CHEM 160 Thermodynamics Chapter 19 Example Problems by img20336

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Thermodynamics
Chapter 19
Example Problems

This list contains problems that will be worked as examples in class and additional problems for students to
try on their own.

1.      Calculate ∆S°rxn for the following reaction at 25°C: 2SO2(g) + O2(g) → 2SO3(g)
Ans. –187.9 J/K

2.      Calculate ∆S°rxn for the following reaction at 25°C: 2O(g) → O2(g)
Ans. –117 J/K

3.      Calculate ∆S°rxn for the following reaction at 25°C: FeCl3(s) → Fe3+(aq) + 3Cl-(aq)
Ans. 320.5 J/K

4.      Calculate ∆Suniv for the synthesis of ammonia at 25°C if ∆Ssys(∆S°rxn) = -199 J/K and
∆Hsys(∆H°rxn) = -92.6 kJ. Is this reaction spontaneous at 25°C?     Ans. 112 J/K, yes

5.      The synthesis of a particular reagent at 50°C leads to an increase in the system’s entropy
by 100 J/K and releases 85 kJ of heat energy. Calculate ∆Suniv for this process. Is this
reaction spontaneous at this temperature?                            Ans. 363 J/K, yes

6.      For a particular reaction, ∆Hrxn = 53 kJ and ∆Srxn = 115 J/K. Calculate ∆G°rxn at (a) 25°C
and (b) 250°C? At which temperature is the reaction spontaneous? (c) At what
temperature does ∆G°rxn = 0?                          Ans. (a) 18.7 kJ, (b) –7.1 kJ, 250°C, 460.9K

7.      Calculate the Gibbs free energy change for a reaction that has ∆Hrxn = 83 kJ and ∆Srxn =
188 J/K at 300°C. Is this reaction spontaneous at this temperature?
Ans. –24.7 kJ, yes

8.      Calculate the entropy change, ∆Sfus (J/K) for the melting of 1 mole of a solid if it’s
melting point is 115°C and it’s heat of fusion, ∆Hfus, is 25 kJ/mol. Ans. 64.4 J/K

9.      Calculate the standard free energy change for the following reaction at 25°C:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
Ans. –818 kJ

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10.   Calculate ∆Grxn for the following reaction if ∆G°rxn = 9.9 x 103 J/mol and (a) [A] = 0.8
M, [B] = 0.5 M, [C] = 0.05 M, and [D] = 0.05 M, and (b) [A] = 0.1 M, [B] = 1 M, [C] =
0.5 M, and [D] = 0.5 M. Is the reaction spontaneous?
2A + B --> C + D
Ans. (a) –2121 J, yes, (b) 17875 J, no

11.   Calculate ∆G°rxn for the ionization of CH3COOH at 25°C. Ka = 1.8 x 10-5. Is the reaction
spontaneous under these conditions?                              Ans. 27 kJ, no

12.   Calculate ∆G°rxn for the neutralization of a strong acid with a strong base. K = 1.0 x 1014
Is this reaction spontaneous under these conditions?                         Ans. –80 kJ, yes

13.   The equilibrium constant for an exothermic reaction is 12.3 at 25°C. Calculate ∆G°rxn. If
∆H°rxn = -20 kJ, does the system become more ordered or disordered?
Ans. –6.22 kJ, more ordered since ∆S = -46.2 J/K

14.   Calculate Keq for a reaction at 25°C if ∆H°rxn = 42.0 kJ and ∆S°rxn = 125 J/K. Is this
reaction reactant-favored or product-favored?                        Ans. 0.15, reactant-favored

15.   Calculate Keq for a reaction at 25°C if ∆G°rxn = -12.0 kJ. Is this reaction reactant-favored
or product-favored?                                                   Ans. 127, product-favored

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