Chemistry (Ch18) Answer Key

Chemistry (ch18) 1. Identify the INCORRECT statement below: a) The symbol H refers to the enthalpy, or "heat content" of the system. b) The ΔH of a reaction tells one the heat liberated or absorbed under constant pressure reaction conditions. c) The internal energy change of a system equals the heat absorbed minus the work done by the system. d) The second law states that ΔS of a system increases in any spontaneous process. e) The symbol G refers to the Gibbs' free energy of the system. Answer: d 2. Identify the INCORRECT statement below: a) In spontaneous changes the universe tends toward a state of greater disorder. b) The Gibbs free energy of a system is increasing in any spontaneous process going on at constant T and P. c) The entropy of a pure, perfect crystal at T = 0 K is zero. d) The entropy of a system can decrease in a spontaneous process, if the surrounding entropy is increasing even more. e) The expansion of a gas into a vacuum is an example of an increase in entropy. Answer: b 3. There are two properties of a reacting system that determine whether a process at constant pressure and temperature can occur spontaneously. These are: a) kinetic and potential energy change b) heat and work c) energy change and enthalpy change d) enthalpy change and entropy change Answer: d 4. The two properties in Question #4 can be combined to give a single thermodynamic state property of a system which dictates whether a process can occur spontaneously. This property is the: a) rate constant b) free energy change c) rate law d) heat content e) exothermicity Answer: b 5. Identify the INCORRECT statement: a) An endothermic process which is spontaneous must have a positive ΔS. b) Every naturally occuring spontaneous process is attended with an increase in the entropy of the universe. c) Every naturally occuring spontaneous process in a system is attended with a decrease in free energy of the system. d) A process which causes the system entropy to decrease can still be spontaneous if an even larger entropy increase takes place in the surroundings. e) Every naturally occuring spontaneous process in a system is attended with an increase in entropy of the system Answer: e 6. Which process is accompanied by a decrease in entropy of the materials? a) expansion of a gas into a vacuum b) solution formation c) crystal formation d) thermal equilization between two regions in thermal contact e) diffusion of solute molecules into a region of low solute concentration Answer : c 7.Estimate the temperature where ΔG = 0 for the following reaction: (Given: ΔH = -176 kJ and ΔS = -284.5 J/K) CAUTION: Beware of units. NH3(g) + HCl(g) ---> NH4Cl(s) a) 582 K b) 467 K c) 619 K d) 634 K e) 680 K Answer: c 8.Which of the following is synonymous with the Law of Conservation of Energy? a) 1st Law of Thermodynamics b) 2nd Law of Thermodynamics c) 3rd Law of Thermodynamics d) Gibb's free energy equation e) none of the above Answer: a 9. Identify the INCORRECT statement below: a) An endothermic process which is spontaneous must have a positive ΔS. b) Every naturally occurring spontaneous process is attended with an increase in the entropy of the universe. c) The Gibb's free energy of the system always increases in a spontaneous process. d) A process which causes the system entropy to decrease can still be spontaneous if an even larger entropy increase takes place in the surroundings. Answer: c 10.A reaction has a ΔH° = +25 kcal/mol and a ΔS° = +0.18 kcal/mol. Remember ΔG = ΔH TΔS. a) This reaction will occur spontaneously at 298 K, and all other temperatures as well. b) This reaction will occur spontaneously at 298 K, but will not at much lower temperatures. c) This reaction will not occur spontaneously at 298 K, but will at much higher temperatures. d) This reaction will not occur spontaneously at any temperature. Answer: b 11. Which one of the following reactions has a POSITIVE entropy change? a) BF3(g) + NH3(g) ---> F3BNH3(s) b) 2 NH4NO3(s) ---> 2 N2(g) + 4 H2O(g) + O2(g) c) H2O(g) ---> H2O(l) d) 2 SO2(g) + O2(g) ---> 2 SO3(g) e) N2(g) + 3 H2(g) ---> 2 NH4(g) Answer: b 12. A process CANNOT be spontaneous (product-favored) if __________. a) the entropy of the universe increases. b) it is endothermic, and there is a decrease in disorder. c) it is endothermic, and there is an increase in disorder. d) it is exothermic, and there is an increase in disorder. e) it is exothermic, and there is a decrease in disorder. Answer: b 13.Determine ΔGo at 25° for the reaction: 2 CO(g) + 2 H2(g) ---> CO2(g) + CH4(g), given that at 25° ΔHo = -247.3 kJ/mol and ΔSo = -256.5 J/mol K. Answers are in kJ/mol. a) 323.9 b) -170.9 c) 76,200 d) -253.7 e) -182.8 Answer: b 14. Which of the following statements regarding the third law of thermodynamics is INCORRECT? a) Pure substances have positive absolute S at T > 0 Kelvin. b) Absolute zero gives a reference point for determining absolute S. c) The absolute S is greater at 300 K than 100 K for a given substance. d) The absolute S is zero at 0 Kelvin. e) The absolute S at 298 K can be positive or negative. Answer: e 15.At 25°C ΔH = 128.9 kJ and ΔG = 33.5 kJ for a reaction. Above what minimum temperature will this reaction become spontaneous? a) 530 K b) 403 K c) 298 K d) 1150 K e) 332 K Answer: b 16. The second law of thermodynamics states: a) All exothermic processes also increase entropy. b) A spontaneous process always increases entropy. c) The entropy of the universe always increases in spontaneous processes. d) The enthalpy of the universe always increases in spontaneous processes. e) DH is less than 0 and DS is greater than 0 for all spontaneous processes. Answer: c 17. Which of the following statements is FALSE? (ΔH, ΔG, and ΔS refer to system changes.) a) ΔS must be positive for a process to be spontaneous. b) ΔG is always positive for nonspontaneous processes. c) If ΔS is negative then ΔH must be negative for a spontaneous process. d) ΔG is always negative for spontaneous processes. e) ΔS is positive for many spontaneous processes. Answer: a 18. Which response includes all the following processes that are accompanied by an INCREASE in entropy, and only those processes? I. boiling water II. freezing water III. N2(g) + 3 H2(g) ---> 2 NH3(g) IV. Br2(l)---> Br2(g) a) I and II b) III and IV c) I and IV d) II, III, and IV e) another one or another combination Answer: c 19. Which one of the following statements is NOT correct? a) When ΔG for a reaction is negative, the reaction is spontaneous. b) When ΔG for a reaction is positive, the reaction is nonspontaneous. c) When ΔG for a reaction is zero, the system is at equilibrium. d) When ΔH for a reaction is negative, the reaction is never spontaneous. e) When ΔH for a reaction is very positive, the reaction is not expected to be spontaneous. Answer: d 20.For the reaction given below, ΔH° = -1516 kJ at 25°C and ΔS° = - 432.8 J/K at 25°C. This reaction is spontaneous __________. SiH4(g) + 2 O2(g) ---> SiO2(s) + 2 H2O(l) a) only below a certain temperature. b) only above a certain temperature. c) at all temperatures. d) at no temperatures. e) cannot tell from the information available. Answer: a 21. A reaction with a ΔG° = -30 kJ/mol: a) has a K = 0 b) has a positive K but K < 1 c) has a negative K d) has a K > 1 e) has a K = -30 Answer: d