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6.1 Electrolytes and Non-electrolytes
Electrolytes Non-electrolytes
•Substances that
can conductelectricity when they
are in molten state
or aqueous solution and undergo
chemical changes.
•Substances that
cannot conductelectricity either in
molten state or
aqueous solution.
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6.1 Electrolytes and Non-electrolytes
Electrolytes Non-electrolytes
•Example:
Sodium chloride,NaCl solution
Molten Lead (II)
bromide, PbBr 2
All ionic compounds
•Example:
Naphthalene
Glucose solution
All covalent
compounds
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6.1 Electrolytes and Non-electrolytes
Electrolytes Non-electrolytes
• Contain freely
moving ions.
• Contain neutral
molecules•Do not contain freely
moving ions.
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6.2 Electrolysis of Molten Compound
What is electrolysis?
• A process whereby the ionic
compounds in molten or aqueousstate are broken down intoconstituent elements by passing
electricity through them.
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WHAT IS ELECTROLYTIC CELL?
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BATTERY
ANODECATHODE
ELECTROLYTE
Electrode whichis connected to
the positive
terminal
Electrode which
is connectedto the negative
terminal
Electrodes
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ANODE CATHODE
PbBr 2
Pb 2+ Br -
Release electron Accept electron
2Br- Br2
+ 2e- Pb2+ + 2e- Pb
Pb2+
+ 2Br- Pb + Br2
2Br- Br2 + 2e-
Pb2+ + 2e- PbHalf equation
Overall ionic equation
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Factors affecting the products of
electrolysis for aqueous solution
A. Position of ions in the
Electrochemical SeriesB. The Concentration of the solution
C. The type of Electrodes
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A. Position in the Electrochemical Series
The lower positionof the ion in theElectrochemical
Series, the easierthe ion to bedischarged duringelectrolysis
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Copper(II)
sulphate
solution
The anions, OH- and SO4
2- move
to the anode. OH- ions are
selectivelydischarged.
Bubbles of
colourless gas isreleased.
Anode Cathode
4OH- (aq) 2H2O(l) + O2 (g) + 4e-
The cations,Cu2+.and H+
move to thecathode.
Cu2+ ions accepttwo electrons.
Brown solid is
formed at thecathode.
Cu2+ (aq) + 2e- Cu (s)
The overall ionic equation :
2Cu2+ (aq) + 4OH-(aq) 2Cu (s) + 2H2O(l) + O2 (g)
CuSO4 solutionCu2+, H+,SO4
2- OH-.
A. Position in the Electrochemical Series
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Concentrated
hydrochloric
acid
B. The Concentration of the solution
The anions,OH- and Cl- move tothe anode.
Cl- ions areselectivelydischargedbecauseconcentration of
Cl-
is higher thanOH- Bubbles of
yellowish gas isproduced.
2Cl- →
Cl2+ 2e
Anode Cathode
The cations,H+ ionsmove to the
cathode. H+ ions
acceptelectron.
Bubbles of
Colourlessgas isproduced.
2H+ + 2e →H2
The overall ionic equation :
2H+
(aq) + 2Cl-
(aq) Cl2 (g) + H2 (g)
Example : Concentrated hydrochloric acid consists ofH+, Cl-,OH-.
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Observation :The blue copper(II)sulphate solution becomesfaded/paler because the
concentration ofcopper(II)ions decreases.
Cu2+
.and H+
move to thecathode.
Cu2+ ions selectivelydischargedbecause Cu2+islower than H+ in ECS
Browndeposit/solid is
producedCu2+ + 2e → Cu
Carbon Carbon
C. The Type of Electrodes (Carbon electrodes)
OH- andSO4
2- move tothe anode.
OH- ions areselectivelydischarged
due to thelower positionin ECS.
Bubbles ofcolourless gas
is released atthe anode.
4OH- 2H2O + O2 +4e-
Anode CathodeUsing Carbon Electrode
The overall ionic equation :
2Cu2+
(aq) + 4OH-
(aq) 2Cu (s) + 2H2O(l) + O2 (g)
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The main source of aluminium is bauxite ore (Aluminium Oxide),
the melting point of aluminium oxide is very high (over 2 000°C),
Cryolite (Na3AIF6) is added to lower the melting point to 980oC.
A. Extraction of Aluminium
Anode : 2O2- O2 + 4e
Cathode : Al3+ + 3e Al
INDUSTRIAL APPLICATIONS OF ELECTROLYSIS
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Coating with a Thin Protective Layer of Metal.
C. Electroplating
Anode : Cu Cu2+ + 2e Cathode : Cu2+ + 2e Cu
Corrodes/
becomes
thinner
Coated/
deposit layer of
brown solid
Blue
solutionremainsunchanged
Copper
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Voltaic Cells
[Galvanic Cell]
A simple voltaic cell is a cell with two different
metals being immersed into an electrolyte and
connected by wire
It converts chemical energy to electrical energy.
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Mechanism of simple voltaic cellAnode (-ve) Cathode(+ve)
more electropositive metal
in ECS
Less electropositive metal in
ECSRelease electrons Accept electrons
Electrons are flow through the external circuit towards the
copper plate.
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- Daniell cell is a voltaic cell.
- Electrodes - Zinc (anode) and copper (cathode)
- The two solutions are separated by the porous pot/salt bridge-A salt bridge is made from a filter paper soaked in a saturated solution
that will not react with the two electrolytes.
-The common electrolyte used are or a solution of sodium or potassium
salts.
Daniell cell using a porous
pot
Daniell cell using a salt
bridge
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Functions of
salt bridge or porous pot :
(a) To prevents the two
electrolytes from mixing.
(b) To allow the flow of ions so
that the electric circuit is
completed.
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Mechanism of Daniell Cell
Anode Cathode
Electrode becomes
thinnerElectrode becomes thicker
Metal releases electronsZn Zn2+ + 2e- Metals accept electronsCu2+ + 2e- Cu
The concentration of copper(II) sulphate solution
decreases. The blue colour of the solution becomes
paler.
The overall ionic equation in Daniell cell is:
Zn + Cu2+ Zn2+ + Cu
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Various types
of
voltaic cell
Lead acid
Accumulator Dry cell Alkaline cellNickel-cadmium
cell
•Portable
•Cheap
•Stable voltage
of 1.5 v
Mercury cell
Portable
•Portable
•Voltage of
1.5 V
•More long
lasting
•Portable
•Rechargeable
up to hundreds
of times
•Rechargeable
•Can be made
to have
higher voltage
Advantages
•Heavy
•Expensive
•
Spilled easily
• Not
rechargeable
• Not rechargeable
•Produces voltage
of 1.3 V only
•Poisonous
• Not
rechargeable
•May leak
•Produce low
voltage of 1.25
•Expensive
•
Heavy
Disadvantages
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SIMILARITIES BETWEEN AN ELECTROLYTIC CELL AND AVOLTAIC CELL
• Consists of an electrolyte, an anode and acathode
• Electrons flow from the anode to the cathode
in the external circuit
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DIFFERENCES BETWEEN AN ELECTROLYTIC CELL AND A VOLTAIC CELL:Type of cell
onversion of
energy
Flow of
electrons
Negative
terminal
Positive
terminal
Types of
electrodes
Voltaic cell
Chemical energy
electrical energy
From positive terminal
to negative terminal
From negative terminal
to positive terminal
Cathode:
Accepts electrons
Anode:
Release of electrons
Anode :Releases electrons
Cathode:Accepts electrons
Two similar
(usually carbon) or
different metals
Two different metals
Electrolytic cell
Electrical energy Chemical energy
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The Electrochemical Series ~
An arrangement of metals based on the abilityof each metal atom to release electrons.
• The higher the position of a metal in theelectrochemical series, the greater the ability of
the metal atoms to release electrons, and themore electropositive is the metal.
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1) The potential difference between two
metals in a voltaic cell.
2) The ability of a metal to displace another
metal from its salt solution.
THE CONSTRUCTION OF THE ELECTROCHEMICAL
SERIES IS BASED ON :
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1) THE POTENTIAL DIFFERENCE BETWEEN TWO
METALS
• In a voltaic cell, two different metals
are used to create a potential difference,
shown by the reading of the voltmeter.
• Thefurther apart
the two metals are in
the electrochemical series, thegreater
the voltage produced by the cell.
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•The metal which is
higher
in theelectrochemical series will be the
negative
terminal.
• The metal which is lower in the electrochemical
series will be the positive terminal
• Electrons flow from negative to positive metal
through the external circuit.
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Magnesium strip
Copper strip
V
Copper(II) sulphate,CuSO4 solution
Voltmeter
Electron flow
Mg Mg2+ + 2eCu2+ + 2e Cu
Overall ionic equation : Mg + Cu2+ Cu + Mg2+
Showing the flow
of electrons
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2) The ability of a metal to displace another
metal from its salt solution
• A metal which is higher in theelectrochemical series is able to displacea metal below it in the
electrochemical series from its saltsolution.
• Example :
Zinc can displace copper from copper(II)sulphate solution.
Zn + CuSO4 Cu + ZnSO4
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The importance of the electrochemical
series
1. To determine the terminals of voltaic cells
2. To compare the standard voltage of the
voltage cell3. To predict the ability of metal to displace
another metal from its salt solution
Example :
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Example :
Figure below shows the cell voltages of a few voltaic cells using different metals
as electrodes.
Predict the negative terminal, positive terminal and cell voltage of the following
voltaic cells.
Voltaic cells Negative terminal Positive terminal Cell voltage/V
Mg/Ag
Mg/Zn
Zn/Pb
Magnesium
Magnesium
Zinc
Zinc
Lead
Silver 3.0 + 0.5 = 3.5
3.0 – 1.1 = 1.9
1.1 – 0.4 = 0.7
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The importance and effects of
electrochemical industries
• Theimportance
of electrochemicalindustries in our daily life is as follows:
Oxygen gas, chlorine gas can be produced byelectrolysis.
Electroplating are widely used to preventcorrosion and enhance the appearance of thematerials.
The industrial manufacture of batteries produces various types of cells and batteriesin different shapes and sizes.
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• The pollution caused by theelectrochemical industries are as follows:
The electroplating industries producepollutants such as poisonous heavy metalions and poisonous cyanide ions.
The use of mercury electrodes inelectrolysis can cause air and water pollution.
• Thus, the waste should be treatedand disposed of in a safe andorderly manner.