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Chexam3 - Version 173 Exam 3 vanden bout(51640 This...

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Version 173 – Exam 3 – vanden bout – (51640) 1 This print-out should have 31 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0points Consider the concentration-time dependence graph for two first-order reactions. [A] 0 Molar Concentration of Reactant Concentration -→ Time -→ Which reaction has the larger rate con- stant? 1. the reaction represented by the upper curve 2. the reaction represented by the lower curve correct 3. Unable to determine Explanation: 002 10.0points Consider the reaction S 2 O 2 4 (aq) SO 2 3 (aq) in basic solution. How many electrons appear in the balanced half-reaction? 1. 3 2. 2 correct 3. 4 4. 6 5. 1 Explanation: 003 10.0points If the two half reactions below were used to make an electrolytic cell, what species would be consumed at the cathode? Half reaction E Cu 2+ (aq) + 2 e -→ Cu(s) +0 . 34 Fe 3+ (aq) + e -→ 2 Fe 2+ (aq) +0 . 77 1. Fe 2+ (aq) 2. Cu 2+ (aq) correct 3. Cu(s) 4. Fe 3+ (aq) Explanation: An electrolytic cell must have a negative cell potential and therefore the cathode reac- tion is the Copper reduction reaction which consumes Cu 2+ (aq). 004 10.0points What is the cathode in Mg(s) | Mg 2+ (aq) || Au + (aq) | Au(s) Mg 2+ + 2 e Mg E red = - 2 . 36 Au + + e Au E red = +1 . 69 and what is the cell type? 1. Au + (aq) | Au(s); an electrolytic cell 2. Mg(s) | Mg 2+ (aq); an electrolytic cell 3. Not enough information is provided. 4. Mg(s) | Mg 2+ (aq); a battery 5. Au + (aq) | Au(s); a battery correct Explanation: The diagram A | B || C | D is read as follows: A B + ne (oxidation)
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Version 173 – Exam 3 – vanden bout – (51640) 2 C + me D (reduction) Since reduction occurs at the cathode, the cathode is Au + (aq) | Au(s). To determine the cell type, calculate E cell: Mg(s) Mg 2+ + 2 e E anode = +2 . 36 V 2 Au + + 2 e 2 Au(s) E cathode = +1 . 69 V 2 Au + + Mg(s) 2 Au(s) + Mg 2+ E 0 cell = 4 . 05 V Since E 0 cell is positive, the reaction is spon- taneous, so this is a galvanic cell (a battery). 005 10.0points For the reaction cyclopropane(g) propene(g) at 500 C, a plot of ln[cyclopropane] vs t gives a straight line with a slope of - 0 . 00067 s 1 . What is the order of this reaction and what is the rate constant? 1. second order; 6 . 7 × 10 2 M 1 · s 1 2. second order; 6 . 7 × 10 4 M 1 · s 1 3. first order; 6 . 7 × 10 2 s 1 4. None of these 5. first order; 6 . 7 × 10 4 s 1 correct 6. first order; 6 . 7 s 1 7. second order; 6 . 7 M 1 · s 1 Explanation: 006 10.0points Consider the 2-step reaction mechanism fast 2 NO + H 2 N 2 O+H 2 O slow N 2 O+H 2 N 2 +H 2 O net 2 NO + 2H 2 N 2 + 2 H 2 O Which of the following rate laws is cor- rect for the given mechanism? Note that the constant k may represent a combination of elementary reaction rate constants. 1. rate net = k [H 2 ] 2 [NO] 2 [H 2 O] correct 2. rate net = k [NO] 2 [H 2 ] 3. rate net = k [NO] [H 2 ] 4. rate net = k [NO] 2 [H 2 ] 2 5. rate net = k [N 2 ] [H 2 O] 2 [H 2 ] 2 [NO] 2 Explanation: The rate law must reflect the rate of the slowest (rate determining) step: Rate = k [N 2 O] [H 2 ] N 2 O is an intermediate, a small amount of
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