Exam 3-solutions - Version 370 – Exam 3 – laude –...

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Unformatted text preview: Version 370 – Exam 3 – laude – (51635) 1 This print-out should have 40 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 6.0 points The Ostwald process is used to produce which of the following? 1. NH 3 2. HNO 3 correct 3. Al 4. H 2 S 5. H 2 SO 4 Explanation: 002 6.0 points Which of the following is not commonly used to protect the world from rust? 1. oxide coatings 2. galvanization 3. salt washings correct 4. sacrificial electrodes Explanation: Salt washings would actually accelerate ox- idative processes. 003 6.0 points How many Coulombs of charge are needed to create 19 . 7 g of Au(s) from Au 3+ (aq)? 1. . 1 C 2. 96 , 485 C 3. . 3 C 4. 9 , 649 C 5. 28 , 945 C correct 6. 289 , 456 C Explanation: This is a 3 electron process. (19 . 7 g Au(s)) parenleftbigg 1 mol 197 g parenrightbigg = 0 . 1 molAu(s) 4 hr = 14 , 400 s It N e F = n p Q = n p N e F = 0 . 1 · 3 · 96 , 485 = 28946 C 004 6.0 points Consider the reaction mechanism below: Step Reaction 1 NO 2 + F 2-→ NO 2 F + F 2 NO 2 + F-→ NO 2 F overall 2NO 2 + F 2-→ 2NO 2 F Which species is a catalyst and which is an intermediate, respectively? 1. none, F correct 2. NO 2 , NO 2 3. NO 2 F, F 4. none, NO 2 F 5. NO 2 , F Explanation: The reactants that precede the slowest for- ward step will appear in the rate law for a reaction. 005 6.0 points In a catalytic converter, one of the general sets of reactions is a reduction. It is . 1. incomplete combustion products being converted to carbon dioxide. Version 370 – Exam 3 – laude – (51635) 2 2. nitrogen oxides being converted to nitro- gen gas. correct 3. carbon dioxide being converted into methane. 4. carbon monoxide being converted to car- bon dioxide. Explanation: The catalytic substrate in catalytic con- verters accelerates severl reactions. These in- clude the reduction of various nitrogen oxides (N x O y ) to nitrogen gas (N 2 ). 006 6.0 points The ’a’ term for this question is 1. A second-order reaction has a rate con- stant of 1 . 25 M − 1 · s − 1 . If the initial reactant concentration is 1.0 M, calculate the time re- quired for 90% reaction. 1. 17 s 2. 7.2 s correct 3. 0.13 s 4. 1.3 s 5. 0.89 s Explanation: 007 6.0 points What is the average current generated in the Cu(s) | Cu 2+ (aq) || Fe 3+ (aq) | Fe(s) electrochemical cell if 50 g of Cu(s) are used up in a 24 hour period? Cu 2+ + 2 e − → Cu E ◦ red = +0 . 22 V Fe 3+ + 3 e − → Fe E ◦ red =- . 04 V 1. 42.17 amp 2. 13.00 amp 3. 111.85 amp 4. 1.76 amp correct 5. 2.64 amp Explanation: m Cu = 50 g t = 24 h The half equation of interest is Cu(s) → Cu 2+ (aq) + 2 e − Find the moles of Cu dissolved: (50 g Cu) 1 mol Cu 63 . 546 g Cu = 0 . 786832 mol Cu Find the amount of charge carried by the electrons: (0 . 786832 mol Cu) × 2 mol e − 1 mol Cu × 96485 C 1 mol e − = 1 . 51835 × 10 5 C Find the average current in 24 hours: Colombs = amp × second amp = Coulomb second = 1 . 51835 × 10 5...
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This note was uploaded on 06/14/2011 for the course MATH 305G taught by Professor Cathy during the Spring '11 term at University of Texas.

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Exam 3-solutions - Version 370 – Exam 3 – laude –...

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