Entropy Practice Problems

AP Chemistry Ch 17 Entropy and Free Energy PRACTICE TEST Which of the following represents an increase in entropy? a) freezing of water (L → S) b) boiling of water (L → G) c) crystallization of salt from a supersaturated solution (S → L) d) the reaction 2NO(g) → N₂O₂(g) e) the reaction 2H₂(g) + O₂(g) → 2H₂O(g) The enthalpy of vaporization of methanol (CH₃OH) is 35.3 kJ/mol at the boiling point. Calculate the entropy change for methanol going from a liquid to vapor. a) 600 J/K • mol b) 551 J/K • mol c) 105 J/K • mol d) -105 J/K • mol e) -551 J/K • mol Calculate the standard entropy change for the following reaction: Cu(s) + ½ O₂(g) → CuO(s)   given that: S°[Cu(s)] = 33.15 J/K • mol S°[O₂(g)] = 205.14 J/K • mol S°[CuO(s)] = 42.63 J/K • mol a) 195.66 J/K b) 93.09 J/K c) -45.28 J/K d) -93.09 J/K e) 195.66 J/K In which of the following reactions do you expect to have a decrease in entropy? a) Fe(s) → Fe(l) b) Fe(s) + S(s) → FeS(s) c) 2Fe(s) + 3/2 O₂(g) → Fe₂O₃(s) d) HF(l) → HF(g) e) 2H₂O₂(l) → 2H₂O(l) + O₂(g) The formation of C₂AB₄ has an enthalpy of formation of -104 kJ and a change in entropy of -60.8 J/K at 30°C. What is ΔG° and spontaneity of the reaction? a) -85.6 kJ, spontaneous b) 18.3 kJ, not spontaneous c) +18.3 kJ, spontaneous d) +85.6 kJ, not spontaneous e) -85.6 kJ, not spontaneous   If ΔH and ΔS are both negative or positive, then ΔG has a ________ sign. a) positive b) negative c) variable d) large e) no At what temperature would a given reaction become spontaneous if ΔH = +119 kJ and ΔS = +263 J/K?   a) 452 K b) 2210 K c) 382 K d) 221 K e) 363 K The free energy change for a given reaction is -36.2 kJ. What is the equilibrium constant at 298 K? a) 0.985 b) 2.22 x 10⁶ c) 1.02 d) 8.32 x 10⁻⁷ e) 3.25 x 10⁶ Given the following information, calculate ΔG° for the reaction below at 25°C. SnCl₄(l) + 2 H₂O(l) → SnO₂(s) + 4 HCl(g) ΔH° = 133.0 kJ and ΔS° = 401.5 J/K a) -252.6 kJ b) -13.4 kJ c) 13.4 kJ d) 122.9 kJ e) 252.6 kJ 10. Given the following information, calculate ΔG° for the reaction below at 25°C. * 2H₂O₂(l) → 2H₂O(l) + O₂(g) * ΔH° = -285.8 kJ * ΔS° = 205.1 J/K/mol Compound ΔH°f (kJ/mol) S° (J/K/mol) H₂O₂(l) -187.8 109.6 H₂O(l) -285.8 69.9 O₂(g) 0 205.1 a) -37700 kJ b) -342.6 kJ c) -233.5 kJ d) -233.5 kJ e) -157.9 kJ 11. For the process at 25°C, I₂(g) → I₂(s), what are the signs of ΔG, ΔH, and ΔS? * **ΔG** | **ΔH** | **ΔS** * a) - | - | - * b) + | - | + * c) - | + | - * d) + | + | - * e) - | - | + 12. If a process is exothermic and not spontaneous, then what must be true? * a) ΔS > 0 * b) ΔH > 0 * c) ΔG = 0 * d) ΔS < 0 * e) ΔH = 0 13. For any reaction at equilibrium, which of the following is true? * a) ΔH < 0 * b) ΔS = 0 * c) ΔS < 0 * d) ΔH = 0 * e) ΔG = 0 14. All of the following have ΔG°f = 0 EXCEPT * a) O₂(g) * b) Br₂(l) * c) H₂(g) * d) Ca(s) * e) Hg(l) 15. Ammonium nitrate spontaneously dissolves in water at room temperature and the process causes the solution to become quite cold. Which of the following is TRUE about the dissolution of ammonium nitrate? * a) The process is exothermic. * b) Its solubility will be greater in warmer water. * c) ΔS for the reaction is negative. * d) All solutions of ammonium nitrate are supersaturated. * e) All solutions of ammonium nitrate are cold.   From the AP Exam formula sheet: THERMOCHEMISTRY   ΔS° = ΣS°(products) - ΣS°(reactants) ΔH° = ΣΔH°f(products) - ΣΔH°f(reactants) ΔG° = ΣΔG°f(products) - ΣΔG°f(reactants) ΔG° = -RT ln K = -2.303 RT log K ΔG = ΔG° + RT ln Q = ΔG° + 2.303 RT log Q ΔG° = ΔH° - TΔS° Answers: C 11. B 12. D 13. E 14. B 15. B