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Chem 162-2007 Chapter 13-Kinetics practice problems

# Chem 162-2007 Chapter 13-Kinetics practice problems -...

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PRACTICE PROBLEMS CHEM 162-2007 EXAM I CHAPTER 13 - KINETICS RATES, RATE CONSTANTS, REACTION ORDERS, HALF-LIFES CONCEPTS RATES,RATE CONSTANTS, REACTION ORDERS, HALF-LIFES CALCULATIONS REACTION MECHANISMS, ACTIVATION ENERGY & CATALYSTS CONCEPTS REACTION MECH., ACTIVATION ENERGY & CATALYSTS CALCULATIONS E. Tavss, PhD 1

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RATES, RATE CONSTANTS, REACTION ORDERS, HALF-LIFES CONCEPTS 8 CHEM 162-2006 FINAL EXAM + ANSWERS CHAPTER 12 – KINETICS RATES, RATE CONSTANTS, REACTION ORDRS, HALF-LIFES CONCEPTS Which one of the following is not true about zero order reactions? A. The rate of the reaction is k B. The rate of the reaction remains constant throughout the reaction. C . A plot of concentration vs. time yields a straight line with a positive slope. D. The units of k are M s -1 . E. The half-life of the reaction is [A 0 ]/2k A. The rate of the reaction is k True. Rate = k[C]o = k. B. The rate of the reaction remains constant throughout the reaction. True. The constant is a constant constant; i.e., the constant doesn’t change. Since Rate = k, the rate doesn’t change. C. A plot of concentration vs. time yields a straight line with a positive slope. False. The integrated rate law is: [C] t = -kt + [C] o . (The formula in the exam syllabus is incorrect; that’s why this problem was considered as being flawed.) This is a straight line. Hence, a plot of [C] t on the y-axis, vs. t on the x-axis will provide a straight line. Since most of the time the kinetics study is disappearance of reactant, the straight line will have a negative slope. (However, in the relatively rare case of plotting the formation of a reactant, the slope would be positive.) D. The units of k are Ms -1 . True. E. The half-life of the reaction is [A 0 ]/2k True. 2
3 Chem 162-2006 Hourly exam I + answers Chapter 12 – Chemical Kinetics Rates, rate constants, reaction orders, half-lives concepts Reaction rates Consider the reaction 4NH 3 (g) + 5O 2 (g) → 4NO(g) + 6H 2 O(g). If the rate of disappearance of O 2 is 0.145 atm/s, what is the rate of appearance of H 2 O? E . 0.174 atm/s The rate of appearance of H 2 O is 6/5 the rate of disappearance of O 2 based on the stoichiometry of the equation. 6/5 x 0.145 atm/s = 0.174 atm/s 4 Chem 162-2006 Hourly exam I + answers Chapter 12 – Chemical Kinetics Rates, rate constants, reaction orders, half-lives concepts Rate laws A graph of 1/[A] vs. time gives a straight line. Which rate law is consistent with this graph? A . r = k[A] 2 B. r = k C. r = k[A] D. r = k[A] -1 E. r = k[A] 3 The graph of 1/[A] vs. time giving a straight line is for a second order reaction: 1/[A] t = kt + 1/[A] o . The rate law for a second order reaction: Rate = k[A] 2 1/[A] t is the Y axis; t is the X axis; k is the slope, and 1/[A] o is the Y-intercept. 3

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5 Chem 162-2006 Hourly exam I + answers Chapter 12 – Chemical Kinetics Rates, rate constants, reaction orders, half-lives concepts Rate constants
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