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Unformatted text preview: arriaga (yga77) HW11 quy (50970) 1 This print-out should have 18 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 10.0 points Which statement below is FALSE? 1. Endothermic reactions may be sponta- neous. 2. The entropy of a system increases when order increases. correct 3. For reactions that release heat to the surroundings, H is negative. 4. For reactions in which the reacting system becomes more disordered, S is positive. 5. If the free energy change of reaction is negative the reaction will occur under the given conditions. Explanation: Entropy ( S ) is high for systems with high degrees of freedom, disorder or randomness and low for systems with low degrees of free- dom, disorder or randomness. When disorder increases, S will increases (be positive). 002 10.0 points Which one of the following reactions has a positive entropy change? 1. H 2 O(g) H 2 O( ) 2. 2 SO 2 (g) + O 2 (g) 2 SO 3 (g) 3. BF 3 (g) + NH 3 (g) F 3 BNH 3 (s) 4. N 2 (g) + 3 H 2 (g) 2 NH 3 (g) 5. 2 NH 4 NO 3 (s) 2 N 2 (g) + 4 H 2 O(g) + O 2 (g) correct Explanation: Processes that result in an increase in the number of moles of gaseous substances have S sys &gt; 0. Only this choice demonstrates an increase in the number of moles of gaseous substances (from 0 moles of gaseous reactants to 7 moles of gaseous products). 003 10.0 points For a given reaction G is very negative. Which of the following is FALSE? 1. S for the reaction may be positive or negative. 2. The entropy of the universe must increase as a result of this reaction occuring. 3. If S is positive, then the reaction will probably be more spontaneous at higher tem- peratures. 4. The reaction should occur very rapidly because G is very negative. correct 5. The reaction is termed spontaneous. Explanation: Thermodynamics determines if a reaction will happen; kinetics determines how fast a reaction will occur. G is a thermodynamic term, not a kinet- ics term. 004 10.0 points The G f for Cu 2 S(s) and SO 2 (g) are- 86 . 2 kJ / mol and- 300 . 1 kJ / mol, respec- tively. What is G for the balanced reaction Cu 2 S(s) + O 2 (g) SO 2 (g) + 2 Cu(s) at 298 K? 1. Information is needed on G f for Cu(s) and O 2 (g) 2. G = +231 . 9 kJ/mol 3. G =- 213 . 9 kJ/mol correct 4. G = +386 . 3 kJ/mol 5. G =- 386 . 3 kJ/mol Explanation: arriaga (yga77) HW11 quy (50970) 2 G f O 2 = 0 G f react =- 86 . 2 kJ / mol G f Cu = 0 G f prod =- 300 . 1 kJ / mol G rxn = summationdisplay n G f prod- summationdisplay n G f rct =- 300 . 1 kJ / mol- (- 86 . 2 kJ / mol) =- 213 . 9 kJ / mol ....
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- Spring '08