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CHM 112
Chapter 17 Worksheet: Thermodynamics
Name: ______________________
Q1. Which of the following processes are spontaneous and which are non spontaneous?
a) spreading of the fragrance of perfume/ air freshener in a room
Spontaneous
b) cleaning your room
c) 2H2O (l) → 2H2 (g) + O2 (g) (at room temperature, 1 atm pressure)
Nonspontaneous
d) Building a house of cards
Q2.
Predict the sign of entropy change (positive or negative) for the following systems:
a) a lake freezing
negative
b) SO2(g) + CaO(s) → CaSO3(s)
c) NH3(g) + HCl(g) → NH4Cl (s) negative
d) 4 CO2 (g) + 2 H2O (g) → 2 C2H2 (g) + 5 O2(g)
e) N2H4 (g) → N2(g) + 2 H2(g) positive
Q3.
Using the values for standard molar entropies (S°) from the Appendix in your text book, calculate ΔS° for the following
reactions.
a) CH3OH (l) → CO (g) + 2 H2(g)
332.3 J/K
b) C2H5OH(l) + O2(g) → HC2H3O2 (l) + H2O (l)
-136.1 J/K
c) 2CH4(g) → C2H6 (g) + H2(g)
-12.7 J/K
Q4.
For the reaction: 2Al(s) + 3/2 O2(g)  Al2O3(s), ∆Horxn = – 1617 kJ/mol and ∆Gorxn = –1577 kJ/mol. Use this data to
calculate ∆S° reaction at 298 K.
-134 J/K
Q5.
What is the minimum temperature required for the spontaneous conversion of CCl4(l) to CCl4(g) when ∆H°(vap) is 57.3
kJ/mol and ∆S°(vap) is 164 J/(mol K) ?
349 K or
76°C
Q6. Using the data in the appendix of your text book, calculate the standard Gibbs free energy change (ΔG°) at 25°C for the
following reactions. In each case, indicate whether the reaction is spontaneous or not at that temperature.
a) 2HCl (g) ⇆ H2 (g) + Cl2(g)
+190.6 kJ
not spontaneous
b) 2 AgCl(s) + Ca(s) ⇆ CaCl2 (s) + 2 Ag (s)
-529.2 kJ
spontaneous
c) SO3(g) + H2O (l) ⇆ H2SO4 (l)
-81.8 kJ
spontaneous
Q7. For each of the reactions listed in Q6, calculate the value of the equilibrium constant K (assuming 1 mole at 25°C).
a) 3.89 × 10-34
b) 5.80 × 1092
c) 2.18 × 1014
Q8.
Given the following data,
4 NO(g)  2 N2O(g) + O2(g)
∆G° = – 139.56 kJ
2 NO (g) + O2(g)  2 NO2 (g)
∆G° = – 69.70 kJ
Calculate ∆G° for the following reaction:
2 N2O(g) + 3 O2(g)  4 NO2 (g)
Q9. Write the reaction for the vaporization of water. Calculate the ΔG⁰rxn at 25.0⁰C.
Is the reaction spontaneous at 25.0⁰C?
H2O (l) → H2O (g)
ΔG° = +8.5 kJ
not spontaneous
Use the values of ΔH⁰f and S⁰ to estimate the value of ΔG⁰rxn at 125⁰C. Is the reaction spontaneous at this temperature? Why?
ΔG° = -3.28 kJ
Spontaneous. At every temperature above the boiling point (100⁰C), ΔG° should be
negative (it will spontaneously boil), but at temperatures lower than that, ΔG° is positive, so it won’t boil spontaneously.
Q10.
Consider the reaction:
2 NO2 (g) ⇄ N2O4 (g)
a) Using the data in your textbook, calculate the Gibbs free energy change (ΔG°) for the reaction at 298 K.
-2.8 kJ
b) Calculate the value of Keq at 298 K.
3.10
c) Calculate ΔG at 298 K when the partial pressures for NO2 and N2O4 are 10.5 and 0.50 atm respectively.
-16.2 kJ
Q11.
Consider the reaction:
C(s) + 2 H2 (g) → CH4 (g)
a) Using the data in your textbook, calculate the standard Gibbs free energy change (ΔG°) for the reaction at 298 K.
-50.5 kJ
b) Estimate ΔG˚ for the reaction at 400 K. (Is the reaction more or less spontaneous at high temperature?)
-42.3 kJ
less spontaneous
c) Calculate the value of ΔG at 298 K when the partial pressures of CH4 and H2 are 25.5 atm and 1.00 atm
respectively. Predict what should happen using Le Chatelier’s principle. Is the calculated result consistent with the
predicted result?
Le Chatelier: More product means shift to the left
New ΔG = -42.5 kJ
matches Le Chatelier
which means the forward reaction is less spontaneous than it was before, which
d) Above what temperature will this reaction be nonspontaneous?
Above 923 K
Q12. At what temperature will the reaction become nonspontaneous?
FeO(s) + CO(g) → CO2(g) + Fe(s)
ΔH = -11.0 kJ; ΔS = -17.4 J/K
Above 632 K
The following are multiple choice questions:
13. Melting of a solid is an example of a process for which
(A) ∆H, ∆S, and ∆G are positive at all temperatures.
(B) ∆H and ∆S are positive.
(C) ∆G is negative at low temperatures, positive at high temperatures.
(D) ∆H = ∆S
14. For the following process: 2I(g)  I2(s)
(A) ∆H is + and ∆S is + for the reaction.
(B) ∆H is – and ∆S is – for the reaction.
(C) ∆H is + and ∆S is – for the reaction.
(D) ∆H is – and ∆S is + for the reaction.
(E) ∆G is + for all temperatures
15. A reaction is spontaneous at all temperatures if
(A) ∆H and ∆S are both positive.
(B) ∆H and ∆S are both negative.
(C) ∆H is positive and ∆S is negative.
(D) ∆H is negative and ∆S is positive.