Exam 4-solutions

# Exam 4-solutions - Version 090 Exam 4 fakhreddine(51615 1...

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Unformatted text preview: Version 090 Exam 4 fakhreddine (51615) 1 This print-out should have 26 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 10.0 points ATTENTION!! PROVIDE YOUR AN- SWER IN YEARS AND USE THE FOL- LOWING CONVERSION: 1 year = 365.25 days Consider the reaction at 25 C 2 NO 2 2 NO + O 2 The overall rxn rate = k [NO 2 ] 2 The rate constant, k = 1 . 4 10 10 M 1 s 1 If 4 . 4 moles of NO 2 are initially present in a sealed 1.00 liter vessel at 25 C, what is the half-life of the reaction? 1. 29.0184 2. 62.8732 3. 56.5859 4. 23.5774 5. 43.5276 6. 30.587 7. 25.7209 8. 26.319 9. 35.3662 10. 23.0963 Correct answer: 25 . 7209 y. Explanation: a = 2 k = 1 . 4 10 10 M 1 s 1 [A] = 4 . 4 M For a second order reaction t 1 / 2 = 1 ak 1 [A] = 1 2 (1 . 4 10 10 M 1 s 1 ) 1 4 . 4 M = ( 8 . 11688 10 8 s ) 1 h 3600 s 1 d 24 h 1 y 365 . 25 d = 25 . 7209 y Remember, leap years account for there being 365.25 days in a year for extended years. 002 10.0 points A catalyst 1. speeds up the reaction but does not change K . correct 2. changes the reaction mechanism to insure that K is increased. 3. speeds up the reaction and increases K to favor product formation. 4. increases K to favor product formation. Explanation: K is a thermodynamic variable based on G . A catalyst lowers the activation energy of the reaction ( E a ), which speeds it up. It does NOT do anything to the equilibrium position of the system, however. 003 10.0 points For the reaction A B the initial concentration of [A] is 0.1 M. How much of compound [A] is left after 60 minutes if k = 4 . 2 10 6 s 1 ? 1. 4 . 1 10 1 2. 9 . 8 10 2 correct 3. 1 . 10 1 4. 8 . 5 10 2 5. 3 . 2 10 2 Explanation: 004 10.0 points In general, the rate of a reaction can be in- creased by 1. All of these correct 2. increasing the temperature. Version 090 Exam 4 fakhreddine (51615) 2 3. increasing the concentration of reac- tants. 4. adding an appropriate catalyst. Explanation: At higher temperatures more particles have the energy necessary for reaction to take place. A catalyst lowers activation energy so more particles have the necessary energy. At higher concentrations a greater number of effective collisions takes place between reac- tants particles. 005 10.0 points ANSWER IN UNITS OF g/mol A metal forms the salt MCl 3 . Electrolysis of the molten salt with a current of 0 . 6 A for 5 . 82 hr produced 4 . 8 g of the metal. What is the molar mass of the metal? 1. 87.3702 2. 44.669 3. 74.5295 4. 41.6282 5. 110.521 6. 49.1984 7. 48.1668 8. 90.756 9. 68.575 10. 186.199 Correct answer: 110 . 521 g / mol....
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Exam 4-solutions - Version 090 Exam 4 fakhreddine(51615 1...

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