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System – the specific part of the universe of interest to us
Surroundings – the part of the universe not contained in the system
Systems and Surroundings
3 types of Systems• open system – exchanges mass and energy• closed system – exchanges energy but no
mass• isolated system – no exchange of either
mass or energy
State of a System (Continued)
State of a system• the system is in a definite state when each of its
properties has a definite value. Change in state
• initial state• final state
Path• initial and final states• intermediate states
Some Definitions
Process• reversible or irreversible transformation
Cyclic transformation• begins and ends at the same state variables.
Some Definitions (Continued)
Work (w)• any quantity that flows across the system’s
boundary and is completely convertible into the lifting of a mass in the surroundings.
How much work was done?
Work
Unit of work = J = 1 kg m/s2
dzFdw z
A single-stage expansion process
Work Done in a Closed, Fluid System
State 1 State 2
Piston(T, P1, V1)
mass (m)
Piston(T, P2, V2)
mass (m)
Dir
ecti
on
of p
isto
n
h2
h1
The work done in the surroundings• wsurr= Pext DV
The work done by the system• wsys = - wsurr = - Pext DV
For an infinitesimal volume change• dwsys = - Pext dV
System and Surroundings
If the system is in equilibrium• Fsys = -Fext
• P = Pext
For a simple system• d wrev = - P dV
Reversible (Multistage) Expansion
Ideal gas as the working fluid.
Reversible Transformation in an Ideal Gaseous System
cc revrev PdVdww
VnRT
P
For an isothermal process (ideal gas as working fluid)
Reversible Transformation (Continued)
1
2lnVV
nRTdwwc revrev
dwirr = -Pext dV for a constant external pressure
Irreversible Transformations
12
2
1
VVP
dVPdww
ext
extc irrirr
Heat - the quantity that flows across the boundary of the system during a change in state• due to temperature difference between
system and surroundings• HOT to COLD (never the other way
around)!!!
Heat
Measured by determining the temperature change of some known object
'Amount of Heat'
C - the heat capacity of the system.
CdTdq
Exothermic - system to surroundings Endothermic – surroundings to system
Heat Flows
surroundingssystem
heat
Heat flows during phase changes - latent heats• Latent heat of vapourisation• Latent heat of fusion
Latent Heats
Subject our system to a cyclic transformation
The Internal Energy
CdTCdTqcycle
cycle
dwdwwcycle
cycle
The following would be true for an exact differential
Cyclic Integrals of Exact Differentials
exact is df if 0df
The infinitesimal change in the internal energy
The Internal Energy
dwdqdU
wqdwdqUc
For a general process
For a system undergoing an isochoric temperature change
Heat Flows Under Constant Volume Conditions
For a macroscopic system
dTCdU V
2
1
T
TVV dTCqU
The Joule Experiment
AT1, Vm,1, P1
B
Stirrer
Valve
Thermal insulationFFOO
CO
CO
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0
20
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60
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The change in the internal energy under isothermal conditions is related to the Joule Coefficient
Internal Energy and the Joule Coefficient
VUT TU
VT
VU
JVT
CVU
Adiabatic Processes for an Ideal Gas
For an ideal gas undergoing a reversible, adiabatic process
1
2
/
2
1,
TT
VV
mvCR
,2 2
1 1
v m
RC RP T
P T
Defining the enthalpy of the system Re-examine the piston with the weight
on top
State Changes Under Constant Pressure Conditions
Piston(T, P, V)
mass (m)
For a system undergoing an isobaric temperature change
Heat Flows Under Constant Pressure Conditions
For a macroscopic system
dTCdH P
2
1
T
TPP dTCqH
The Joule-Thomson Experiment
Porous Plug
Thermal insulation
T1, P1, Vm,1T2, P2, Vm,2
FFOO
CO
CO
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100
80
0
20
20
40
60
60
40
FFOO
CO
CO
50
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100
80
0
20
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The partial derivative
The Joule-Thomson Coefficient
is known as the Joule-Thomson coefficient, JT.
HPT
The change in the enthalpy under constant pressure conditions is related to the Joule-Thomson Coefficient
Relating H to the Joule-Thompson Coefficient
PHT TH
PT
PH
JTPT
CPH
Enthalpy Changes for Reactions
The shorthand form for a chemical reaction
J
JJ0
J = chemical formula for substance JJ = stoichiometric coefficient for J
Reaction Enthalpy Changes
The enthalpy change for a chemical reaction
JHnHJ
mJr
Hm [J] = molar enthalpies of substance J
nJ = number of moles of J in the reaction
The Enthalpy Change
Reaction beginning and ending with equilibrium or metastable states
JHn
HHH
JmJ
initialfinalr
Note – Initial and final states have the same temperature and pressure!
A Standard State Reaction
A reaction that begins and ends with all substances in their standard states
The degree sign, either or
• P = 1.00 bar• [aqueous species] = 1.00 mol/ kg• T = temperature of interest (in data tables -
25C or 298 K).
Standard Reaction Enthalpies
We note that for 1 mole of a reaction under standard conditions
JHHJ
mJr
The Formation Reaction
A "chemical thermodynamic reference point."
For CO and CO2
C (s) + O2 (g) CO2 (g)
C (s) + ½ O2 (g) CO (g)
The Formation Reaction
The formation reaction• 1 mole of a compound • constituent elements • stable state of aggregation at that temperature.
Formation of 1.00 mole of Na2SO3(s)
2 Na(s) + S(s) + 3/2 O2 (g) Na2SO3 (s)
‘Formation enthalpy of Na2SO3(s)’, fH°[Na2SO3 (s)]
The Significance of the Formation Enthalpy
fH° is a measurable quantity!
Compare CO (g) with CO2 (g)
C (s) + 1/2 O2 (g) CO (g)
fH° [CO(g)] = -110.5 kJ/mole
C (s) + O2 (g) CO2 (g)
fH° [CO2(g)] = - 393.5 kJ/mole
Formation Enthalpies
Formation enthalpies - thermodynamic reference point! • Ho
m [J] = fH [J]
• Hm [elements] = 0 kJ / mole.
Use the tabulated values of the formation enthalpies
The General Equation
The enthalpy change for a given reaction is calculated from the formation enthalpies as
Notes Reverse a reaction Multiply a reaction by an integer
JHHJ
fJr
The Calorimeter
A calorimeter - device containing water and/or another substance with a known heat capacity
Calorimeters – either truly or approximately adiabatic systems
Relating H and U
The enthalpy and the internal energy both represent quantities of heat.
U = qv.
H = qp.
Relate the two state functions using the following relationship
U = H - PV
Other Important Enthalpy Changes
Enthalpy of solution Enthalpy of dilution Enthalpy of fusion Enthalpy of vapourisation
The Solution Enthalpy
solH - heat absorbed or released when a quantity of solute is dissolved in fixed amount of solvent
solH = Hm(sol’n) – Hm(component)• H(component) = Hm(solid) + Hm(solvent)
Two definitions• Standard• Limiting
The Dilution Enthalpy
For the process,
HCl (aq, 6 M) HCl (aq, 1 M). The Enthalpy of dilution of the acid.
dilH = Hm(sol’n 2) – Hm(sol’n ,1)
Reaction Enthalpy Changes With Temperature
Differentiate the reaction enthalpy with temperature
JHHJ
mJr
JHdTd
dTHd
JmJ
r
Internal Energy Changes in Chemical Reactions
Examine a chemical reaction.
C (s) + O2 (g) CO2 (g)
U = U[CO2 (g)] – U[C(s)] – U[O2(g)]
Note - rH = -393.5 kJ/mole
RTnUH
JUU
grr
JfJr
Enthalpies and Hess’s Law
Use tabulated values of formation enthalpies to obtain rH°.
May also estimate reaction enthalpies using an indirect method.
Hess’s Law
Hess’s Law – • the enthalpy change for a given reaction is
the same whether the reaction occurs in a single step or in many steps.
Bond Energies
Examine the following reactions H2 (g) ® H (g) + H (g) DU° = 433.9 kJ
Cl2 (g) ® Cl (g) + Cl (g)DU° = 239.5 kJ
Bond dissociation energies. Enthalpy changes are designated D (H-
H) and D (Cl-Cl).
For Polyatomic Molecules
CO2 (g) ® C (g) + 2 O (g)DU = 740 kJ DH of this reaction D(C=O) What about dissociating methane into C
+ 4 H’s?CH4(g) ® C(g) + 4 H(g) DU° = 1640 kJ
4 C-H bonds in CH4 \ D (C-H) 410 kJ/mol
Make or Break!!
Note: all chemical reactions involve the breaking and reforming of chemical bonds• Bonds break - we add energy. • Bonds form - energy is released.
rU° S D(bonds broken) - S D(bonds formed)