Solutions/Answers to Exercises
Exercise 1. Indicate if entropy is positive or negative for the following:
a) sublimation; positive
b) condensation; negative
c) melting; positive
b) condensation; negative
c) melting; positive
Exercise 2. Indicate if entropy is increasing or decreasing in the following:
a) 2 NH3 (g) → N2 (g) + 3 H2 (g); increasing
b) C2H4 (g) + Cl2 (g) → C2H4Cl2 (l); decreasing
c) Cl2 (g) + H2C=CH2 (g) → ClCH2CH2Cl (l); decreasing
b) C2H4 (g) + Cl2 (g) → C2H4Cl2 (l); decreasing
c) Cl2 (g) + H2C=CH2 (g) → ClCH2CH2Cl (l); decreasing
Exercise 3. Indicate if reactions with the following ΔH and ΔS values are spontaneous, nonspontaneous, endothermic, or exothermic.
a) ΔH = -48 kJ; ΔS = 132 J/K at 395 K
Both ΔH and ΔS are favorable; The process would be spontaneous at all temperatures. The process is exothermic.
b) ΔH = -126 kJ; ΔS = 38 J/K at 485 K
Both ΔH and ΔS are favorable; The process would be spontaneous at all temperatures. The process is exothermic.
c) ΔH = 78 kJ; ΔS = 98 J/K at 785 K
ΔH is unfavorable and ΔS is favorable; The process would be spontaneous at higher temperatures and nonspontaneous at lower temperatures. The reaction is endothermic.
d) ΔH = 64 kJ; ΔS = -135 J/K
Both ΔH and ΔS are unfavorable; The process would be nonspontaneous at all temperatures. The process is endothermic
Both ΔH and ΔS are favorable; The process would be spontaneous at all temperatures. The process is exothermic.
b) ΔH = -126 kJ; ΔS = 38 J/K at 485 K
Both ΔH and ΔS are favorable; The process would be spontaneous at all temperatures. The process is exothermic.
c) ΔH = 78 kJ; ΔS = 98 J/K at 785 K
ΔH is unfavorable and ΔS is favorable; The process would be spontaneous at higher temperatures and nonspontaneous at lower temperatures. The reaction is endothermic.
d) ΔH = 64 kJ; ΔS = -135 J/K
Both ΔH and ΔS are unfavorable; The process would be nonspontaneous at all temperatures. The process is endothermic
Exercise 4.
A process has ΔH = 32 kJ and ΔS = 56 J/K. At what temperature will it change from nonspontaneous to spontaneous?
Exercise 5.
Consider the following reaction:
ΔH = -218 kJ, ΔS = -234 J/K
Calculate ΔG° for the reaction at 25°C.