1. Chapter 19: Spontaneous Change: Entropy and Free Energy
Chemistry 140 Fall 2002
General Chemistry
Principles and Modern Applications
Petrucci • Harwood • Herring
10th Edition
Chapter 19: Spontaneous Change: Entropy and Free Energy
King Faisal University
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2. 19-1 Spontaneity: The Meaning of Spontaneous Change
Chemistry 140 Fall 2002
19-1 Spontaneity: The Meaning of Spontaneous Change
A spontaneous process is a process that occurs in a system left to itself; once started, no action from outside the system (external action) is necessary
to make the process continue.
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General Chemistry: Chapter 19

3. General Chemistry: Chapter 19
Spontaneous Process
A process that occurs in a system left to itself.
Once started, no external actions is necessary to make the process continue.
4 Fe(s) + 3 O2(g) → 2 Fe2O3(s)
H2O(s) H2O(l)
A non-spontaneous process will not occur without external action continuously applied.
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General Chemistry: Chapter 19

4. General Chemistry: Chapter 19
Chemistry 140 Fall 2002
Spontaneous Process
For mechanical system potential energy decreases.
For chemical systems the internal energy U is equivalent to potential energy.
Berthelot and Thomsen 1870’s
Spontaneous change occurs in the direction in which the enthalpy of a system decreases (heat is given to the surroundings)
Mainly true but there are exceptions.
Bertholet and Thomsen concluded that exothermic reactions should be spontaneous. In fact, many exothermic processes are spontaneous, but some are not. Also, some endothermic reactions are spontaneous. Thus, we cannot predict whether a process is spontaneous from its enthalpy change alone. Here are three examples of spontaneous, endothermic processes
the melting of ice at room temperature
the evaporation of liquid diethyl ether from an open beaker
the dissolving of ammonium nitrate in water
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General Chemistry: Chapter 19

5. 19-2 The Concept of Entropy
Chemistry 140 Fall 2002
19-2 The Concept of Entropy
Entropy, S.
The greater the number of configurations of the microscopic particles among the energy levels in a particular system, the greater the entropy of the system.
ΔS > 0 spontaneous
ΔH = ΔU + Δ PV
ΔH = ΔU=0
This means that the expansion is not caused by the system dropping to a lower energy state.
Entropy usually refers to the idea that everything in the universe eventually moves from order to disorder, and entropy is the measurement of that change.
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General Chemistry: Chapter 19

6. General Chemistry: Chapter 19
Entropy Change
ΔS =
qrev T
Notice that it is directly proportional to the quantity of heat because the more energy added to a system (as heat), the greater the number of energy levels available to the microscopic particles.
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General Chemistry: Chapter 19

7. 19-3 Evaluating Entropy and Entropy Changes
Phase transitions.
Exchange of heat can be carried out reversibly.
ΔS = ΔH
Ttr
H2O(s, 1 atm) H2O(l, 1 atm) ΔHfus° = 6.02 kJ/mol at K
ΔSfus =
ΔHfus
Ttr =
6.02 kJ mol-1 K
= 2.20 10-2 kJ mol-1 K-1
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General Chemistry: Chapter 19

8. General Chemistry: Chapter 19
Chemistry 140 Fall 2002
Trouton’s Rule
A useful generalization known as Troutons rule states that for many liquids at their normal boiling points, the standard molar entropy of vaporization has a value of about 87 J mol-1 K-1
ΔS =
ΔHvap
Tbp
~ 87 kJ mol-1 K-1
Instances in which Trouton s rule fails are also understandable. In water and in ethanol, for example, hydrogen bonding among molecules produces a lower entropy than would otherwise be expected in the liquid state. Consequently, the entropy increase in the vaporization process is greater than normal.
Failures of this rule are understandable:
Water and hydrogen bonding. Sliquid is lower than expected so ΔS is larger than 87.
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General Chemistry: Chapter 19

9. An entropy based of Raoult’s Law
Chemistry 140 Fall 2002
An entropy based of Raoult’s Law
PA =  APA
Section 1
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General Chemistry: Chapter 19

10. General Chemistry: Chapter 19
Chemistry 140 Fall 2002
Absolute Entropies
Third law of thermodynamics.
The entropy of a pure perfect crystal at 0 K is zero.
Standard molar entropy.
The standard molar entropy is the entropy content of one mole of substance under a standard state.   
Changes in entropy are associated with phase  transitions and chemical reactions.   Chemical equations make use of the standard molar entropy of reactants and products to find the standard entropy of reaction
 are the stoichiometric coefficients.
ΔS = [ΣpS°(products) - ΣrS°(reactants)]
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General Chemistry: Chapter 19

11. Vibrational Energy and Entropy
More complex molecules (e.g., with more atoms per molecule) produce grater molar entropies.
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General Chemistry: Chapter 19

12. General Chemistry: Chapter 19
19-4 Criteria for Spontaneous Change: The Second Law of Thermodynamics.
ΔStotal = ΔSuniverse = ΔSsystem + ΔSsurroundings
The Second Law of Thermodynamics:
ΔSuniverse = ΔSsystem + ΔSsurroundings > 0
All spontaneous processes produce an increase in the entropy of the universe.
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General Chemistry: Chapter 19

13. Free Energy and Free Energy Change
Chemistry 140 Fall 2002
Free Energy and Free Energy Change
Hypothetical process:
only pressure-volume work, at constant T and P.
qsurroundings = -qp = -ΔHsys
Want to evaluate the system only, not the surroundings.
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General Chemistry: Chapter 19

14. Free Energy and Free Energy Change
For the universe:
TΔSuniv. = TΔSsys – ΔHsys = -(ΔHsys – TΔSsys)
-TΔSuniv. = ΔHsys – TΔSsys
For the system:
G = H – TS G: Gibbs Free Energy
ΔG = ΔH – TΔS ΔG : Gibbs Free Energy Change
ΔGsys = - TΔSuniverse
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General Chemistry: Chapter 19

15. Criteria for Spontaneous Change
ΔGsys < 0 (negative), the process is spontaneous.
ΔGsys = 0 (zero), the process is at equilibrium.
ΔGsys > 0 (positive), the process is non-spontaneous.
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General Chemistry: Chapter 19

16. Table 20.1 Criteria for Spontaneous Change
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General Chemistry: Chapter 19

17. 19-5 Standard Free Energy Change, ΔG°
The standard free energy of formation, ΔGf°.
The free energy change for a reaction in which a substance in its standard state is formed from its elements in reference forms in their standard states.
The standard free energy of reaction, ΔG°.
ΔG° = [∑p ΔGf°(products) - ∑r ΔGf°(reactants)]
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General Chemistry: Chapter 19

18. 19-6 Free Energy Change and Equilibrium
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General Chemistry: Chapter 19

19. Free Energy Change and Equilibrium
Chemistry 140 Fall 2002
Free Energy Change and Equilibrium
Section 1 Tuesday
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General Chemistry: Chapter 19

20. Relationship of ΔG° to ΔG for Nonstandard Conditions
2 N2(g) + 3 H2(g) NH3(g)
ΔG = ΔH - TΔS
ΔG° = ΔH° - TΔS°
Isothermal expansion
For ideal gases ΔH = ΔH°
ΔG = ΔH° - TΔS
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General Chemistry: Chapter 19

21. ΔG and the Equilibrium Constant Keq
ΔG = ΔG° + RT ln Q
ΔG = ΔG° + RT ln Keq= 0
If the reaction is at equilibrium then:
ΔG° = -RT ln Keq
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General Chemistry: Chapter 19

22. Criteria for Spontaneous Change
Every chemical reaction consists of both a forward and a reverse reaction.
The direction of spontaneous change is the direction in which the free energy decreases.
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General Chemistry: Chapter 19

23. Significance of the Magnitude of ΔG
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General Chemistry: Chapter 19

24. 19-7 ΔG° and Keq as Functions of Temperature
ΔG° = ΔH° -TΔS°
ΔG° = -RT ln Keq
ln Keq =
-ΔG° RT =
-ΔH°
TΔS° +
ln Keq =
-ΔH° RT
ΔS° R +
ln =
-ΔH°
RT2
ΔS° R +
RT1 - = 1 T2 T1
Keq1
Keq2
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General Chemistry: Chapter 19

25. Temperature Dependence of Keq
ln Keq =
-ΔH° RT
ΔS° R +
slope =
-ΔH° R
-ΔH° = R  slope
= J mol-1 K-1  2.2104 K
= -1.8102 kJ mol-1
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General Chemistry: Chapter 19

26. ΔS > 0 spontaneous process
Summary
19-1 Spontaneity: The Meaning of Spontaneous Change A process that proceeds without need of external intervention is said to be a spontaneous process. Anon spontaneous process cannot occur without external intervention. A process that is spontaneous in one direction is non spontaneous in the reverse direction.
19-2 The Concept of Entropy Entropy is a thermodynamic property related to the distribution of a system’ s energy among the available microscopic energy levels. Entropy usually refers to the idea that everything in the universe eventually moves from order to disorder, and entropy is the measurement of that change.
ΔS > 0 spontaneous process
ΔS =
qprocess T
19-3 Evaluating Entropy and Entropy Changes The third law of thermodynamics states that the entropy of a pure, perfect crystal at 0 K is zero.
The entropy of a substance in its standard state is called standard molar entropy S0, Standard molar entropies of reactants and products can be used to calculate standard entropy changes in chemical reactions.
ΔS = [ΣpS°(products) - ΣrS°(reactants)] ν : Coefficient factor in the chemical reaction
Trouton’s rule, states that the standard entropy of vaporization at the normal boiling point is approximately constant and equal to 87 joul / mol.K
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General Chemistry: Chapter 20

27. ΔG° = [∑p ΔGf°(products) - ∑r ΔGf°(reactants)]
Summary
19-4 Criteria for Spontaneous Change: The Second Law of Thermodynamics
G = H – TS G: Gibbs Free Energy
ΔG = ΔH – TΔS ΔG : Gibbs Free Energy Change
19-5 Standard Gibbs Energy Change ΔG0
The standard free energy of reaction, ΔG°.
ΔG° = [∑p ΔGf°(products) - ∑r ΔGf°(reactants)]
ν : Coefficient factor in the chemical reaction
19-6 Gibbs Energy Change and Equilibrium The relationship between the standard Gibbs energy change and the equilibrium constant for a reaction is
At equilibrium:
ΔG° = -RT ln Keq
ΔG = ΔG° + RT ln Keq= 0

28. General Chemistry: Chapter 19
Chapter 19 Questions
5,7,9,10,11, 15
17,19,23,24,25,
27,28,33, 39
41,47,49,55.
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General Chemistry: Chapter 19