alkaline medium and in the presence of oxygen: 2H2O+ O2 + 4e-→ 4 OH
-
Corrosion of iron produces Fe+2
and OH- ions at the anode and cathode sites respectively. These ions
diffuse towards each other.2Fe2+
+4OH- →2Fe (OH) 2(ferrous hydroxide)
4Fe (OH) 2+O2 +2H2O →2 Fe2O3.3 H2O
Hydrated ferric oxide (yellow rust)
In the presence of limited oxygen, ferrous hydroxide is converted into magnetic oxide of iron (Fe3O4)3Fe
(OH) 2+½ O2→Fe3O4.3H2O
(Black rust)
Factors influencing the corrosion rate:
Primary Factors:
1. Relative size of anodic and cathodic areas:
Corrosion of a metal occurs fast if the anodic area is small and the cathodic area is larger.
For example,tin is cathodic to iron as can be inferred from its position in the electrochemical series. In the
3
plating of tin on iron, if some areas are not covered or some pin holes are left,a small anode and a large
cathode areas are formed. An intense localized corrosion occurs at the exposed anodic area.
On the other hand, zinc plating on iron gives an anodic coating to iron. Even if zinc plating peels off at
some points, the rate of corrosion of iron is low. This is because of the formation of large anodic and
small cathodic area.
2. Nature of the metal:
The electrode potential is the main factor, which determines the rate of corrosion in differential metal
corrosion. When two different metals are exposed to the atmosphere are in contact with each other, the
more active metal with lower electrode potential values are more reactive and undergoes corrosion very
fast.The noble metals such as Silver, Gold, Platinum with higher electrode potential values are less
susceptible for corrosion.The rate of corrosion depends upon the difference in potential .Larger is the
potential difference;higher is the rate of corrosion.For eg: the potential difference between iron and
copper is 0.78V which is more than that between iron and tin 0.3V, therefore, iron corrodes faster in
contact with copper.
3. Nature of the corrosion product:
Most of the metals form their oxides as the corrosion product.This oxide film forms athin film on the
surface of the metal and its nature decides the extent of further corrosion.
If the corrosion product is stoichiometric in composition, insoluble and nonporous with low ionic and
electronic conductivities, then the layer effectively prevents further corrosion.
Ex:-Al,Ti,Cr metals develop such protective layer and prevent further corrosion.
If the corrosion product is non-stoichiometric, highly soluble and porous with higher ionic and electronic
conductivities, the layer cannot effectively prevent further corrosion.Ex: Fe, Zn.
Secondary factors:
4. pH;The rate of corrosion increases with decrease in pH. If the pH is less than three, greater corrosion
occurs even in the absence of air, due to evolution of hydrogen at cathode.
2H+ + 2e
-→ H2
4
At pH greater than 10, corrosion of iron practically ceases due to the formation of a protective coating of
insoluble metal hydroxides. Between pH 10 and 3 presence of oxygen is essential for corrosion.Ex:-Ships
submerged in seawaterremain unaffected.
Types of Corrosion
According to electrochemical theory, corrosion occurs due to formation of large number of minute
galvanic cells. There are different ways in which galvanic cell can be formed on the metal surface.
Accordingly, the following types of corrosion have been observed.
1) Differential Metal corrosion:
When two dissimilar metals are in contact with each other, the metal with lower reduction potential
becomes anodic and undergoes corrosion. Whereas, the metal with higher potential becomes cathodic
and remains unaffected. This kind of corrosion is called differential metal corrosion or galvanic
corrosion. The rate of corrosion depends primarily on the amount of current passing from the anode to
the cathodeand also on the difference in potential between the metals. Higher the difference, faster is the
rate of corrosion.
Example:
a) Std. potential of Fe(-0.44V) is less than that of Cu(0.34V). Therefore, when iron is in
contact with copper, iron becomes anodic and undergoes corrosion whereas copper
becomes cathodic and remains unaffected.
b) Std. potential of Zn(-0.76V) is less than that of Fe(0.44V). therefore, when iron is
in contact with Zn, Zinc undergoes corrosion whereas Fe remains unaffected.
5
2. Differential aeration corrosion:
When metal is exposed to different concentrations of air(O2), part of the metal exposed to lower
concentration of O2 becomes anodic and undergoes corrosion. Whereas, other part of the metal
exposed to higher concentration of O2 becomes cathodic and remains unaffected. This kind of
corrosion is called as differential aeration corrosion.
Ex:Iron /zinc rod partially immersed in NaCl solution. The part of the metal immersed in solution is
exposed to lower concentration of oxygen becomes anodic area and undergoes corrosion. Whereas,
part of the metal outside water is exposed to more oxygen becomes cathodic and remains unaffected.
Examples of differential aeration corrosion, a) Waterline corrosion, & b) Pitting corrosion.
a) Water line corrosion:
It is observed in steel or iron water tanks partially filled with water. Metal just below the water line,
whichis exposed to lower concentration of O2, becomes anodic and undergoes corrosion.
Part of the tank just above the water line which is exposed to higher concentration of O2becomes cathodic
and remains unaffected. This type of corrosion is also observed in ships floating in seawater.
6
b) Pitting corrosion:
Pitting corrosion results when dust particles get deposited on a metal surface. The portion covered by the
dust will be less aerated compared to the exposed surface. The covered portion thus becomes anodic with
respect to the exposed surface. In the presence of electrolytes and moisture corrosion starts beneath the
dust resulting in a pit. Once a pit is formed, the corrosion progress rapidly. This is because of the
formation of small anodic area (pit) compared to a large cathodic area (surface).
Differential aeration corrosion is observed in
1) Less aerated areas in machinery
2) Less aerated areas in metals as for example cracks
3) Less aerated points of contact in a wire screen
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Stress Corrosion:
If two samples of steel, one of which is under stress, are placed in a corrosive environment, the stressed
specimen corrodes at a faster rate. Stress may be due to mechanical operations such as poor design,
riveting, cold working, welding, bending, pressing and quenching. In a corrosive environment, the
stressed portion is anodic with respect to the unstressed portion and undergoes corrosion. At the stressed
portion, the atoms are slightly displaced creating an anodic zone of higher potential. These zones are very
active and are attacked even by mild corrosive agents.
An example of stress corrosion is intercrystalline corrosion (or) caustic embrittlement.Mild steel boilers
under corrosion at the stressed portion when the pressure is 10 to 20 atm. Fine hairline cracks may be
present at the stressed portion of the boiler. Boiler water containing alkaline impurities passes into the
cracks by capillary action. Later water evaporates leaving behind caustic soda in the cracks. When the
concentration of caustic soda reaches 10%, an electrochemical cell is set up between the iron under stress
and the iron in the main body.
The iron surrounded by dilute
sodium hydroxide is the cathode and the iron under stress acts as the anode and gets corroded resulting in boiler
failure.
8
Iron under stress/ Con.NaOH / DilNaOH/ Iron (cathode)
Anode In cracks In water (Main body)
Na2CO3+ H2O 2NaOH + CO2
3Na2FeO2+4H2O 6NaOH + Fe3O4 + H2
Caustic embrittlement can be prevented by the addition of compounds such as sodium sulphate, tannin,
lignin, phosphates which block the cracks, thereby preventing the infiltration of alkali.
Corrosion control Techniques:
Corrosion can be controlled by preventing the formation of galvanic cells
1) Protective coatings
a) Metal coatings - anodic and cathodic
b) Surface conversion coatings – anodizing and phosphating
a) Metal coatings
Corrosion of metals can be controlled by isolating them from the environment. This can be brought
about by covering the substrate or base metal with a layer of another metal. The process is referred
to as metal coating. Metal coatings are of two types:
Anodic metal coating
Cathodic metal coating
i) Anodic metal coating:
Anodic metal coatings are coatings, which are anodic to the base metals. Examples are Aluminum,
magnesium, Zinc and cadmium coatings on iron. These metals are above iron in the galvanic series and
undergo corrosion thereby protecting iron. A characteristics feature of anodic coatings is that the base
metal on which the coating is done will not get corroded even if the coatings peal off. This is due to the
formation of large anodic and small cathodic areas. Hot dipping or galvanising is an example of metal
coating.
The process of coating zinc on iron by hot dipping is called galvanizing. It involves the following steps:
1.The metal surface is washed with organic solvents to remove organic matter on the surface.
2. Rust and other deposits are removed by washing with dilute sulphuric acid.
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3.The article is then dipped in a bath of aqueous solution of ZnCl2 and NH4Cl which acts as a flux and
then dried. Finally, it is dipped in molten zinc, maintained at 430-4700C. The flux of ammonium chloride
also prevent the oxidation of molten zinc.
4. The excess zinc on the surface is removed by passing through a pair of hot rollers, which wipes out
excess zinc coating and produces a thin coating.
Galvanization is used extensively to protect iron from corrosion in the form of roofing sheets, fencing
wire, buckets, nuts, nails, pipes, tubes, etc.
Galvanized articles are not used for preparing and storing foodstuffs, since zinc dissolves in dilute
acids producing toxic zinc compounds.
ii) Cathodic Metal Coating
Cathodic metal coatings are coatings, which are cathodic to the base metal. Examples are tin, nickel,
chromium and copper on iron.
In this type of coating, base metal should be completely covered and there should not be any gap left over
it.The coating is given in such a way that the applied coat completely covers the surface without leaving
even pinholes. Otherwise, rapid corrosion of the base metal takes due to the formation of large cathodic
and small anodic areas.
Tinning, the process of coating tin on iron is described below:
1. The sheet is first washed with organic solvents to remove grease or oil deposits.
2. Then treated with dilute sulphuric acid to remove rust and scale deposits. Finally, it is washed
well with and air-dried.
3. Then it is passed through molten zinc chloride and ammonium chloride flux and then dried. The
flux helps to adhere on the metal surface.
4. It is passed through a tank that contains molten tin at 3300C.
5. Finally, passed through a series of rollers immersed in palm oil to prevent oxidation of tin.The
roller wipe the excess tin deposits and produce a continuous, thin coating of tin on the sheet.
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The largest single use of tin is in coating of steel for manufacturing containers used for storing foodstuffs
like jam, instant food, milk products, pickles, etc.Copper utensils are coated with tin to prevent
contamination of foodstuffs with poisonous copper.
b)Surface conversion coatings
Anodizing:
It is the process of oxidation of outer layer of metal to its metal oxide by electrolysis.Oxide layer
formed over the metal itself acts as a protective layer.
Passive metals, which are capable of forming non-porous, non-conducting oxide layer,can be protected
fromcorrosion by subjecting to anodizing. This phenomenon is also known asanodic oxidation. Other
metals such as magnesium, titanium, also form anodic films. But these have not found commercial
applications.
In the anodising of aluminium, it is cleaned, degreased, and polished and taken as anode in an electrolytic
cell. Steel or copper is taken as cathode.The electrolyte consists of 5-10% chromic acid. The temperature
ofthe bathis maintained at 350C. A potential is applied and gradually increased from 0 to 40V during the
first 10mins. Anodizingis continued for 20mins at 40V. After 20mins, the potential is increased to 50V
and held at this potential for 5mins. An opaque oxide layer of 2-5 micrometre thickness is obtained. For
higher thickness, 10% sulphuric acid is used as the electrolyte at 22 oC and at a potential of 24V.The
article is dyed immersing for about 20mins in a solution of the dye at 50-60oC . Finally, the article is
treated with nickel acetate followed by boiling water.
Anodized aluminium is also used as an attractive,highly durable exterior for roofs, walls, buildings, and
in computer hardware exhibit displays, scientific instruments and in a range of home appliances and
consumer products. It also finds applications in satellites for their protection in space environment.
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Cathodic protection :Corrosion of metal occurs at anodic area, where metal is oxidised liberating
electrons.These electrons flow to the cathodic area of the metal, which remains unaffected. In cathodic
protection, electrons are supplied from an external source to prevent the oxidation of metal at anodic area
and thus making the whole metal cathodic.When whole metal becomes cathodic, itdoesn’t undergo
corrosion.
The process of protecting a metal against corrosion by making the entire metal to be cathodic by
providing electrons from an external source is called as cathodic protection.There are two important
methods of cathodic protection.
a)Sacrificial anodic method:In this method, zinc or magnesium which are anodic to the specimen metal
is used.The specimen to be protected is usually made of iron,copper, or brass. The anodic metal like zinc,
or magnesium when connected to the specimen undergoes oxidation and provide electrons to the
specimen.Thereby,the entire specimen metal is converted to be cathodic and hence protected from
corrosion.Anodic metal undergoes corrosion and sacrifices itself to protect the specimen.Therefore, it is
called sacrificial anode.Sacrificial anodes have to be replaced in due course of time.This method is used
to protect buried pipelines,ships,water tanks,iron rods.
b)Impressed current method :
In this method,the electrons are supplied from a direct current source.Metal specimen to be
protected is connected to negative terminal of direct current (DC) source. Potential greater than
potential of anodic reaction is applied in the reverse direction to prevent anodic reaction.Thus,
whole metal becomes cathodic and hence protected from corrosion.The positive terminal of the
battery is connected to inert anode like platinised titanium.
This method is used for protecting marine structures, water tanks, offshore rigs,and oil pipelines.
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METAL FINISHING
The term metal finishing covers a wide range of processes carried out to modify the surface
properties of a metal by deposition of a layer of another metal, a polymer or by the formation of an
oxide film.
Metal Finishing Techniques
1) Electroplating of metals, alloys.
2) Electrolesspating of metals, alloys.
3) Immersion plating of metals.
4) Electrophoretic painting involving painting on electrically charged conducting surface.
5) Chemical conversion coatings. Ex: Anodizing, Phosphating.
Importance of Metal Finishing
Metal finishing finds extensive applications in industries, electronics, engineering and metal processing
companies.The technological importance of metal finishing is
1. To give decorative surfaces.
2. To impart abrasion and wear resistance.
3. To offer corrosion resistance.
4. Imparting hardness.
5. To impart thermal resistance.
6. To offer the surface a thermal or optical reflectivity
7. To provide electrical or thermal conduction
8.In the manufacture of electrical and electronic components such as printed circuit boards, capacitors and
contacts.
9. To manufacture metal articles by electroplating
10. To produce gramophone records.
11. In electrochemical machining, polishing and etching.
Polarization, decomposition potential and over voltage with respect to metal finishing:
1. Polarization
Polarization is an electrode phenomenon. The electrode potential is determined by the Nernst equation:
E = Eo+ 0.0591 log(M
n+)
n
whereMn+
is the concentration of the metal ions surrounding the electrode surface at equilibrium. When
there is a passage of current, the metal ion concentration in the vicinity of the electrode surface decreases
owing to the reduction of metal ions. Therefore, there is a shift in the equilibrium and a change in the
electrode potential.
13
The concentration gradient between the bulk of the solution and the area surrounding the electrode
surface leads to diffusion of ions from the bulk of the solution towards the electrode surface. This re-
establishes the equilibrium.
If the diffusion is slow, the electrode potential changes and the electrode is now said to be polarized.
Polarization is defined as a process in which there is a variation in electrode potential due to slow
supply of ions from the bulk of the solution to the electrode. Polarization depends on
1) Nature of the electrode i.e. size, shape and composition.
2) Electrolyte concentration.
3) Temperature.
4) Products formed at the electrodes.
5) Rate of stirring of the electrolyte.Effect of polarization can be minimized by stirring the electrolyte
continuously and by using electrodes of larger surface area and electrolytes of higher conductivity.
2) Decomposition Potential: It is defined as the minimum voltage that must be applied to bring
about continuous electrolysis of an electrolyte.Decomposition potential can be determined by using an
electrolytic cell. The cell consists of two platinum electrodes immersed in a dilute solution of an acid or a
base. The voltage is varied by moving the contact maker D along the wire AB and the current passing
through the cell is measured using an ammeterAt low voltage no reaction occurs and there is a very slight
increase in the current. On increasing the voltage to slightly above 1.7V, there is an abrupt increase in the
current and sudden evolution of H2& O2. The applied voltage of 1.7V is the decomposition voltage for
dilute acids & bases.A plot of the current flowing between the electrodes against the applied voltage is
shown in thebelow fig
.
There is a very slight increase in the current at low voltages. Beyond the decomposition voltage
electrolysis begins and there is an abrupt increase in the current.
Consider the electrolysis of a 1M solution of ZnI2 which produces Zn and I2 at the respective electrodes.
For this electrolysis, the decomposition potential is found to 1.30V which is in agreement with theoretical
value. The products accumulated at the electrodes and the setup of the following cell is as follows;
Zn | Zn2+
|| I2 |I-
14
This cell exerts a back emf and offers resistance to the flow of current. For electrolysis to occur, the
applied voltage (Eo) should be at least equal to the back emf.
Zn | Zn2+
|| I2 |I-
4) Over –Voltage:
It is defined as the excess voltage that has to be applied above the theoretical decomposition
potential for continuous electrolysis.This is denoted by η. The applied voltage has to exceed the
theoretical value by at least of 1V for continuous electrolysis. This is known as over-voltage or over
potential.
η= Experimental decomposition potential - theoretical decomposition potential
ED = ECathode - EAnode + η
This cell exerts a back emf and offers resistance to the flow of current. For electrolysis to occur, the
applied voltage (Eo) should be at least equal to the back emf.
Over Voltage depends on several factors:-
1) Nature and physical state of the metal electrodes.
2) Nature of the substance deposited.
3) Current density.
4) Temperature.
5) Rate of stirring of the electrolyte.
Hydrogen overvoltage is the overvoltage required for the liberation of hydrogen at the cathode during
electrolysis. It is large for soft metals like lead; mercury shows the highest overvoltage among metals.
Hydrogen overvoltage is a measure of the liberation of hydrogen gas from the electrode surface; a lower
overvoltage indicates a quick release of hydrogen gas. The liberation of H2 on electrode surface takes
place in three steps ;
H3O+ H
++ H2O
H+ + e
- H
H + H H2
The deposition of metals over electrode surface takes place in one step.
M n +
+ e M
Therefore the overvoltage for metal deposition is small compared to hydrogen overvoltage.
Factors influencing the nature of electrodeposit:
1)Current density :
Current density is the current per unit area of the electrode surface. It is expressed in mA/cm2 (or)
A/dm2
(or) A/m2 .
When the current density is low, metal is deposited slowly and the deposits produced are gross grained.
When the current density is high, the rate of electro deposition increases but the electro deposit is loose,
15
rough and brittle. In general, it is desirable to use as high current density as practicable and thereby to
increase the rate of electroplating.
When the current density is above the optimum limiting value, one more possibility is the formation of a
burnt and spongy deposit.
This is caused by the occurrence of hydrogen discharge at the cathode. Therefore, an optimum current
density should be applied to obtain a fine and smooth deposit.
2) Concentration of metal ions: In general, a decrease in metal ion concentration decreases the crystal
size and results in a fine adherent coating films.This can be achieved either by the addition of a compound
with a common ion or by the formation of complex compounds and ions. For example, when copper is
deposited from copper sulphate bath, sulphuric acid is added to the solution
CuSO4 Cu2+
+ SO42-
H2SO4 2H+
+ SO42-
Due to common ion effect of SO42-
from sulphuric acid, the concentration of cupric ions in the solution is
reduced, using a solution of sodium cuprocyanide,
Low concentration of cuprous ions is formed through the formation of [Cu(CN)2]- and this releases
cuprous ions continuously for the deposition of copper.
3)pH;
For a good electrodeposit, the pH of the bath must be properly maintained by using a suitable buffer at a
definite range.
In general, the cyanide baths are alkaline with pH values varying from about 9 for silver to about 13 for
cadmium. For non-cyanide baths, slightly basic or acidic conditions are preferred.
At a higher pH, precipitation of hydroxides of the metal may take place.
At very low pH.more hydrogen evolution takes place on the cathode, giving burnt deposit.
Therefore, the required pH is maintained by using suitable buffers by having optimum pH value of 4-8.
Ex: Electroplating of nickel-Buffer- 4.0 - 4.5
5) Temperature :
Rate of electrodeposition increases with increase in temperature. But, at higher temperatures evolution of
hydrogen gas, corrosion of equipment also increases and organic additives may be decomposed. At lower
temperature, rate of electrodeposition is lower. Hence, plating is generally carried out at moderate
temperature of 30-600C.
16
5)Additives( organic additives and complexing agents):
Organic additives:These are organic compounds added in very small quantity to plating bath to
improve the quality of deposit.Their mode of action is specific.There are four classes of organic
additives.
i)Brightners;These are used in plating bath to obtain bright deposit.They produce fine deposit as light
falling on the deposit gets reflected and hence the surface becomes bright.Ex;thiourea.
ii) Levelers:These are used to obtain an even electrodeposit on the irregular surface by getting adsorbed
at regions where rapid deposition takes place.Thus,the adsorbed additives avoid uneven deposition.Ex:
sodium allylsulphonate.
iii)Structure modifiers:(stress relievers): Electrodeposits are associated with internal stress and results
in micro cracking of the deposit.The structure modifiers modify the structure of the deposit in such a way
as to reduce the internal stress.Ex: saccharin
iv) Wetting agents:Common side reaction during electroplating is liberation of hydrogen gas at cathode
surface,ie 2H+ + 2e
- H2
These gas bubbles get entrapped within the deposit and try to escape after the plating is over.This leads to
burnt deposit. Wetting agents release effectively hydrogen gas bubbles from the deposition and hence
prevent them from entrapment. Ex:sodium lauryl sulphate.
Complexing Agents
Complexing agents are used for the following purposes:-
1. To maintain low metal ion concentration.
2. To prevent the chemical reaction between cathode metal & plating ions.
3. To prevent the passivation of anode.
4. To improve the throwing power of the plating bath.
5. To enhance the solubility of the slightly soluble metal salts.
The most common complexing agents used in electroplating are hydroxide, cyanide and sulphate ions.
Throwing power of plating bath and its determination by Haring Blum cell:
Throwing power is defined as the degree of uniformity of metal distribution or evenness of deposit
thickness obtained on a cathode of irregular shape. The throwing power depends on the shape of the
cathode. It is maximum in regular shaped cathode whereas it is minimum, if the cathode shape is
irregular. Factors affecting throwing power Conductivity of the electrolyte- Solutions with higher
conductivity will have higher throwing power. 2. Presence of addition agents- Additives like levelers and
brighteners ensure an even deposit and hence increase the throwing power. 3. Agitation
Haring Blum Cell:-Throwing power is determined by Haring- Blumcell.This electrolytic cell consists
of a box of PVC of 15x5 cm dimension and it consists of an anode placed between 2 cathodes.Cathode 1
17
is placed at a distance of X1 from anode and cathode 2 is placed at a distance of X2 from anode in such a
way that X1 >X2 . All the electrodes are completely immersed in plating bath whose throwing power has
to be estimated. Electroplating is carried out for a fixed time and weight of metal deposited over two
cathodes is measured.If the weight of the metal deposited over C1 and C2is W1 and W2 respectively.
Throwing power is calculated by using the equation
%TP = (K-M) / [K+M-2] x 100
where,
K=X1/X2& M= W2/W1
W1& W2 are the weights of the metal deposited on the cathodes.
When W1=W2, that is the amount deposited is same irrespective of the placement of the electrodes,
throwing power is 100% and is considered to be very good.
ELECTROPLATING:
Electroplating can be defined as a process in which a base metal is coated with a thin and uniform
layer of another metal by electrolytic deposit.
The components include:
1) An electrochemical bath containing a conducting salt and the metal to plated in a soluble form as well
as a buffer and additives.
2) The electronically conducting cathode ie the article to be plated.
3) The electronically conducting anode, the coating metal itself or an inert material of good electrical
conductivity like graphite.
4) An inert vessel to contain the above mentioned materials made of either rubber lined steel, plastic,
concrete or wood.
18
5) A D.C- electrical power source.
Electroplating is the process of electrolytically depositing a layer of metal on to a surface. The object to
be plated is made the cathode in an electrolytic bath containing metal ion Mn+
, so that the simplest
reaction at the cathode is
Mn+
+ ne- M
The preferred anode reaction is the dissolution of the same metal in the solution:
M Mn+
+ ne-
However if the anode is made of some inert material that does not pass into the solution, the electrolytic
salt is added continously in order to maintain optimum metal ion concentration in the solution.
Electroplating of gold :It is generally carried out using cyanide bath.Gold plating bath is classified into
three categories based on pH range.
Alkaline cyanide bath- pH over 10
Neutral cyanide bath- pH 6-9
Acid cyanide bath- pH 3.5-5
This electroplating process is carried for plating printed circuit boards,connectors,and for plating semi-
conductors.
Alkaline cyanide and neutral cyanide baths are employed for plating semi-conductors while acid cyanide
is used for plating printed circuit boards,connectors.
Alkaline cyanide bath:
Plating bath: potassium gold cyanide,dipotassium phosphate and potassium cyanide.
pH: 12
Current density: 3-5 ASF
Temperature: 49-71OC
Anode: Stainless steel
Cathode: Object to be plated with gold
Time to plate 0.0001’’ thickness: 8 minutes.
Reactions:-
Au(CN)4-+ 2e Au(CN)2
-+ 2CN
-
Au(CN)2-+e Au + 2CN
-
19
Gold metal when made anode dissolves very fast in cyanide bath. Therefore, inert anode is used.AuCl3is
added periodically to react with cyanide ions released during the reaction. This helps in maintaining the
concentration of Au(CN)4- in the bath. When gold is directly plated on copper, the copper atoms have the
tendency to diffuse through the gold layer,causing tarnishing of its surface and formation of an oxide
layer.Therefore,a layer of a suitable barrier usually nickel to be deposited on the copper substrate,before
gold plating.
Applications:Used for decorative purposes in jewellery,in the electrical industry, printed circuits,
contacts, in electronics, transistors and in aerospace industry.
Electroless Plating
It is defined as the controlled deposition of a continuous film of a metal from its salt solution on to a
catalytically active surface by a suitable reducing agent without using electrical energy.
Mn+
+ Reducing agent M + Oxidized product
Pre-treatment Process- preparation of catalytically active surface:
1) Acid Treatment- The substrate surface is given an acid treatment with Cr2072-
and H2SO4.
2) Electroplating a thin layer of the metal to be plated.
3) For non-conducting surfaces such as plastics (or) PCB or glass the surface is treated with Stannous
Chloride solution containing hydrochloric acid at 250C followed by dipping in palladium solution.The
surface is dried to get a layer of Pd.
Electroless plating of Copper
Copper from its solution gets deposited spontaneously in presence of a suitable reducing agent on metals
such as gold, silver, platinum, palladium, rhodium. Insulators like plastics & glass are to be activated
before subjecting to electroless plating.
An important application of electroless copper plating is in Printed Circuit Boards(PCB).
Plating Bath
Plating bath solution: Solution of copper sulphate.
Reducing Agent: Formaldehyde
Buffer: NaOH & Rochelle Salt.
Complexing Agent: EDTA
20
pH: 11
Temperature: 25oC
Cathode: Cu2+
+ 2e Cu
Anode: 2HCHO + 4OH- 2HCOO
- + 2H2O + H2 + 2e
Overall Reaction: Cu2+
+ 2HCHO + 4OH- Cu + 2HCOO
- + 2H2O + H2
The PCB base made of polymeric material is initially cladded with thin foils of copper on both sides. Then it is
etched as per circuit specifications to get circuit track. Holes are drilled through the circuit tracks as per circuit
specifications for double-sided PCB. The holes are givenpre-treatment and arecatalytically activatedby dipping
in stannous chloride and palladium chloride. The catalytically activated PCB is dipped in plating bath
solution for electroless plating of holes. The holes are coatedby dipping in stannous chloride and
palladium chloridewith thin layer of copper resulting in double sided PCB
Distinction between Electroplating and Electrolessplating
Electroplating Electrolessplating
Driving force Current Autocatalytic redox
reaction
Anode Separate anode Catalytic surface of
Substrate
Cathode Object to be plated(treated to remove
surface impurities)
Object to be plated(treated
to make surface
catalytically active)
Reducing agent electrons Chemical reagents
Applicability Only on conductors Conductors and non
conductors
21
Throwing
power
Not Satisfactory with irregular shape Satisfactory in all parts
Cost Not economical Most economical
Reactions Oxidation takes place at anode and
reduction at cathode
Both oxidation and
reduction takes place at
catalytically active surface
1
Module 3
Chemical Energy Sources and Photovoltaic Cells Introduction
Energy resources play a vital role in the economy of the country. Energy consumption increases
with the complexity of life one chooses. In early days the energy demands were met by muscular
effort, wind and water currents, fuel-wood and direct solar warming. Today, energy consumption
is related to the extent of industrialization of a country and the standard of living of its people.
With the progress of industrial civilization, fuels are used in transportation, communication,
illumination, manufacturing and a host of other applications. The subject of interest in recent
times is energy production, consumption and management. With the increase in population there
has been a ten-fold growth in energy consumption over the past decades and expected a thirty-
time increase by 2050, which becomes a challenging issue to be discussed. Energy resources
may be conventional, non- conventional, renewable and non-renewable.
At present, the major energy sources are hydroelectric, coal, petroleum, natural gas and the
product of their processing, water, biomass and nuclear. The sun, wind and the tides also meet
with a part of the energy needs.
Due to the eminent exhaustion of these resources one need to understand ways for their efficient
use and in developing an alternate energy source.It is in this context an engineer has to
understand “fuels” is of utmost importance.
Chemical fuel
Definition:
A chemical fuel is a substance, which on combustion in air or oxygen produces significant
amounts of heat, which can be conveniently used for useful domestic and other purposes.
Fuels + O2 Products + Heat
The principle elements present in chemical fuels are carbon and hydrogen. When a chemical fuel
undergoes combustion in air, carbon gets converted to carbon dioxide and hydrogen gets
converted to water.
The examples of chemical fuels include wood, coal, crude oil, natural gas etc. chemical fuel are
primarily used as source of heat and power. Apart from this they are raw material in the
production of several industrially important organic compounds.
Classification of Fuels
2
On the basis of their origin, Chemicals fuels are broadly classified as primary and secondary
fuels. These are subdivided into solid, liquid and gaseous fuel based on their physical state in
which they are used.
A primary fuel is the one which occurs naturally and requires no chemical processing before
utilization. Example of primary fuel include wood, coal, crude petroleum and natural gas.
A secondary fuel is produced from naturally occurring substance by subjecting them to
treatments which alter their chemical composition and often improve their calorific value and
utility. Examples of secondary fuels include coke, coal gas and gasoline.
Table: 1 Classification of fuels
Physical State Primary Fuels(Natural) Secondary Fuels(Derived)
1 Solid
Wood, Coal, Peat
Charcoal, coke
2
Liquid
Petroleum (Crude Oil) Petrol, Diesel, Kerosene, Synthetic petrol
3 Gas Natural Gas Producer gas, Water gas, LPG, Biogas
Importance of Hydro-carbon fuels
Petroleum, coal and natural gas are excellent hydrocarbon fuels. They are called “fossil fuels”.
These fuels are excellent hydrocarbon fuels as they contain carbon and hydrogen as major
elements. When they are burnt in adequate supply of air, they are oxidized to CO2 and water and
release significant amount of energy in the form of heat which can be converted into a suitable
form to perform useful work. Among these fossil fuels natural gas(mainly methane) is cheap and
it is a superior fuel. It burns cleanly, leaves no residue and produces less CO2 per unit energy.
Unlike coal and petroleum it produces no NOX, SOx, CO etc as pollutants it is therefore safest
and excellent fuel available.
Development in chemical industry saw a parallel increase in the demand for petroleum as a raw
material. Petroleum is used in manufacture of many organic chemicals, polymers, fertilisers,
drugs, dyes etc. thus, the present day world is heavily dependent on hydrocarbon fuels for the
energy needs in transportation(vehicles, trains, aeroplanes), industrial processing, heating,
cooling buildings(refrigeration), generating electric power(Thermal power plant) etc.
Calorific Value
Calorific value of a fuel gives information about its heating efficiency.
3
Calorific value: Calorific value of a fuel is defined as “the total quantity of heat liberated, when
a unit mass or volume of the fuel is burnt completely in air or oxygen and the products of
combustion are cooled to ambient temperature ”.
Units:
The calorific value is normally expressed in Calories/gram in cgs units.
(1cal/g = 4.182kJ/kg)
SI unit for solid fuels J/ kg
For gaseous fuels J/m3
Gross(higher) calorific value (HCV) :
Gross or HCV is the “ total amount of heat produced, when unit mass/volume of the fuel has
been burnt completely and the products of combustion have been cooled to room temperature”
Usually all fuels contains some hydrogen and when the calorific value of hydrogen containing
fuel is determined experimentally, the hydrogen is converted into steam. If the products of
combustion are condensed to the room temperature, the latent heat of condensation of steam also
gets included in the measured heat. Therefore, it is always higher than the net calorific value.
GCV = Sensible heat + Latent heat of condensation of water
Net (lower) calorific value (LCV):
LCV is “the net heat produced, when unit mass/volume of the fuel is burnt completely and the
products are permitted to escape”. In actual practice, combustion products are not cooled to
temperature but simply let off into the atmosphere. Since this calorific value does not include the
latent heat of steam, hence, net calorific value is always lower than gross calorific value.
NCV = GCV - (Latent heat of water vapour formed)
= GCV - (Mass of H2 x 9 x latent heat of steam)
= GCV - H2 (in percent) x 0.09 x latent heat of steam
Determination of Calorific Value of a solid or liquid fuel using Bomb Calorimeter
PRINCIPLE: A known weight of the sample (solid or liquid fuel) is burnt completely in excess
of oxygen. Surrounding water and calorimeter absorbs the liberated heat. Thus, the heat liberated
during the combustion of fuel is equal to the heat absorbed by water and copper calorimeter. The
higher calorific value of the fuel is calculated from the data.
4
Construction And Working
1. A known mass of fuel (Solid/liquid) is taken in clean steel crucible and placed inside
cylindrical steel vessel called bomb.
2. The bomb is air tight and has inlet for oxygen.
3. A magnesium wire is connected to the electrodes in contact with the fuel sample.
4. The bomb is placed in a copper calorimeter containing a known weight of water which is
surrounded by air jacket and water jacket to prevent heat loss to surroundings. Water is
stirred continuously using electrical stirrers.
5. The initial temperature of water is recorded using Beckmann‟s thermometer.
6. The other ends of electrodes are connected to 6V battery to ignite the fuel.
7. On combustion, heat liberated = heat absorbed by water and calorimeter. The maximum
temperature attained by water is recorded.
Bomb calorimeter
Observation And Calculation:
Mass of the fuel = m kg
Mass of water taken in the calorimeter = W kg
Water equivalent of calorimeter = w kg
Initial temperature of water = t10C
5
Final temperature of water = t20C
Specific heat of water = s kJ/kg/0C (4.187 kJ/kg/
oC)
Latent heat of steam i.e, L = 2454kJ/kg
Heat liberated on combustion of fuel = Heat absorbed by water
Heat liberated on combustion of unit mass of fuel = GCV
Therefore, heat liberated on combustion of mass „m‟kg of the fuel = m x GCV
Heat required to raise temperature of 1kg of water by1oC = s =4.187 kj/kg/
oC
Therefore, heat absorbed by (W+w) mass of water to raise its
temperature from t10C to t2
0C = (W+w) x (t2-t1) x s
Hence, m x GCV = (W+w) x (t2-t1) x s
GCV = (W + w) (t2 – t1) S kJ/kg
m
NCV = GCV - (% of H2 x 0.09 x latent heat of steam) kJ/kg
NUMERICALS
1. 0.85 g of coal sample (C=90%. H2 =5%, ash=5%) was subjected to combustion in bomb
calorimeter. Mass of water taken in calorimeter was 2000g and the water equivalent of
calorimeter is 600g. The rise in temperature was found to be 3.5oC. Calculate the gross
and net calorific values of the sample [latent heat of steam=2.454 KJ/g] [specific heat of
water =4.2 kJ/kg/oC]
Soln.: Mass of coal sample = 0.85 g = 0.85
1000= 0.85x10−3kg
% of H2 =5%
Mass of water = 2000g = 2000/1000 g = 2 kg
Water equivalent = 600 g = 600/1000 kg =0.6 kg
(W+w) = 2.6 kg
∆t = 3.5oC
Qgross = W + w XSX∆t
m=
2.6X4.2X3.5
0.85X10−3KJ/kg = 44965 KJ/kg
QNet = QGross − Latent heat of water formed
6
= QG − 0.09x5%x2454 = 43860KJ/kg.
2. Calculate the gross and Net calorific values of a coal sample from the following data
obtained from a bomb calorimeter experiment.
Mass of coal =0.73 g
Mass of water taken in the calorimeter = 1500 g
Water equivalent of calorimeter = 470g
Initial temperature = 25.0oC
Final temperature = 27.3oC
Percentage of hydrogen in coal sample =2.5%
Latent heat of steam =2454 kJ kg-1
Specific heat of water=4.2 kJkg-1o
C-1
Soln.:
Qgross = W + w XSX∆t
m=
1500 + 470 x10−3kgx4.2kJkg−1 oC−1x 27.3 − 25.0 oC
0.73X10−3kg
=266068.77 kJ kg-1
Use Eqn.
QNet = QGross − Latent heat of water formed
=25988.07 kJ kg-1
-0.09 x 2.5 x 2454 kJ kg-1
= 26068.77 kJ kg-1
-552.157 kJ kg-1
= 25516.6 kJ kg-1
3. Calculate GCV and NCV of a fuel from the following data.
Mass of fuel =0.83g, W=3500g. , W = 385 g, t1 =29.20C, t2 = 26.5
0C, % H2 = 0.7 and
S = 4.2 kJ/kg/oc
GCV = (W+w) x t x S
M
= (3.5 + 0.385) x (29.2 – 26.5) x 4.2
0.83 x 10-3
GCV = 53079.39 kJ/kg
NCV = GCV –0.09 x H x 587 x 4.184
= 53079.39 – 0.09 x 0.7 x 587 x 4.2
NCV = 52924.07 kJ/kg
Practice problems:
7
Q: Calculate the gross and net calorific value of a coal sample from the following data
obtained from bomb calorimeter experiment
1) Weight of coal = 0.73g
2) Weight of water taken in calorimeter = 1500g
3) Water equivalent of calorimeter = 470g
4) Initial temperature = 25oC
5) Final temperature = 27.3oC
6) Percentage of hydrogen in coal sample = 2.5%
7) Latent heat of steam = 587cal/g
Answer: HCV = 25969.3x103J/kg
NCV = 25416.3x103J/kg
Cracking
Primary distillation of petroleum (crude oil) yield less amount of gasoline. The demand for petrol
is increasing steeply. The present day the cracking process heavily supplements gasoline
demand.
Cracking is defined as the process of breaking of high boiling, higher molecular weight
hydrocarbons (heavy oil fractions) into low boiling, low molecular weight hydrocarbons .
Heat and pressure
Eg: C14H30 C7H16 + C7H14
Absence of air
Cracking process involves breaking of carbon – carbon and carbon – hydrogen bonds.The rate of
cracking and the end products are strongly dependent on the temperature and presence
of catalysts. Cracking is the breakdown of a large alkane into smaller, more
useful alkanes and alkenes.
These reactions are endothermic and hence favoured at high temperature.
Cracking process aims at:
1. To convert low demand, high boiling fractions into low boiling fractions suitable for
automobiles.
2. To produce raw materials like gasoline in large quantities for petrochemical industries.
8
FLUIDISED (MOVING) BED CATALYTIC CRACKING:
Principle: In fluidized bed catalytic cracking, the finely divided catalyst is kept agitated by gas
stream (cracking fuel) and can be pumped as a true liquid. There is a good contact between the
catalyst and the reactant. Cracking takes place in fluidized state and there is good contact
between the catalyst and heavy oil.
REACTION CONDITIONS:
Feed stocks : Gas oil, heavy oil fractions.
Catalyst used : Al2O3 + SiO2 (FLUIDISED)
Temperature : 5500c
Pressure : Little above the normal pressure.
Yield : 70-80%
PROCESS:
1) The finely divided catalyst bed is fluidized by the upward passage of feed stock vapors in
a cracking chamber (reactor)
2) Cracked vapors are withdrawn continuously from the top of the cracking chamber and
directly fed into a fractionating column to separate into gases, gasoline and uncracked
oils.
3) The uncracked oil may be cracked in a second stage cracking, there by increasing the
overall yield of the cracked products.
9
4) Spent catalyst is drawn continuously from the cracking chamber, transported in air stream
to a regeneration chamber in which elemental carbon deposited on catalyst is removed
and catalyst is mixed with fresh feed stock and returned to the cracking chamber.
REFORMING OF PETROL:
The object of reforming is to upgrade the octane number of petrol fraction and to produce
hydrocarbons for use as feed stocks in the synthesis of petrochemicals.
Reforming is the process of modifying the structure of hydrocarbon without any change in the
number of carbon atoms to form a new compound to improve its antiknocking characteristics
and octane rating of fuel.
The process is brought by passing the petroleum fractions at about 500oC over platinum
coated on alumina a catalyst
In reforming process, linear hydrocarbons with lower octane values are converted to
branched, cyclic and aromatic hydrocarbons having higher octane values.
Process : Reaction condition
Feed stock – Gasoline(Crude petrol)
Catalyst – Platinum supported on aluminium silica base
Temperature – 470-525oC
Pressure – 15-50 atmosphere
The main reactions during the catalytic reforming process are.
1. Isomerization: Conversion of straight chain hydrocarbons to branched chain.
CH3-CH
2-CH
2-CH
2-CH
2-CH
3CH
3-CH-CH
2-CH
2-CH
3
n- Hexane
2-methyl pentane
CH3
2. Dehydrogenation: Cycloalkanes are converted into aromatic hydrocarbons
Cyclohexane Benzene
-3H2
10
methyl cyclohexane Toluene
3. Dehydrocyclization: Dehyrogenation followed by cyclization of alkanes
-H2 -3H2
n-Heptane methyl cyclohexane Toluene
4. Hydrocracking:
CH3-(CH
2)8-CH
3 2 CH3-(CH
2)3-CH
3
n- Decane n- Pentane
Pt
KNOCKING:
Knocking is defined as production of shock waves in an IC engine due to explosive combustion
of fuel – air mixture due to increase in compression ratio beyond certain level leading to rattling
sound.
Reason for knocking
Compression ratio
The efficiency of power production in spark ignited internal combustion engines is related to the
compression ratio (CR).
CR = 𝑉1
𝑉2 = Cylinder volume at the end of suction stroke
Cylinder volume at the end of compression stroke
Fig: Variation of IC engine efficiency with compression ratio
The efficiency of power output increases with CR
11
𝑉1
𝑉2> 1
The power output and efficiency of IC engine depends on CR(compression ratio)
Knocking mechanism:
Under ideal conditions: In IC engine the petrol air mixture drawn into the cylinder
undergoes compression and is ignited by electric spark. The hydrocarbons in petrol
undergoes complete combustion and flame propagates smoothly.
C2H6 + 7/2 O2 2CO2 + 3H2O + energy
Knocking occurs because the chain reaction proceeds at fast rate. The hydrocarbon molecule
combine with O2 to form peroxides. The unstable peroxides decompose readily to give number
of gaseous compounds, which gives rise to pressure waves and knocks against walls.
Under knocking conditions, the above reaction takes place in 4 different steps:
1. C2H
6 + O
2 CH
3-O-O-CH
3
2. CH3-O-O-CH
3 CH
3-CHO + H
2O
3. CH3-CHO + 3/2 O
2 HCHO + CO
2 + H
2O
4. HCHO + O2 H
2O + CO
2 The knocking tendency of gasoline decreases in the following order
Straight chain hydrocarbons > Branched chain hydrocarbons > Alkenes > Cycloalkanes >
Aromatic
ADVERSE EFFECT OF KNOCKING:
1. It produces undesirable rattling noise.
2. It increases the fuel consumption.
3. It results in decreased power output.
4. It causes mechanical damage due to overheating, to engine parts such as spark-plug,
piston and engine walls.
5. The driving becomes unpleasant.
REMEDIAL MEASURES:
1. A suitable change in engine design.
2. By using high rating gasoline.
3. By using critical compression ratio.
4. By using anti-knocking agents.
12
OCTANE NUMBER:
Knocking capacity of petrol is measured in terms of octane number.
Definition: Octane number of a fuel is defined as the percentage by volume of isooctane in a
mixture of isooctane and n-Heptane blend, which has the same knocking characteristic as the
gasoline under test.
CH3-(CH2)5-CH3
n-Heptane ON=0
Isooctane ON=100
(2,2,4-trimethyl pentane)
Higher the octane number lower is the tendency to knock and better is the quality of petrol.
Branched chain compounds produce low knocking while straight chain compounds produce high
knocking. Isooctane, a branched chain hydrocarbon, which has an excellent combustion
characteristics and very little tendency to knocking is given an octane number 100. While n-
Heptane, a straight chain hydrocarbon, which has poor combustion characteristics and knocks
badly, is given octane number zero.
Thus if the octane number of a gasoline is 70 it means that its knocking characteristics are
similar to that of the knocking characteristics of a mixture of 70% isooctane and 30% n-Heptane.
Higher the octane number of gasoline, least is the knocking.
NOTE: Octane number of the hydro carbons decreases in the following order
Benzene > Alkenes > Cycloalkane > Branched chain hydrocarbons > Straight chain
hydrocarbons
Diesel Knock
Diesel knock is the clanking, rattling sound emitted from a running Diesel engine. The noise is
caused by the compression of air in the cylinders and the ignition of the fuel as it is injected into
the cylinder. This is much the same as a gasoline engine suffering from pre-ignition.
In diesel engine the fuel and air mixture is ignited by the application of heat and pressure and not
by the spark. Air is drawn into the to about 350oC. The diesel oil is injected in the form of spray
CH3-C-CH
2-CH-CH
3
CH3
CH3
CH3
13
which instantaneously gets ignited and develops pressure. However the combustion of fuel in
diesel engines is not instantaneous but there exists an interval of time between fuel injection and
start of ignition. This interval of time is called “ignition delay” or “ignition lag”. The shorter
ignition leads to burning of fuel at the rate at which it is injected. Longer ignition delay results in
accumulation of the fuel in the engine and causes an explosive combustion when ignited. This is
called „diesel knock‟. Longer the ignition delay, larger is the diesel knock.
The diesel knock can be minimized by using fuels having following characteristics
The fuels should have straight chain structure
The fuel must have short ignition delay
The fuel ignition temperature should be less than that of compressed air
The fuel must have high cetane rating.
The cetane rating of diesel fuel can be raised by additives such as ethyl nitrate,
amyl nitrate.
CETANE NUMBER:
Quality of diesel oil is measured in terms of cetane number.The Cetane number is a measure of
the ease with which the given diesel fuel will undergo compression ignition.
-Methylnaphthalene and n- Cetane are specified as standards, since n-Cetane or
hexadecane(C16H34) has low ignition lag, is an ideal diesel fuel, its cetane number is fixed as
100, while -Methylnaphthalene has long ignition lag, poor diesel fuel and its cetane number is
fixed as zero.
C10H7-CH3 CH3-(CH2)14-CH3
-Methylnaphthalene n-Cetane
Cetane number=0Cetane number=100
Definition: Cetane number is defined as the percentage of n-Cetane in a mixture of n-Cetane and
-Methylnaphthalene that has the same ignition characteristics as the diesel fuel under test.
The cetane number of a diesel fuels can be raised by the addition of small quantity of
ethyl nitrite, isoamylnitrite and acetone peroxide which helps in starting ignition.
Cetane number of Aromatic hydrocarbons < Alkenes < Cycloalkanes < straight
chain alkanes
14
Anti-knocking agents:
Knocking tendency can be reduced by adding certain compounds to fuels which increase the
octane number of fuels. Such compounds are called anti-knocking agents. The anti-knocking
agents are
1) TML (Tetra methyl lead) Pb(CH3)4
2) TEL (Tetra ethyl lead)Pb(C2H5)4
3) Ethyl methyl lead
They are used along with ethylene dichloride or ethylene dibromide.
TEL and TML get converted to Pb or PbO and get deposited on the engine parts or the
exhaust pipe causing damages. But if they are used along with ethylene dichloride or
dibromide, Pb and PbO are converted to volatile PbCl2 or PbBr2 that escape as gases into
atmosphere.
UNLEADED PETROL:
Petrol whose Octane number is increased by addition of substance other than lead
compounds is called as unleaded petrol.
The octane value can be increased by adding MTBE or ETBE
CH3-C-OCH
2CH
3
CH3
CH3
Ethyl tertiary butyl ether() ETBE
CH3-C-O-CH
3
CH3
CH3
Methyl tertiary butyl ether
They contain oxygen in the form of ether group and supplies oxygen for complete
combustion of the petrol in internal combustion engines thus reducing the formation of
peroxy compounds.
ADVANTAGES OF UNLEADED PETROL:
1.Eliminates the pollution level of lead in atmosphere.
2.This permits the attachment of catalytic converters to the exhaust pipe in automobiles.
3.The catalyst converts the toxic exhaust gases like CO and NO to non- toxic gases CO2 and N2
respectively. Consequently, the pollution level is reduced to a great extent.
15
POWER ALCOHOL
Alcohol used for the generation of power is called power alcohol. Power alcohol is a blend of
absolute ethyl alcohol and petrol used as fuel in IC engines. Blend contains 10-85% of absolute
ethanol and 90-15% petrol by volume.
Ethyl alcohol used in the generation of power in internal combustion engines is called power
alcohol.
MANUFACTURE OF POWER ALCOHOL: The raw materials for the manufacture of power
alcohol or ethyl alcohol are saccharine materials (molasses, sugar cane, sugar beets etc.) starchy
materials (such as potatoes, starch etc.) cellulose materials (sulphite liquor from paper mills) and
hydrocarbon gases.
Advantages of alcohol blended petrol:
1. The power output is good.
2. Alcohol blended petrol possess better anti-knocking properties.
3. Because of the higher octane number (anti-knocking properties), alcohol blended petrol can
be used in engines with higher compression ratio.
4. There is no starting difficulty with alcohol petrol blend.
5. Lubrication in case of alcohol petrol blend and pure petrol is the same.
6. Air required for complete combustion is less.
Disadvantages of alcohol blended petrol:
1. Alcohol lowers the calorific value of petrol.
2. Alcohol is easily oxidized to acids hence alcohol may cause corrosion.
3. Alcohol absorbs moisture and as a result separation of alcohol and petrol layers takes
place especially at low temperature. To avoid this, blending agents such as benzene or
toluene are used.
BIODIESEL
Biodiesel is a mixture of mono-alkyl esters of long fatty acid. It is obtained by
trans-esterification of vegetable oil in NaOH.
It is a clean burning fuel obtained from renewable resources such as vegetable oils like
soyabean oil, palm oil, peanut oil, sunflower oil, jatropha oil which are triglycerides.
Synthesis:
16
Vegetable oils are convert into biodiesel by trans-esterification reaction with excess methanol in
presence of NaOH. The products are monoalkyl esters of long chain fatty acids and glycerine.
R is long chain fatty acids in oil. Glycerol is soluble in water and hence can be separated.
Advantages:
1. It is completely biodegradable, non-toxic and free from sulphur compounds.
2. It is made by using renewable sources.
3. Ecofriendly products are formed
4. It has higher cetane number(60) compared to diesel(45)
5. It contains more oxygen and hence undergoes complete combustion compared to diesel
and emits lesser amount of CO and hydrocarbons
6. Use of bio-diesel reduces greenhouse gases and is non-toxic.
SOLAR ENERGY
Solar energy is clean and renewable source of energy and alternative to fossil fuels.
Utilization of solar energy
1. Direct utility – when solar energy is directly converted to electric energy. Example:
PV cells
2. Indirect utility: Conversion of Solar energy to chemical energy, which later used as
energy sources. Example: Photosynthesis, Photovoltaic cell
They are referred to as semiconductor devices that convert sunlight (electromagnetic radiations)
into direct current electricity. As long as light is shining on the solar cell, it generates electrical
power. When the light stops, electricity stops. Solar cells never need recharging like a battery.
Principle:
The energy associated with photon (E) is given as-
E = hν = hc/λ
17
Where h is planks constant
C is velocity of light λ is wavelength of light
Construction and working of PV cells.
Consists of Ultra thin wafers of n type(phosphorous doped) on to top of boron doped Si
(p-type)
A metallic grid forms one of the electrical contact of the diode and allows the light to fall
between grid lines
An antireflective layer between grid lines increases the amount of transmitted light.
The cells other electrical (bottom) contact is formed by Ag metal.
When light radiations falls on p-n junction diode, e- - hole pairs are generated by
absorption of radiations.
The electrons are drifted and collected to n-type. The holes are drifted and collected at p-
type end.
When these ends are electrically connected the current flows between the two ends
through external circuits.
Importance of Photo Voltaic cells
Photovoltaic cells can generate electricity for a wide variety of applications
1. Solar energy, being unlimited, in exhaustible and renewable photo voltaic cells can be
considered as continuous energy source.
2. Electricity can be generated in rural areas, individual families which are living far away
from electric grid connections and in remote areas such as mountains.
3. They can be used for domestic lighting, spinning of fans, grinding grains, transistor
radios, small TV sets and tape recorders.
4. In agricultural sectors it is used for irrigation.
18
5. In production activities such as milling, sawing and sewing.
6. During the operation of solar cells, there is no harmful emission or transformation of
matter (generation of pollutants) nor any production of noise or other by-products.
7. Photo Voltaic cells provide power for space craft and satellites, an extra terrestrial
dimension of photo voltaics.
Advantages
1. Low operating cost (no fuel)
2. No moving parts and no wear and tear
3. Ambient temperature operation
4. Modular
5. High public acceptance and excellent safety record
Limitations
1. Energy can be produced only during day time
2. High installation costs
3. Poor reliability of auxiliary elements includingstorage
4. Lack of widespread commercially available system integration and installation
Solar Grade Silicon
Elemental silicon is used in photovoltaic cells as the main semi-conductor material converting
light into electricity. The commercial silicon feedback available to solar cell is of 2 types;
1. The metallurgical grade silicon (silicon metal)
2. Semi conductor grade silicon (poly silicon)
Impurities are present in high concentration prevent the use of metallurgical grade silicon in solar
cells. Semi conductor grade silicon is more refined and impurity levels are at ppb level.
Silicon used for solar cells can tolerate higher concentrations of impurities than in semiconductor
grade, without affecting the efficiency of the cell considerably.
The Silicon having these impurities in substantial amount but within tolerable limits is called the
solar grade silicon. It lies in between the metallurgical grade silicon and semi conductor grade
silicon.
Preparation of solar grade Silicon by Union Carbide process
Preparation of Si for use in Photo Voltaic cells involves several steps
1. Preparation of metallurgical grade Si from naturally occurring quartz
19
A mixture of quartz (SiO2) and C is taken in a crucible. Two electrodes are dipped in
the mixture and an electric arc is struck. The mixture is heated to high temperature of
1700 OC
SiO2(s) + 2 C (s) Si (l) + 2CO (g)
CO is oxidized to CO2 and let into atmosphere
Si obtained is in the molten state and is contaminated with Al, Ca and Mg
The impurities are removed as slag by the addition of silica.
4 Al + 3 SiO2 3 Si + 2 Al2 O3
2 Ca + SiO2 Si + 2 CaO
2 Mg + SiO2 Si + 2 MgO
The slag is removed mechanically and molten Si is poured into moulds and solidified.
The Si obtained is called metallurgical grade Silicon (98.5%)
2. Synthesis of silicon hydride from the Silicon
Metallurgical grade Si is heated to 300 OC in presence of dry HCl. Trichlorosilane
and a small amount of tetrachlorosilane are formed as given below
Si + 3 HCl HSiCl3 + H2
Trichlorosilane
Si + 4 HCl SiCl4 + 2H2
Tetrachlorosilane
SiCl4 is converted into HSiCl3 by treating with H2 at 1000 OC
SiCl4 + H2 HSiCl3 + HCl
HSiCl3 is passed through ion exchange resin containing quaternary ammonium salts
to give dichlorosilane and tetrachlorosilane. Dichlorosilane subsequently forms
trichlorosilane and silane (SiH4)
2HSiCl3 H2SiCl2 + SiCl4
3H2SiCl2 SiH4 + 2HSiCl3
The products are separated by distillation
Tetra chlorosilane and trichlosilane are recycled to hydrogenation reactor and ion
exchange resin respectively.
3. Purification of SiH4
20
SiH4 is further purified by distillation and passed into the reactor containing heated Si
seed rods. Silane gets pyrolysed to form polysilicon (semiconductor grade Si)
SiH4 Si + 2H2
Si obtained above is purified further by Zone refining
Zone refining:
Silicon of 99.9% purity can be obtained by Zone refining.
Principle: When solid is melted, the impurities tend to concentrate in the molten zone.
Process:
A Si rod is clamped vertically and heated by a RF(Radiofrequency) coil in a reducing
atmosphere.
The RF coil is slowly moved from top to bottom.
As the molten zone moved down, the impurities also are swept down with the molten
material.
When the process is repeated, all impurities concentrate at the bottom portion of the rod.
It is cut and removed,
By this technique the impurities concentration can be decreased to 1atom/1012
atoms of
Si(Ultrapure). The purified Si rod is polycrystalline in nature.
21
WATER TECHNOLOGY
INTRODUCTION
Water is most abundantly distributed throughout the biosphere. But 97%of it is locked in seas and
oceans which is too salty to be used for drinking,agriculture and for industrial purposes.Around 2.4% is
trapped in polar ice caps and glaciers.Only 0.3% of the world's water resources is available for human
usage(domestic,industrial&agriculture use).Water suitable for human usage is present in
rivers,streams,lakes,ponds,etc.
Water is not only essential for existence of plant&animal life but also very much essential for the
industrial development.One cannot imagine the development of modern industry without water.It is
known that water is a universal solvent.Thus it is used as important ingredient in many industries like
food processing ,chemical , fertilizer,pharmaceutical,polymeric and many more industies.Water plays an
important role in producing electricity through nuclear and thermal power plants in form of steam
.Hydrogen an effective fuel can be an answer for the energy crisis and environmental pollution
confronted by todays world .Water can provide huge amount of hydrogen through electrolysis and
thereby can solve energy crisis in the world.
BOILER FEED WATER:-
One of the most important use of water in industries for steam production. Boiler feed water is the water
supplied to the boiler for the generation of steam or hot water. Boilers are used in many manufacturing
industries as well as in power generating units. Steam is used for power generations, heating, drying and
sterilization,etc. The boiler feed water contains impurities like dissolved salts, dissolved gases, etc. These
impurities create many problems in boiler. Use of impure water in the boilers leads to different types of
boiler problems that impair the effective use of boilers and also affect the quality of steam produced.
Impure water can cause three important problems in boilers which are:-
i) Scale and sludge formation
ii) Priming and foaming
iii) Boiler corrosion
i) Scale and sludge formation:-
Scales:-
When water is converted into steam, the suspended matter and the dissolved salts present in it may get
deposited on the inner walls of the boiler, which are called scales. The scales formed mainly due to the
dissolved salts like MgCl2, MgHCO3 , CaSO4,CaHCO3&silica.The hard deposits adhered to the inner
surface of boilers which are difficult to remove are called scales.
Mg(HCO3)2------->Mg(OH)2 + CO2
Ca(HCO3)2-------->CaCO3 + H20 + CO2
Mg(OH)2 and CaCO3 have low solubility so they form scales.
Problems caused by scale formation:-
a) Wastage of fuel:-
Scale is a bad conductor of heat therefore heat cannot be transformed uniformly. This leads to wastage of
fuel.
b) Boiler explosion:-
On heating metal expands faster than scales, this can lead to the boiler explosion.
c) Decrease in efficiency:-
The deposition of scales in tubes,valves,etc decreases the efficiency of boiler.
d) Decrease in boiler safety:-
Overheating of the boiler makes the boiler metal soft,weak, and leads to distortion of the metal ,therefore
it is unsafe to bear the steam pressure, particularly in high pressure boilers.
e) Expenses of cleaning:-
Scales must be removed regularly and this cleaning process is very much expensive.
Prevention of boiler scales:-
i) By external treatment:- Salts are removed before feeding into the boiler.
ii) By internal treatment:-water is treated with HCl or EDTA.
iii) Blow down:-removal of concentrated water and replacing with fresh water.
iv) Loose scales are removed by using wooden scraper.
Sludge:-The loose collection of suspended solid sand less adherent ppt formed in the boiler is called
sludge. It is formed due to salts such as Ca3(Po4)2 CaCl2, MgCl2,MgCO3,soluble in hot water but gets
precipitated in cooler parts of boiler.If this sludge is not removed frequently, it settles down to form
scales. Sludge can be easily removed by blowdown method.
Sludge Scale
1) Loose, slim, non-adherent precipitate 1) Hard, thick , strong adherent precipitate
2) Due to salts like MgSO4, MgCl2 2) Due to salts like CaSO4, Ca(HCO3)2 silica
3)Due to poor conductance, they decrease
theboiler efficiency to lesser extent and
causingchocking in the pipelines.
3) Due to poor conductance, they decrease
theboiler efficiency to maximum extent,
causereduced fuel economy, improper boiling, boiler explosion etc.
4) It can be prevented by periodical
replacement of concentrated hard water
byfresh water, this process is known as
“blowdown” method.
4) It can be prevented by special methods like
i)External treatment of ion exchange,
ii) Mechanical hard scrubbing methods.
ii) Priming & Foaming:-
When water is heated in the boiler, steam rises from the surface of water. If steam carries small droplets
of water with it then it is called wet steam. These tiny droplets moving along with steam can carry some
suspended and dissolved impurities present in the boiler water. The process of carrying small droplets of
water along with impurities by steam is called carry over. This carry over is mainly due to priming and
foaming.
PRIMING:-It is the process of very rapid boiling of water in the boiler which makes some water
droplets to be carried away along with steam. Priming is caused by
i) Presence of dissolved and suspended impurities.
ii) Scale& sludge formation.
iii) Very high level water in the boiler.
iv) Defective boiler design.
Problems caused by priming:-
1.It reduces the heating efficiency of steam.
2. It causes corrosion on turbine blades and other parts.
Prevention:-
Well designed boiler.
Optimum water level.
Regulating steam generation.
Minimizing foaming.
FOAMING: Foaming is production of foam or bubbles on boiler water. It causes priming and causes due
to presence of organic matter,oils,alkalis,fatsetc. in water.Foaming is the formation of small but
persistent bubbles on the surface of boiler water.
Prevention:-
i) Using antifoaming agents like castor oil,polyamides
ii) Removal of silica using ferrous sulphate.
iii) Removal of suspended matter like oil ,grease by adding NaAl2O3
C) Boiler corrosion:- The process of degradation of the boiler surface by the attack of boiler feed water
is called as boiler corrosion.The dissolved gases like oxygen, and carbon di oxide present in the boiler
feed water cause boiler corrosion.
Reactions causing boiler corrosion:
Dissolved oxygen: - Dissolved oxygen of more than 7ppm in water attacks on Fe or steel of the boiler
surface as follows causing rust .
4Fe + 2H2O + O2------>4Fe(OH)2
2Fe(OH)2 + 1/2O2------>Fe2O3+2H2o
Carbon dioxide: It is present in the boiler water either from air or due which produced due to the
decomposition of HCO3 salts react with water to produce carbonic acid which causes corrosion.
Co2 + H2O----->H2CO3
Corrosion also occurs when feed water becomes acidic in nature.Under this condition, acid may be
formed due to the presence of magnesium compounds in water.
MgCl2:- MgCl2 on hydrolysis produce HCl which causes corrosion.
MgCl2 + 2H2O ------->Mg(OH)2 + 2HCl
Control of boiler corrosion:-
1) By removing oxygen using reducing agents like hydrazines or sodium sulphite.
N2H4 + O2 ------> N2 + 2H2O
Na2SO3 + 1/2 O2 -------> Na2SO4
2) By removing CO2 using lime or NH4OH.
Ca(OH)2 + CO2 -------> CaCO3 + H2O
NH4OH + CO2 -------> (NH4)2CO3 + H2O
COD – Chemical Oxygen Demand
COD can be defined as “the amount of oxygen consumed for the complete oxidation of organic and inorganic
impurities present in 1 litre of waste water sample in presence of strong oxidising agent like acidified
K2Cr2O7.”
Principle : - The method involves the addition of excess amount of acidified K2Cr2O7 to the known volume of
waste water and titrating unconsumed K2Cr2O7 against std. FAS solution( Back titration).
Method :-
Weight out certain crystals of FAS and add 10 ml of dil sulphuric acid and make it up upto the mark with
distilled water in 250 ml standard flask. Determine the strength of prepared FAS solution by calculating
normality.
Normality = weight of FAS x 4 = X (N)
Equi wt (392)
Pipette out 25cm2 of (V) waste water into a clean conical flask. Add 10cm3 of std. K2Cr2O7 and 10cm3 of 1:1
H2SO4 ,and add 2 drops of indicator –ferroin and start titrating against standard FAS.The end point is
bluish-green to reddish-brown. Let the volume of FAS consumed be ‘a’ cm3. Perform blank titration in the
same way without adding waste water sample. Let the FAS consumed be ‘b’ cm3 Therefore net volume of
FAS=(b-a)cm3.
1000 cm3 of 1N FAS Solution = 1 Equivalent of oxygen=8 g of oxygen.
1cm3 of 1N FAS Solution =8 mg of oxygen
(b-a) cm3 of X (N)FAS Solution =(b-a) x Xx 8mg of oxygen
25 cm3 of waste water requires =(b-a) x Xx 8mg of oxygen
Therefore 1000 cm3 of waste water requires=1000 x 8 x (b-a) x X
25
COD = 8000 x(b-a) x X = …… mg of oxygen / litre.
25
In case of industrial waste water,add 1g AgSO4 and 1g of HgSO4.to the conical flask and attach the conical
flask to a reflex condenser and reflux it for half an hour cool and titrate it against std FAS solution using
indicator. The end point is bluish-green to reddish-brown and the calculations have to be done in same
manner.
PROBLEMS ON COD
1) In a COD test , 25cm3 and 17.5cm3 of 0.052N FAS solution were required for blank and sample titration
respectively. The volume of test sample used is 25cm3. Calculate the COD of the sample solution.
Sol:-
COD of the sample = N x (b-a) x 8000
V
= 0.052 x (25-17.5) x 8000
25
= 124.8 mg of oxygen / dm3
Softening of Water:
The process of removal of Calcium, Magnesium, Iron salts and other metallic ions (which form insoluble
soaps) from water is called softening of water.Three important industrial methods used for water softening
are:
1. Cold and hot lime- soda process
2. Permutite or zeolite process
3. Ion exchange or demineralization
Softening of water by Ion exchange Method (Demineralization)
In this method, softening of water is done by exchanging the ions causing hardness of water with desired ions
from an ion exchange resin. Ion exchange resins are high molecular weight, cross linked polymers with a
porous structure.
The functional groups which are attached to the chains are responsible for ion exchange properties. The resins
containing acidic groups which are capable exchanging H+ (or Na+) ions for cations (Ca 2+ or Mg 2+) present in
water are known as cation exchange resins (RH+). The resins containing basic groups which are capable of
exchanging OH- for anions (Cl-, SO4 2-) present in water are known as anion exchange resins (ROH-)
Process: In this process, cation and anion exchange resin are packed in separate columns. Hard water is first
passed through cation exchange resin where cations like
Cations present in water are removed from hard water by exchanging with H+ ions as follows: Ca2+ + 2R -
H+-------> R2 - Ca2+ + 2H+
Mg2+ + 2R - H+------->R2 - Mg2+ + 2H+
Hard water is then passed through anion exchange resin where ions are exchanged with OH - ions as follows:
R - OH- + Cl- -------> R+- Cl- + OH-
R – HCO3- + Cl- -------> R+- HCO3 + OH-
2R - OH- + SO4-2 -------> R2 SO4 + 2OH-
These H+ and OH- ions released combine to form water molecule. Thus water coming out of two resins is free
and called as ion- exchanged or deionised or demineralized water.
Regeneration: When the resins are exhausted and lose their capacity to exchange ions, they are regenerated by
dilute acids and base as follows.
R - M + HCl = R - H + MCl
R - X + NH4OH- = R -OH + NH4Cl
Potable water:
Water that meets all the required parameters of pure and fit consumption is called potable water. Most of the
oil rich countries depend upon sea water for drinking. But sea water contains about 3-5% of dissolved salts and
it is unfit for drinking. Salts water can be converted into portable water by subjecting into desalination.
Desalination of water:
It is a process of removal of dissolved salts present in water. There are mainly 2 methods available for
desalination.
A. Reverse osmosis
B. Electro dialysis
In this method, salts present in water are removed using ion selective membranes, and by applying direct
current. An electro dialysis unit consist of three compartments partitioned by cation and anion selective
membranes. Two electrodes, a cathode and an anode are placed at two extreme ends. Salt water is taken in the
middle compartment. When direct current is applied, anions move towards anode through anion selective
membrane and cation move towards cathode through cation selective membrane. Thus ions get concentrated
in anode and cathode compartments.Hence in the middle compartment concentration of salts decreases and
pure water is obtained.
Sewage treatment : - Sewage can be classified into two types – Domestic sewage and industrial Sewage.
Domestic sewage contains organic waste, pathogenic bacteria,colour,bad smell etc. If such water without
treatment is sent into water bodies (rivers,lakes) they contaminate and cause depletion of dissolved oxygen,
impart colour and smell , cause diseases.
The domestic sewage is treated before discharging into water bodies. The treatment involves the following
steps :
1. Primary treatment(physical method)
2. Secondary treatment (biological treatment)
3. Tertiary treatment ( Chemical treatment)
1. Primary treatment(physical method):
It involves
a) Screening: it is a physical process which removes large suspended or floating matter in the sewage. This is
obtained by using mesh screens which retains the floating and suspended particles when sewage water is
passed through it.
b) Slit and Grit removal: Sand, broken glass, etc., are removed by passing sewage through grit chambers, in
which the velocity of flow of sewage is reduced. Being heavier, slit and grit particles settle down at the
bottom.
c) Removal of oil and grease: Oil and grease are converted into a soapy mixture by blowing compressed air
through sewage water in skimming tanks, and lifted to the surface in the form of foam. The floating substance
is skimmed off.
d) Sedimentation process: The finer suspended impurities are removed in sedimentation tank by adding
coagulants like alum, ferrous sulphate etc.
2. Secondary treatment – activated sludge method
The waste water after primary treatment is allowed to flow into large tanks where biological treatment is
carried out called ‘ activated sludge method’ which is carried as follows.
Air is passed vigorously from the bottom of the tank so that aerobic oxidation of organic matter takes
place. Aerobic oxidation can be enhanced by adding a part of the sludge forms during primary
treatment.
The added sludge is called activated sludge , since it contains a large number of aerobic bacteria.
Activated sludge and sewage water are agitated by passing air for several hours.
After the process is completed , water us sent into a sedimentation tank where sludge settles and the
water free from organic matter is drawn out.
The water obtained from secondary treatment has less organic impurities which after chlorination is
discharged into water bodies.
Instrumental Methods of Analysis
Conductometric Titrations
Principle: Conductometric titration is based on the fact that during acid - base titration, one
type of ions are replaced by the other type of ions and invariably these two ions
differ in the ionic conductivity with the result that conductivity of the solution
varies during the course of titration. The equivalence point may be located
graphically by plotting the change in conductance as a function of the volume of
titrant added.
Theory: The electrical conductance C of a solution is a measure of ability of the solution to
conduct electricity. It is otherwise defined as inverse of resistance.
C = 1/R and its unit is Siemens.
When AC current is passed in a solution the resistance of a solution containing a fixed
concentration of an electrolyte at constant temperature is directly proportional to distance
between two electrodes (‘l’ cm) and inversely proportional to area of cross section(‘a’ cm2).
R α l/a
R = ρ (l/A), Where ρ is Specific resistance
1/ ρ = (1/ R) x (a/l) = κ (specific conductance)
κ = (1/ R) x (a/l) ....... Ω-1cm-1 or S cm-1
Where κ is specific conductance, it is defined “Conductance of a solution placed between two
electrodes which are unit distance apart and having unit cross sectional area”.
Instrumentation:
It consists of conductivity cell having two platinum electrodes placed at 1cm apart and each
having a unit area of cross section. The cell is dipped in the electrolytic solution taken in the
beaker and its connected to the conductance measuring device. In conductivity meter we usually
measure specific conductance.
Application of conductometry in acid-base titration
a) Strong acid with a Strong Base:
During the titration of strong acid with a strong base ( Eg. HCl Vs NaOH), the following
reaction takes place
(H+ + Cl-) + (Na+ + OH-) -----------(Na+ + Cl-) + H2O
The highly conducting H+ ions are initially present in the solution are replaced by Na+ ions
having much smaller ionic conductance. Therefore, the conductance first falls, and after
theequivalence point has been reached, rapidly rises with further addition of NaOH which
furnishes Na+ ions and OH- ions. The intersection of the two straight lines gives the end point.
b) Strong acid with a weak Base
During the titration of strong acid with a weak base (Eg. HCl Vs NH4OH), the following
reaction takes place
(H+ + Cl-) + (NH4+ + OH-) ----------- (NH4++ Cl-) + H2O
The highly conducting H+ ions are initially present in the solution are replaced by NH4+ ions
having much smaller ionic conductance. Therefore, the conductance first falls, and after the
equivalence point has been reached, the conductance almost remains constant, since the excess
of ammonium hydroxide added, does not ionize in the presence of ammonium chloride
(common ion effect). The plot is curved near the end point, due to the hydrolysis of the salt of
strong acid and weak base (NH4Cl).
c) Weak acid with a strong Base
On titrating strong base like NaOH with weak acid like CH3COOH, the conductance
decreases initially not only due to the replacement of H+ ions by Na+ ions, but also
suppresses due the decrease in dissociation of CH3COOH due to common ion effect
(CH3COONa).But, very soon conductance increases on adding NaOH, as NaOH neutralizes
the undissociated acetic acid molecules to sodium acetate, this increase in conductance raises
up to equivalencepoint. Beyond the equivalence point, further addition of strong base
introduces OH- ion concentration and thereby a sharp increase in conductance.
Colorimetry
Principle: “The variation of color of the solution with change in concentration of a solute
component in the solution forms the basis of colorimetry”. The color is either inherent color of
the constituent itself or due to the formation of colored complex by addition of suitable reagent.
Theory: When a monochromatic light of intensity Io traverses through a transparent colored
medium a part of light is absorbed (Ia), a part is reflected (Ir) and remaining is transmitted (It).
Then, Io = Ia+Ir+It, for air glass interface Ir is negligible, therefore Io = Ia+It.
Colorimetry is governed by two laws,
Beer’s Law: According to Beer‟s law when monochromatic light passes through the colored
solution, the amount of light transmitted decreases exponentially with increase in concentration
of the colored substance.
It = Ioe-εc
Lambert’s Law: According to Lambert‟s law the amounts of light transmitted decreases
exponentially with increase in thickness of the colored solution.
It = Ioe-ε t
A ( absorbance) = log It/Io = εct
A= εct
Where, ε = Molar absorptivtiy coefficient, c = concentration and t = thickness of the solution
Instrumentation of colorimeter:
Colorimeter is an instrument which compares the amount of light getting through unknown
solution and the amount of light getting through a pure solvent. The source light is
passedthrough filter (to get desired frequency light) and concentrated using lens. Then this
light is targeted on a sample (which is kept in cuvette). The light transmits through the sample
and the transmitted light is measured by detector. As the intensity of the initial light is known,
we can find the difference between - intensity of initial light and intensity of transmitted light.
Applications:
Application of colorimetry in the estimation of copper :
When a solution of cupric ions is treated with ammonia solution, a characteristic deep blue
colored complex is formed. The intensity of the blue color depends on
concentration of copper.
Cu2+ + 4NH 3+----------- [Cu(NH3)4]2+
Procedure:
1. Prepare 100ml of 0.1M CuSO4 solution
2. Draw out 5, 10, 15, 20 and 25 ml of CuSO4 stock solution into separate 50ml standard flask and take
test solution in another standard flask.
3. Add 5ml of 1:1 to all the standard flasks and dilute it upto the mark with distilled water and mix for
uniform concentration.
4. Prepare blank solution taking only 5ml of 1:1 ammonia and makeup the solution upto the mark
with distilled water.
5. Measure the absorbance of all the solutions against the blank solution at 620 nm.
6. Draw the calibration curve by plotting absorbance against concentration (in mg/ml). Fromthe graph
determine the copper in the test solution.
Flame photometry
Principle: The compounds of alkali and alkaline earth metals can be thermally dissociated in
flame. Some of the atoms that are produced will be further excited to higher level; radiation is
emitted when the excited atoms return to the ground state. This radiation falls in the visible
region of the electromagnetic spectrum. The measurement of emitted radiation forms the basis of
flame photometry.
Theory: The test solution containing metal ions is passed into the atomizer through the
nebulizer. The test solution sucked through the nebulizer, air from compressor and fuel gas from
the fuel cylinder mix together in the mixing chamber. Now, the mixture is introduced into the
flame. Here, all the excited atoms emit radiations of different wavelength. If the emitted
radiation is passed through optical filter then, the optical filter permits only the characteristic
wavelength of the element under study. The transmitted light falls on the photocell, amplifier
and then on the digital recorder to get the data.
Instrumentation: It consists of an atomizer, mixing chamber, burner, filter, detector and
reading-out device. Pressurized air passed into atomizer and the suction and suction pump draws
the solution of the sample into the atomizer. Inside the atomizer it mixes with air as a fine mist
and passes in to the mixing chamber. In the mixing chamber it mixes with the fuel and passes
into the burner, where the mixture is burnt, the radiation from the resulting flame passes through
the lens and finally through the filter, which permits only characteristic wavelength of the
element under study to pass through the detector. The output from the detector is readout on a
suitable read-out system
Application of flame photometry in the estimation of sodium (Na)
Sodium imparts a bright yellow color to the Bunsen flame. Therefore to estimate sodium, a
filter which transmits yellow light is used.
1) Prepare 1 ppm NaCl solution by dissolving required amount of NaCl( 2.542g) in one litre of
distilled water.
2) Transfer 2,4,6,8 and 10 ml of standard of NaCl solution into 100 ml strandard flasks anddilute up
to the mark with distilled water. This gives 0.02, 0.04, 0.06, 0.08 and 0.1 ppm of
NaCl solution respectively. Also dilute the given test solution up to the mark and mix well. 3) Place the
distilled water in the sample vessel. Aspirate the solution through the capillary and set the emission
intensity reading to zero.
4) Place standard solution of highest concentration in the sample vessel. Aspirate the solution
through the capillary and set the emission intensity reading to 100.
5) Then place 0.02 ppm standard solution in the sample vessel and meaure the emission
intensity.
6) Remove the 0.02 ppm solution and place distilled water and set the reading to zero again.
7) Repeat the procedure and measure the emission intensities for the other standard solutions and
also for the test solution.
8) Draw a graph by plotting the emission intensity against concentration of NaCl in ppm.
9) From the graph, find out the concentration of NaCl in ppm of the test solution and calculate the
amount of Na.
***************************
POLYMERS
The term polymer is derived from the ancient greek word, polus meaning many and meros
meaning parts, and refers to a molecule whose structure is composed of multiple repeating units.
The term was coined by Jons Jacob Berzelius.
(1) Monomers:
―Building blocks of polymer‖- These are simple molecules that combine with each other to form
polymer. Eg. Ethylene.
(2) Polymer:
It is a macro molecule formed by the covalent linkage of a large number of monomer units. Eg.
Polyethylene
(3) Polymerization
The process of formation of high molecular weight long chain compounds by the combination of
repeating units (monomers) is called as polymerization.
n M ---(M)n---
TYPES OF POLYMERIZATION
(i)Addition or chain growth polymerization: The process in which a polymer is obtained from
simple addition reactions of monomers without elimination of any byproduct is called as
addition polymerization.kes place by chain reaction.
n CH2=CH2CH2=CH2 n
Ethylene Polyethylene
(ii) Condensation or step growth polymerization; The process in which a polymer is obtained
by condensation reaction of monomers with continuous elimination of by products (water, HCl,
ammonia, alcohol...Etc) is called condensation polymerization.
Eg; (1) Formation of polyester, terylene from terephthalic acid and ethylene glycol.
Terephthalic acid
ethylene glycol Terylene
HOOC-C6H
4-COOH + HO-C
2H
4-OH O-C-C
6H
4-C-O-C
2H
4-O
n
OO
C6H
5CH-CH
2o
Cl
CH=CH2
Cl
+ C6H
5CH-CH
2
Cl
CH-CH2
o
Cl
1
C6H
5CH-CH
2
Cl
CH-CH2
o
Cl
1CH=CH
2
Cl
+ C6H
5CH-CH
2
Cl
CH-CH2
o
Cl
2
C6H
5CH-CH
2
Cl
CH-CH2
o
Cl
x -1
CH=CH2
Cl
+ C6H
5CH-CH
2
Cl
CH-CH2
o
Cl
x
C6H
5CH-CH
2
Cl
CH-CH2
o
Cl
y-1
CH=CH2
Cl
+ C6H
5CH-CH
2
Cl
CH-CH2
o
Cl
y
(2) Formation of nylon-6,6 from adipic acid and hexamethylene diamine.
Adipic acid Hexamethylene diamine Nylon 6,6
Mechanism of addition polymerization of vinyl chloride
Vinyl chloride undergoes addition polymerization in the presence of dibenzoyl peroxide as an
initiator to form poly vinylchloride
Vinyl chloride Polyvinylchloride
The reaction can be explained by the following mechanism,
1. Generation of the free radical: Free radicals are generated by thermal or photochemical
decomposition of initiator (dibenzoyl peroxide)
C6H
5-C-O-O-C-C
6H
5
O O
h or 2 C
6H
5-C-Oo
O
2C6H
5o
2. Initiation of chain reaction: Free radicals generated initiate the chain reaction by reaction with
vinyl chloride further generating a new free radical.
C6H
5o + CH=CH
2
Cl
C6H
5CH-CH
2o
Cl
3. Newly formed free radical attacks more vinyl chloride monomers leading to the propagation
of the chain length
n(CH2=CHCl) (CH
2-CHCl)
n
n HOOC-(CH2)4-COOH + n H2N-(CH2)6NH2 C-(CH2)4-C-N-(CH2)6-N
O O H H
n
4. Termination of chain reaction: The termination of polymer chain takes place in two ways,
a. Termination by radical coupling:
C6H
5CH-CH
2
Cl
CH-CH2
o
Cl
xC
6H
5CH2-CH
Cl
oCH2-CH
Cly
+
Coupling
C6H
5CH-CH
2
Cl
CH-CH2
Cl
xC
6H
5CH2-CH
Cl
CH2-CH
Cly
b. Termination by disproportination: In disproportination termination two polymers are formed
one of which will be unsaturated.
C6H
5CH-CH
2
Cl
CH-CH2
o
Cl
xC
6H
5CH2-CH
Cl
oCH2-CH
Cly
+
Dispropotination
C6H
5CH-CH
2
Cl
C=CH2
Cl
x + C6H
5CH2-CH
Cl
CH3-CH
Cly
Glass transition temperature (Tg):-
The temperature at which the polymer is transformed from stiff, hard and glassy state to soft,
flexible and rubbery state called Tg
Glassy state Viscoelastic state Viscofluid state
(Hard and brittle) (Rubbery) (Polymer melt)
Tg Tm
Tm is melting point of polymer. The temperature below which it is in rubbery state and above
which it behaves like a viscous liquid is melting point of polymer.
Parameters influencing Tg value:-
1. Flexibility:
Higher flexibility of a polymeric chain leads to higher segmental mobility and hence lower will
be the Tg. Thus, flexibility of the chain and Tg are always inversely related. A polymeric chain
becomes flexible if there exists a free rotation along the chain. Linear polymer chain made ofC-
C, C-O and C-N single bonds have higher degree of freedom of rotation, increases chain
flexibility and thus decrease Tg.Presence of rigid structures in polymer chain such as aromatic or
cyclic structure hinder freedom of rotation thus lowering of chain flexibility and increase in Tg.
Eg:Tg of polyethylene -1100C
Tg of polystyrene 1000C
2. Intermolecular forces
Presence of intermolecular forces in the polymer chain due to polar groups, dipoles, exerts
strong forces of attraction like hydrogen bonding with neighboring chains. Strong intermolecular
cohesive forces restrict molecular mobility. This leads to an increase in Tg.
Eg: Tg of polypropylene -180C
Tg of nylon 6,6 570C
iii.Branching and cross-linking
A high density of branching brings the polymer chains closer, lowers the free volume thus
reducing the chain mobility and resulting in an increase in Tg whereas small branching provides
more free volume for free rotation, therefore lowers Tg value.
Eg:Tg of polyethylene -1100C
Tg of polystyrene 1000C
iv.Molecular mass:-
The Tg values of all polymers, increase with molecular weight, because when molecular mass is
high,long polymer chain coil and entangle with one another.This restricts free mobility of chains
and Tg increases.Tg increases with increase in molecular mass upto 20,000. And beyond it, has
negligible effect on Tg.
Significances of Tg
Tg can be used to evaluate the flexibility of a polymer and predict its response to mechanical
stress.
Polymers show an abrupt change in their physical properties at their Tg.Thus values of heat
capacity, refractive index,coefficient of thermal expansion determine the usefulness of
polymer over a temperature range.
Processing of polymers like moulding, calendring, and extrusion is decided only with the
knowledge of Tg.
Polyurethanes (PU): Polyurethanes are characterized by the presence of urethane linkage –
[NHCOO]- in the molecular chains.
Synthesis: PU‘s are made by addition polymerization of di-isocyanates with a di-ol. During
the addition, the ‗H‘ atom of -OH group migrates and adds to nitrogen atom of di-isocyanate
to form urethane linkage as follows.
Properties:
1] Due to the presence of oxygen atom in the chain they are flexible in nature.
2] They are resistant to water, oil and corrosive chemicals.
3] PU‘s are spongy transparent linear thermoplastics.
Applications:
1] PU foams are widely used in cushions for furniture.
2] PU fibres are used in light weight water repellent garments like swim suits.
3] They are used in tyre treads.
4] As PU is abrasion resistant they are used as floor coatings of gymnasium, dance floors, etc.
TEFLON: [POLY TETRA FLUORO ETHYLENE (PTFE)]:
PREPARATION:It is obtained by heating the monomer under pressure with ammonium per
sulphate as initiator.
PROPERTIES:
(i) It is highly crystalline and very tough polymer. (ii)It is a linear polymer and a thermoplastic.
(iii) It is a very good insulator and high chemical resistance to almost all solutions.
(iv) It is not wetted by oil or water. (v) High Boiling point and high density.
(vi) High thermal stability and mechanical strength. Teflon is therefore called ―wonder plastic‖
n. C=N-R-N=C
OO
+ n. HO-R-OH
C-N-R-N-C
OO
-O-R-O
H Hn
n CF2=CF2
Ammonium persulphate / PeroxideCF2=CF2 n
APPLICATIONS:
(i) It is used as insulating material for motors, generators, coils, transformers and capacitors.
(ii) Anticorrosive coating in army weapons
(iii) As dry lubricant.
(iv) Oil and water do not wet Teflon and this property is used in coating nonstick cooking wears.
(v)Used as lining for food processing equipments, in making gaskets.
POLYMER COMPOSITES:
A combination of two or more distinct dissimilar components to form a new class of
material suitable for structural applications is referred to as composite materials.
Polymer composites are produced by suitably bonding a fiber material with polymer resin
matrix and curing the same under pressure and heat. While each component retains its parent
constituents, particularly in terms of mechanical properties.
(i)The common fiber material used in polymer composites are glass fiber, boron filament,
carbon/graphite fibers, kevlar etc.
(ii)The common resin matrixes used are polyesters, epoxy, phenolic, silicon, vinyl derivatives
and polyamides.
(iii) The fibre is embedded in between two matrix layers.It exhibits properties superior to either
of two components.
Modern technologies like aerospace, speed boating, submarines require materials with unusual,
unique combination of properties. Polymer composites have light weight, high strength,
stiffness, good abrasion resistance, impact resistance and corrosion resistance. Any single metal,
alloy ,ceramic or polymeric material cannot offer the combination of above properties.
PROPERTIES: (i)They have low density. (ii)They have high strength to weight ratio.
(iii)They are much stronger and durable than conventional metals like steel and aluminium.
(iv)They are most suitable for aerospace applications due to their light weight.
(v)They have good corrosion resistance.
(vi) They have good abrasion resistance and impact resistance.
IMPORTANT TYPE OF FIBER-REINFORCED POLYMER COMPOSITE :
Kevlar: It is an aromatic polyamide with the name poly(para-phenyleneterepthamide).It is
prepared by condensation reaction of para-phenylenediamine and terepthaloylchloride. Kevlar is
used as fiber in preparing polymer composites materials with epoxy resin as matrix. The linkage
through para positions of the phenyl rings gives Kevlar a strong ability to stretch and hence its
extra strength.
Applications:
It forms even better fibres than non-aromatic polyamides,since it has very light weight. It is
used in lightweight boathulls, aircraft panels, and race cars.
It has high tensile strength and stiffness than fibre glass.
These are used for structures which require stiffness, high abrasion resistance and corrosion
resistance.
Applications include use in bridge structures, bullet proof vests and puncture resistant
bicycle tyres.
Biodegradable polymers: are a specific type of polymer that breaks down after its intended
purpose to result in natural byproducts such as gases (CO2, N2), water, biomass, and inorganic
salts.
Synthesis: Polylactic acid is prepared by the condensation polymerization of lactic acid at
temperatures below 200ᵒC, giving out one molecule of water for each condensation.
OH
OH
OCondensation Polymerisation
- H2OO
OHO
O
O
O
nLactic acid Polylactic acid
NH2
NH2
+
OCl
O Cl
-nHCl
NH
O n
n n
Applications:
PLA is used as a feedstock material in desktop fused filament fabrication 3D printers.
Being able to degrade into non-toxic lactic acid, PLA is used as medical implants in the form
of anchors, screws, plates, pins, rods, and as a mesh.
It is useful for producing loose-fill packaging, compost bags, food packaging, and disposable
tableware.
In the form of fibers and nonwoven fabrics, PLA also has many potential uses, for example
as upholstery, disposable garments, awnings, and diapers.
Pure poly-L-lactic acid (PLLA), on the other hand, is the main ingredient in Sculptra, a long-
lasting facial volume enhancer, primarily used for lipoatrophy of cheeks.
*****************************************************************************
NANOTECHNOLOGY
Definition:
This is a term that has entered into the general and scientific vocabulary only
recently but has been used at least as early as 1974 by Taniguchi. Nanotechnology is defined as
a technology where dimensions and tolerances are in the range of 0.1-100 nm (from size of the
atom to about the wavelength of light) play a critical role.
Popular definition for Nano technology is: ―Nano technology relates to the ability to build
functional devices based on the controlled assembly of nano scale objects for specific
technological applications.‖
Introduction:
Some notes on scale
1A0=10
-10m
1 nm=10-9
m 1 µm=10-6
m 1mm=10-3
m
Atomic/crystallographic micro structure macro structure
1cm=10-2
m
The word ―Nano‖ means dwarf in Greek language. Use it as a prefix for any unit like a second
or a meter and it means a billionth of that unit. A nanosecond is one billionth of a second. And a
nanometer is one billionth of a meter—about the length of a few atoms lined up shoulder to
shoulder.
Role of Bottom-up and Top-Down approaches in Nanotechnology
Figure: Schematic representation of the building up of Nanostructures.
The biggest problem with top down approach is the imperfection of surface structure and
significant crystallographic damage to the processed patterns.
These imperfections which in turn leads to extra challenges in the device design and
fabrication.
But this approach leads to the bulk production of nano material. Regardless of the defects
produced by top down approach, they will continue to play an important role in the synthesis
of nano structures.
When structures fall into a nanometer scale, there is a little chance for top down approach. All
the tools we have possessed are too big to deal with such tiny subjects. Bottom up approach also
promises a better chance to obtain nano structures with less defects, more homogeneous
chemical composition. On the contrary, top down approach most likely introduces internal
stress, in addition to surface defects and contaminations.
Classification of Nanomaterials based on dimension
1. Zero dimensional (0-D): These nanomaterials have Nano-dimensions in all the three
directions. Metallic nanoparticles including gold and silver nanoparticles and semiconductor
such as quantam dots are the perfect example of this kind of nanoparticles. Most of these
nanoparticles are spherical in size and the diameter of these particles will be in the1-50 nm
range. Cubes and polygons shapes are also found for this kind of nanomaterials.
2. One dimensional (1-D): In these nanostructures, one dimension of the nanostructure will be
outside the nanometer range. These include nanowires, nanorods, and nanotubes. These
materials are long (several micrometer in length), but with diameter of only a few nanometer.
Nanowire and nanotubes of metals, oxides and other materials are few examples of this kind of
materials
3. Two dimensional (2-D): In this type of nanomaterials, two dimensions are outside the
nanometer range. These include different kind of Nano films such as coatings and thin-film-
multilayers, nano sheets or nano-walls. The area of the nano films can be large (several square
micrometer), but the thickness is always in nano scale range
4. Three Dimensional (3-D): All dimensions of these are outside the nano meter range. These
include bulk materials composed of the individual blocks which are in the nanometer scale (1-
100 nm)
Size dependant properties of nanomaterials
Catalytic properties
When we compare bulk material with that of nano material of same weight, the surface area
available in case of nanomaterials is some thousand times more than bulk material.
This enhanced surface of the nanomaterials makes them active catalysts in many reaction
Many nano materials are used as heterogeneous catalysts.
Certain nano structured metal clusters have shown effective catalysis in hydrogenation
reactions.
They can be made as electrodes for certain reactions leading to selective product formation
Thermal properties
As size decreases surface energy increases also melting point decreases – This is due to the fact
that the surface atoms requires less energy to move as they are in contact with less atoms with
the surface.
E.g. Nano crystals of CdSe of 3mm size have melting point 700K whereas the CdSe bulk
material will melt at 1678 K
Optical properties:
Optical properties of material depends upon it electronic structure, changes in the band gap (VB-
CB) and zone structure of the material leads to changes in absorption and emission of the light
by the material.
Eg. Bulk gold appears yellow in color whereas nano sized gold appears different color based on
their size. – This is explained as in bulk gold electrons are free to move throughout the metal
hence they are free and absorption wavelength resulting in yellow color.
However in case of nano gold electrons are restricted for their movement resulting in absorption
of light at wavelength other than that the bulk gold will absorb.
Liquid solid reactions (Precipitation method)
Precipitation is a technique in which two reagents mixed to get an insoluble material in the solid
form which precipitates out of the solution and settles down.
The precipitation technique of preparation of nano materials involves two steps
Formation of the nuclei: Here the nuclei refers to group of atoms together – Initially as the
precipitation starts few particles will approach each other forming a small mass (called
nuclei).
Growth step: In the growth step the above formed nuclei attracts the particles present in the
solution thus increasing its size itself.
The growth step decides that the material to be formed is a nano material or bulk material.
By monitoring and restricting the growth of the nuclei we can prepare nano size particles. If
the growth is not controlled, leads to precipitation of bulk mass.
The growth of the nuclei can be controlled by maintaining suitable concentrations, pH of the
medium and temperature.
For example, TiO2 powders have been produced with particle sizes in the range 70-300 nm from
titanium tetraisopropoxide.
Chemical Vapour Condensation (CVC)
Fig: A schematic of a typical CVC reactor
Chemical vapor condensation (CVC) was developed in Germany in 1994. It involves pyrolysis
of vapors of metal organic precursors in a reduced pressure atmosphere.
In Chemical vapour condensation technique precursor (mixture of metal/ oxides etc in
solvent) material is heated by passing through hot surface resulting in vaporization of
material (to be prepared in nano scale eg. ZrO2, Y2O3 and nanowhiskers)
In the hot tube chemical reaction happens between the precursor reactants in the vapour
phase.
The compounds vaporized forms clusters on cooling, which are swiped by the carrier gas to
the cool finger where they undergo condensation and settle on the surface.
The material settled on the surface of the cold finger is removed by scrapping.
The thickness and size of the nano material to be formed is controlled by adjusting the flow
rate of the carrier gas swiping the reactants.
More is the resident time of the reactants more is the size and vice versa.
General applications of nanomaterials
Nano clusters of inorganic materials like SiO2, ZnO, CdS etc., were found to be photo
initiators while their corresponding bulk material doesn‘t show this property.
ZnS clusters and its aggregates acts as effective photo catalyst in the reduction of organic
compounds
The nano clusters of cerium oxide (CeO2-x) have been found to possess a significant
concentration of Ce3+ and oxygen vacancies resulting in excellent poisoning resistance
against H2O and CO2
Applications of Fullerenes
They act as powerful antioxidants, can find applications in health and personal care areas
Due to the small size, room temperature operation and high sensitivity – they find
application as coating materials on some chemical sensors.
Field emission display (FED‘s) using carbon nanotubes are the next generation replacement
for LCD, Plasma with decreased cost and increased power.
Fullerenes blended with polymers are used in phogtovoltaics.
Their use in water purification, catalysis and fuel cell are also being explored
Applications of nanowires
They find lot of applications in electronics optics magnetic medium and sensor devices.
They are used in magnetic information storage medium.
They are used as Rectifiers junction diodes memory cells and switches transistors LED‘s and
inverter etc. ,
Nano wires are used coat titanium implants which reduce the risk of implant failure in
biomedical field.
Applications of nanorods
They are used in display technology- Because of the their change in reflectivity with change
of orientation with an applied electric field.
They also used as (MEMS) Micro-electromechanical.
Nano rods are selectively absorbed by cancerous cells and get heated up when is irradiated
with IR radiation and gets destroyed.
They will be employed in energy harvesting system (semiconductor nanorods) and in LED‘s
Sl.
No.
Question RBT Level
1 With suitable reactions explain the free radical mechanism of addition
polymerization
L1 and L2
2 Define glass transition temperature. Illustrate with suitable examples the
factors flexibility and intermolecular forces that influence glass transition
temperature.
L1, L2 and
L3
3 Differentiate between addition and condensation polymerization L2 and L3
4 Give the synthesis of polyurethane and Teflon, give any four applications
of each
5 What are polymer composites? Give the synthesis of kevalr with any four
applications of them
L1 and L2
6 Explain any two size dependant properties of nanomaterials L1 and L2
7 Explain the precipitation method of synthesis of nanomaterials L1 and L2
8 Explain the Chemical vapor deposition method of synthesis of
nanomaterials
L1 and L2
9 Give any four applications of nanomaterials L1
