1. Chapter 16 Thermodynamics: Entropy, Free Energy, and Equilibrium
Chemistry: McMurry and Fay, 6th Edition
Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
11/29/2018 6:19:03 PM
Chapter 16
Thermodynamics: Entropy, Free Energy, and Equilibrium
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2. 16.1 Spontaneous Processes
Chemistry: McMurry and Fay, 6th Edition
Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
16.1 Spontaneous Processes
11/29/2018 6:19:03 PM
Spontaneous Process: A process that, once started, proceeds on its own without a continuous external influence.
Will the reverse reaction spontaneous?
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3. 16.1 Spontaneous process
Spontaneity reaction always moves a system toward equilibrium
Both forward and reverse reaction depends on
Temperature
Pressure
Composition of reaction mixture
Q < K; reaction proceeds in the forward direction
Q>K; reaction proceeds in the reverse direction
Spontaneity of a reaction does not identify the speed of reaction

4. Gibbs free energy and spontaneity
Gibbs free energy and Reaction spontaneity
i. The spontaneity of a chemical reaction is the possibility of the reaction occurring or
not without the aid of outside energy.
ii. If a reaction occurs without a net input of energy, it is called “spontaneous.” This
type of reaction actually releases (“gives off”) useful energy.
iii. If a reaction requires energy in order to occur, it is called “nonspontaneous.” This
type of reaction “takes in” energy when it occurs.
iv. Depiction of spontaneity with a reaction energy diagram
v. Gibbs free energy is the energy that is “free” to do work. “Free” doesn’t mean “costs
nothing,” it means “available,” as in “Are you free tonight at around 8:00?

5. Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
11/29/2018
Free Energy
Using the second law and DG = DH - TDSsys
DG < 0 The reaction is spontaneous.
DG > 0 The reaction is nonspontaneous.
DG = 0 The reaction mixture is at equilibrium.
Thermodynamic Standard State: Most stable form of a substance at 1 atm pressure and at a specified temperature, usually 25 °C; 1 M concentration for all substances in solution
DG° = DH° - TDS°sys
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6. The effect of H, S and T on Spontaneity
Low Temperature
High Temperature - +
Spontaneous (G<0)
Nonspontaneous (G > 0)
Spontaneous (G< 0)
nonSpontaneous (G>0)
Nonspontaneous (G>0)

7. Entropy and Gibbs free energy
Like Horxn, at standard condition
DSo = So(products) So(reactants)
DGo = DGof (products)  DGof (reactants)

8. Chemistry: McMurry and Fay, 6th Edition
Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
11/29/2018 6:19:03 PM
Copyright © 2011 Pearson Prentice Hall, Inc.

9. 16.5 Standard Molar Entropies and Standard Entropies of Reaction
Chemistry: McMurry and Fay, 6th Edition
Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
11/29/2018 6:19:03 PM
16.5 Standard Molar Entropies and Standard Entropies of Reaction
Calculate ΔSo for the following reaction
CO(g) + 2H2(g) = CH3OH(l)
Predicted Sign for Δ So? __________
p652
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10. Standard Free Energies of Formation
Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
Standard Free Energies of Formation
11/29/2018
Using table values, calculate the standard free-energy change (ΔGo) at 25 °C for the reduction of iron(III) oxide with carbon monoxide:
2Fe(s) + 3CO2(g)
Fe2O3(s) + 3CO(g)
Fe2O3(s)
3CO(g)
2Fe(s)
3CO2(g)
Gof (kJ/mol)
-743.5
-137.2
-394.4
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11. Example
What is the standard free-energy change, ΔoG, for the following reaction at 25.0oC?
2KClO3(g)  2KCl(s) + 3O2(g)
ΔHo = -78.0kJ ΔSo =493.9J/K
Is the reaction spontaneous at standard-state condition?
Does the reaction become nonspontaneous at higher temperature?

12. 16.9 Standard Free-Energy Changes for Reactions
Chemistry: McMurry and Fay, 6th Edition
Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
11/29/2018 6:19:03 PM
16.9 Standard Free-Energy Changes for Reactions
Calculate the standard free-energy change at 25 oC for the Haber synthesis of ammonia using the given values for the standard enthalpy and standard entropy changes:
2NH3(g)
N2(g) + 3H2(g)
DHo = 92.2 kJ
DSo = 198.7 J/K
p658
Negative value for DG° means it’s spontaneous.
Spontaneous doesn’t mean fast, however. Rate of reaction is kinetics.
Will the reaction become nonspontanous at higher temperature?
If so, then determine at what temperature will it become nonspontanous?
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13. Entropy and the Second Law of Thermodynamics
Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
Entropy and the Second Law of Thermodynamics
11/29/2018
DStotal = DSsystem + DSsurroundings or
DStotal = DSsys + DSsurr
DStotal > 0 The reaction is spontaneous.
DStotal < 0 The reaction is nonspontaneous.
DStotal = 0 The reaction mixture is at equilibrium.
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14. Entropy and the Second Law of Thermodynamics
Chemistry: McMurry and Fay, 6th Edition
Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
11/29/2018 6:19:03 PM
Entropy and the Second Law of Thermodynamics
DSsurr a  DH
DSsurr a T 1
DSsurr = T
 DH
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15. Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
11/29/2018
Free Energy: a thermodynamic quantity defined by G = H - TS
DG = DH - TDSsys
Using:
DStotal = DSsys + DSsurr
Dssurr = T
- DH
DG = -TDStotal
DStotal = DSsys - DH T
Free energy was introduced in Ch 8 (Thermochemistry: Chemical Energy).
Constant pressure and temperature
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16. Example Consider the combustion of propane gas:
C3H8(g) O2(g)  3 CO2(g) H2O(g)
a. Calculate the entropy change in the surrounding associated with this reaction occurring at 25.0oC
b. Determine the entropy change for the system
c. Determine the entropy change for the universe (entropy total). Will the reaction be spontaneous?
So (J/K mol)
ΔHof (kJ/mol)
C3H8(g)
270.2
-103.8
O2(g)
205.0
CO2(g)
213.6
-393.5
H2O(g)
188.7
-241.8

17. 16.3 Entropy and Probability
Chemistry: McMurry and Fay, 6th Edition
Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
11/29/2018 6:19:03 PM
16.3 Entropy and Probability
S = k ln W
k = Boltzmann’s constant
= 1.38 x 1023 J/K
W =
The number of ways that the state can be achieved.
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18. 16.4 Entropy and Temperature
Chemistry: McMurry and Fay, 6th Edition
Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
11/29/2018 6:19:03 PM
16.4 Entropy and Temperature
ΔS increases
when increasing the average kinetic energy of molecules
Total energy is distributed among the individual molecules in a number of ways
Botzman- the more way (W) that the energy can be distributed the greater the randomness of the state and higher the entropy
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19. 16.4 Entropy and Temperature
Chemistry: McMurry and Fay, 6th Edition
Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
11/29/2018 6:19:03 PM
16.4 Entropy and Temperature
Third Law of Thermodynamics: The entropy of a perfectly ordered crystalline substance at 0 K is zero.
Lower temperature
Less molecular motion
Narrower distribution of
individual molecular energies
Less randomness
Lower entropy
Higher temperature
Greater molecular motion
Broader distribution of
individual molecular energies
More randomness
Higher entropy
Look back at S = k ln W. If W = 1 (only one way to order a perfectly ordered crystalline substance), then ln W = 0.
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20. 16.6 Entropy and the Second Law of Thermodynamics
Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
11/29/2018
16.6 Entropy and the Second Law of Thermodynamics
First Law of Thermodynamics: In any process, spontaneous or nonspontaneous, the total energy of a system and its surroundings is constant.
Helps keeping track of energy flow between system and the surrounding
Does not indicate the spontaneity of the process
Second Law of Thermodynamics: In any spontaneous process, the total entropy of a system and its surroundings always increases.
Provide a clear cut criterion of spontaneity
Direction of spontaneous change is always determined by the sign of the total entropy change
The first law was defined in Ch 8 (Thermochemistry: Chemical Energy). Entropy was also introduced in that chapter.
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21. 16.11 Free Energy and Chemical Equilibrium
Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
16.11 Free Energy and Chemical Equilibrium
11/29/2018
DG = DG° + RT ln Q
When the reaction mixture is mostly reactants:
Q << 1 RT ln Q << 0 DG < 0
The total free energy decreases as the reaction proceeds spontaneously in the forward direction.
When the reaction mixture is mostly products:
Q >> 1 RT ln Q >> 0 DG > 0
The total free energy decreases as the reaction proceeds spontaneously in the reverse direction.
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22. Chemistry: McMurry and Fay, 6th Edition
Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
11/29/2018 6:19:03 PM
Copyright © 2011 Pearson Prentice Hall, Inc.

23. Free Energy and Chemical Equilibrium
Chemistry: McMurry and Fay, 6th Edition
Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
11/29/2018 6:19:03 PM
Free Energy and Chemical Equilibrium
DG = DGo + RT ln Q
DG = Free-energy change under nonstandard conditions.
At equilibrium, DG = 0 and Q = K.
DGo = RT ln K
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24. Free Energy Changes and the Reaction Mixture
Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
11/29/2018
Free Energy Changes and the Reaction Mixture
Calculate DG for the formation of ethylene (C2H4) from carbon and hydrogen at 25 °C when the partial pressures are 100 atm H2 and 0.10 atm C2H4.
C2H4(g)
2C(s) + 2H2(g)
Is the reaction spontaneous in the forward or the reverse direction?
C(s)
H2(g)
C2H4(g)
∆ Gof (kJ/mol)
61.8
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25. Free Energy and Chemical Equilibrium
Chemistry: McMurry and Fay, 6th Edition
Chapter 16: Thermodynamics: Entropy, Free Energy, and Equilibrium
11/29/2018 6:19:03 PM
Free Energy and Chemical Equilibrium
Calculate Kp at 25 oC for the following reaction:
CaO(s) + CO2(g)
CaCO3(s)
CaCO3(s)
CaO(s)
CO2(g)
Gof (kJ/mol)
-603.5
-394.4
Problem p668
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26. Example
The value of ΔGof at 25oC for gaseous mercury is kJ/mol. What is the vapor pressure of mercury at 25oC?
Hg(l) Hg(g)