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Unformatted text preview: Su, Yung Exam 4 Due: Dec 7 2007, 4:00 pm Inst: FAKHREDDINE 1 This print-out should have 17 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. The due time is Central time. Each question is worth one point. 001 (part 1 of 1) 12 points Consider the equation NH 4 Br(s) NH 3 (g) + HBr(g) , and H = +188.3 kJ. Which response de- scribes the thermodynamic spontaneity of the reaction? 1. The reaction is not spontaneous at any temperatures. 2. The reaction is spontaneous only at rela- tively high temperatures. correct 3. We cannot tell from the information given. 4. The reaction is spontaneous only at rela- tively low temperatures. 5. The reaction is spontaneous at all tem- peratures. 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. S(g) > S ( ) > S (s) . A reaction is spontaneous only when G is negative. H is positive for this reaction and S is positive. G = H- T S = (+)- T (+) = (+)- T G will be negative (spontaneous reactions) only at high values of T . 002 (part 1 of 1) 10 points In the reaction Cu 2+ + Fe Cu + Fe 2+ 1. iron is reduced. 2. copper is oxidized. 3. iron is oxidized. correct 4. copper(II) is the reducing agent. Explanation: The oxidation number of an element in a monatomic ion is simply the charge on the ion, so Cu in Cu 2+ and Fe in Fe 2+ both have oxidation numbers of +2. The oxida- tion number of a free, uncombined element is zero. Oxidation is the loss of electrons, which corresponds to an algebraic increase in oxi- dation number. The oxidation number of Fe changes from 0 to +2, so Fe is oxidized. 003 (part 1 of 1) 12 points Is the reaction Cl 2 (g) + 2HBr(g) Br 2 ( ) + 2HCl(g) spontaneous at 25 C and one atmosphere pressure? The standard molar Gibbs free en- ergy of formation of HBr(g) at 25 C is- 53 . 43 kJ/mol and that of HCl(g) is- 95 . 30 kJ/mol. 1. No, because G for the reaction is neg- ative. 2. Cannot tell unless we are given the stan- dard molar Gibbs free energies of formation at 25 C of Br 2 ( ) and Cl 2 (g). 3. Yes, because G for the reaction is pos- itive. 4. No, because G for the reaction is posi- tive. 5. Yes, because G for the reaction is neg- ative. correct Explanation: Su, Yung Exam 4 Due: Dec 7 2007, 4:00 pm Inst: FAKHREDDINE 2 G f HBr(g) =- 53 . 43 kJ/mol G f HCl(g) =- 95 . 30 kJ/mol G f Br 2 = 0 kJ/mol G f Cl 2 = 0 kJ/mol G rxn = X n G f prod- X n G f rct = 2(- 95 . 30 kJ / mol)- 2(- 53 . 43 kJ / mol) =- 83 . 74 kJ / mol For spontaneous reactions, G is negative....
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