Chemistry 142
Chapter 17: Free Energy and Thermodynamics
Outline
I. First Law of Thermodynamics
II. Entropy
III. Second Law of Thermodynamics
IV. Third Law of Thermodynamics
V. Gibbs Free Energy
Sign Conventions
How does a chemical reaction do work?
Thermodynamics vs. Kinetics
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What is a state function?
What is chemical spontaneity?
4 Gas Molecules
Relative Probability
4 Gas Molecules
Relative Probability 1:4:6:4:1
How does entropy change?
Chapter 17 – Thermodynamics
Example – Entropy
17.1 Predict the sign of the entropy change for
each of the following processes:
a.
Solid sugar is added to water to make
a solution.
b. Iodine vapor condenses on a cold
surface to form crystals.
How does a change in state change the entropy?
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Entropy Change and State Change
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How does a temperature change effect entropy?
What effects the relative standard entropy?
Substance
S°,
(J/mol∙K)
H2O (l)
70.0
H2O (g)
188.8
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What effects the relative standard entropy?
Molar
S°,
Substance
Mass (J/mol∙K)
Ar (g)
39.948
154.8
NO (g)
30.006
Substance
210.8
S°,
(J/mol∙K)
KClO3(s)
143.1
KClO3(aq)
265.7
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Chapter 17 – Thermodynamics
Example – Entropy
17.2 Calculate ΔS˚ at 25˚C for the reaction:
2 NiS (s) + 3 O2 (g)  2 SO2 (g) + 2 NiO (s)
Given the following values:
Substance
S˚ (J/mol K)
SO2 (g)
248
NiO (s)
38
O2 (g)
205
NiS (s)
53
Chapter 17 – Thermodynamics
Example – Entropy
17.3 Calculate ΔS˚ for the reduction
aluminum oxide by hydrogen gas:
Al2O3 (s) + 3 H2 (g)  2 Al (s) + 3 H2O (g)
Given the following values:
Substance
S˚ (J/mol K)
Al2O3 (s)
51
H2 (g)
131
Al (s)
28
H2O (g)
189
of
How are DG, DH, and DS related?
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Chapter 17 – Thermodynamics
Example – Free Energy
17.4 At what temperature is the following
process spontaneous at 1 atm? What is
the normal boiling point of liquid bromine?
Br2 (l)  Br2 (g) ΔH˚ = 31.0 kJ/mol
ΔS˚ = 93.0 J/K mol
Chapter 17 – Thermodynamics
Example – Standard Free Energy
17.5 Use the following data at 25 ˚C to calculate
ΔG˚ for the reaction:
Cdiamond (s)  Cgraphite (s)
ΔG˚ = ?
Given:
Cdiamond (s) + O2 (g)  CO2 (g)
ΔG˚ = -397 kJ
Cgraphite (s) + O2 (g)  CO2 (g)
ΔG˚ = -394 kJ
Chapter 17 – Thermodynamics
Example – Standard Free Energy
17.6
A chemical engineer wants to determine the
feasibility of making ethanol (C2H5OH) by reacting
ethylene (C2H4) and water according to the reaction:
C2H4 (g) + H2O (l)  C2H5OH (l)
Is the reaction spontaneous? Given:
Substance ΔGf˚ (kJ/mol)
C2H5OH (l) -175
C2H4 (g)
-237
H2O (l)
68
Chapter 17 – Thermodynamics
Example – Standard Free Energy
17.7
One method for synthesizing methanol (CH3OH) involves
reacting carbon monoxide and hydrogen gases:
CO (g) + 2 H2 (g)  CH3OH (l)
Calculate ΔG at 25 ˚C for the reaction, if carbon monoxide
is at 5.0 atm and hydrogen gas is at 3.0 atm.
Substance ΔGf˚ (kJ/mol)
CH3OH (l)
-166
CO (g)
-137
H2 (g)
0
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K<1
How are K and
DG related?
K>1
K=1
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Chapter 17 – Thermodynamics
Example – Free Energy and Equilibrium
17.8
The overall reaction for the corrosion of iron by oxygen
gas is:
4 Fe (s) + 3 O2 (g)  2 Fe2O3 (s)
Using the following data at 25 ˚C calculate the
equilibrium constant, K.
Substance
ΔHf˚ (kJ/mol)
S˚ (J/mol K)
Fe (s)
0
27
Fe2O3 (s)
O2 (g)
-826
0
90
205
Chapter 17 – Thermodynamics
Example – Free Energy and Equilibrium
17.9
Calculate ∆H°rxn and ∆S°rxn for the decomposition of
carbon dioxide into carbon monoxide and oxygen gases.
2 CO 2 (g)  2 CO (g) + O2 (g)
Using the following data:
T(K)
1500
2500
3000
Kc
5.5E-09
4E-01
40.3
Chapter 17 – Thermodynamics
Example – Free Energy and Equilibrium
T(K)
Kc
1500 5.5E-09
2500 4.0E-01
3000 40.3
Calculate ∆H°rxn and ∆S°rxn for the decomposition of
carbon dioxide into carbon monoxide and oxygen gases.
2 CO 2 (g)  2 CO (g) + O2 (g)
Using the following data:
1/T (1/K)
0.0006667
0.0004000
0.0003333
Van’t Hoff Plot
ln(K) versus Reciprical Kelvin Temperature
ln(Kc)
-19.02
-0.92
3.696
5.00
0.00
0
0.0001
0.0002
0.0003
0.0004
0.0005
0.0006
0.0007
-5.00
ln(K)
17.9
-10.00
-15.00
-20.00
-25.00
1/T (1/K)
ln(K) = -68070 K/T + 26.353
R² = 1
Chapter 17 – Thermodynamics
Example – Free Energy and Equilibrium
17.10 Automobiles and trucks pollute the air with nitrogen
monoxide. At 2000 °C, Kc for the reaction
N2(g) + O2 (g)  2 NO (g)
is 4.10 x 10-4, and ΔH˚ = 180.6 kJ. What is the value of Kc
at 25 °C?
What is the energy tax?
• you can’t break even!
• to recharge a battery with 100 kJ of
useful energy will require more than
100 kJ
• every energy transition results in a
“loss” of energy
– conversion of energy to heat which is
“lost” by heating up the surroundings
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What is the heat tax?
fewer steps
generally results
in a lower total
heat tax
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What happens in real reactions?
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