Exam 4 - Version 115 Exam 4 Holcombe (52460) This print-out...

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Version 115 – Exam 4 – Holcombe – (52460) 1 This print-out should have 28 questions. Multiple-choice questions may continue on the next column or page – fnd all choices beFore answering. 001 10.0 points Magnesium metal is placed in contact with an underground steel storage tank to inhibit the corrosion oF the tank because 1. magnesium has a lower oxidation poten- tial than iron. 2. magnesium metal is a better reducing agent than iron. correct 3. magnesium is cheaper than gold. 4. magnesium has a higher reduction poten- tial than iron. 5. magnesium Functions as the cathode in the electrochemical cell instead oF the iron. Explanation: 002 10.0 points The experimentally determined rate law For the reaction (CH 3 ) 3 CBr + OH (CH 3 ) 3 COH + Br is rate = k [(CH 3 ) 3 CBr]. A postulated mech- anism is (CH 3 ) 3 CBr (CH 3 ) 3 C + + Br (1) (CH 3 ) 3 C + + OH (CH 3 ) 3 COH (2) Which reaction represents the rate de- terming step? 1. (CH 3 ) 3 CBr (CH 3 ) 3 C + + Br correct 2. (CH 3 ) 3 C + + OH (CH 3 ) 3 COH 3. More inFormation is needed. Explanation: Since the experimentally-determined rate law is zeroth order with respect to OH , any reaction involving it must not a±ect the rate; i.e. , the frst reaction must be slower and the rate-determining step. As soon as (CH 3 ) 3 C + is Formed it quickly reacts with OH to Form (CH 3 ) 3 COH. 003 10.0 points In the electrolytic refning oF copper, the pure copper 1. serves as the positive electrode. 2. is electroplated onto the anode. 3. serves as the anode. 4. is electroplated onto the cathode. cor- rect 5. remains oxidized in solution. Explanation: 004 10.0 points WITHDRAWN 005 10.0 points A catalyst 1. increases K to Favor product Formation. 2. changes the reaction mechanism to insure that K is increased. 3. speeds up the reaction and increases K to Favor product Formation. 4. speeds up the reaction but does not change K . correct Explanation: 006 10.0 points The rate constant For the second order reac- tion 2 NO 2 N 2 O 4 is 2 . 79 L / mol · min at 48 C. IF the initial con- centration oF NO 2 is 2 M, what is the halF-liFe?
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Version 115 – Exam 4 – Holcombe – (52460) 2 1. 14.9 s 2. 5.38 s 3. 21.5 s 4. 0.54 min 5. 58.2 s 6. 10.8 s correct Explanation: 007 10.0 points How long will it take for a current of 3.00 amperes to plate 3 . 177 grams of Cu out of a solution of Cu(NO 3 ) 2 ? (1 Faraday = 96,485 coulombs, Cu = 63 . 55 g/mol) 1. None of these 2. 26.8 min 3. 1.79 hr 4. 2.68 hr 5. 53.6 min correct 6. 12.9 sec Explanation: I = 3.0 A m Cu = 3 . 177 g MW Cu = 63 . 55 g/mol The reaction that is occuring is Cu 2+ + 2 e Cu(s) . Number of moles of Cu(s) plated out
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This note was uploaded on 04/22/2011 for the course CHEM 302 taught by Professor Mccord during the Spring '10 term at University of Texas at Austin.

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Exam 4 - Version 115 Exam 4 Holcombe (52460) This print-out...

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