230_Sp10_L31_post - CHEM 230 Sp10 Lecture 31 Chapter 13:...

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CHEM 230 Sp10 Lecture 31 Chapter 13: Chemical Kinetics 1) Order and Integrated Rate Laws 2) Half-Life 3) Mechanism: a) intermediates b) rate-determining step c) elementary reactions Session ID 230
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Reaction Order Reminder Zero-order reactions Rate = k Concentration independent First-order reactions Rate = k [A] Linear relationship between rate and [A] k = slope of line Second-order reactions Rate = k [A] 2 or k [A][B] Exponential relationship between rate and [A] Rate [A] initial [A] initial [A] initial
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The Units of k The units of k must cancel out in rate law to give units of rate (usually rate will be M/s) The units of k will depend on the order of the reaction: Zero-order: Rate = k First-order: Rate = k [A] Second-order: Rate = k [A] 2 n -order: Rate = k [A] n L mol s = 1 M s 1 s mol L s = M s L n- 1 mol n- 1 s = 1 M n " 1 s
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Integrated Rate Law [A] t = [A] 0 1 + [A] 0 kt Zero order First order Second order “THE” Rate = Rate law Integral Integrated rate law Solve for [A] t " d [ A ] dt = k [ A ] 2 " d [ A ] [ A ] 2 [ A ] o [ A ] t # = kdt 0 t # 1 [ A ] t " 1 [ A ] 0 = kt d [ A ] [ A ] [ A ] o [ A ] t " = # kdt 0 t " d [ A ] [ A ] o [ A ] t " = # kdt 0 t " " d [ A ] dt = k [ A ] " d [ A ] dt = k [ A ] t " [ A ] 0 = " kt [ A ] t = " kt + [ A ] 0 ln [ A ] t [ A ] 0 " # $ % = ( kt [ A ] t = [ A ] 0 e " kt A B
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Determine order by graph Zero order First order Second order Graph Slope = -k -k k 1 [A] t = 1 [A] 0 + kt 1 [A] t vs t [ A ] t = " kt + [ A ] 0 [ A ] t vs. t ln[ A ] t = " kt + ln[ A ] 0 ln[ A ] t vs. t
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Two ways to determine order Run multiple trials; vary concentrations of each reactant systematically and measure initial rate. Run one trial and measure concentration of each reactant with time. Plot [] vs. t, ln[] vs. t, and 1/[] vs. time
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Kinetics Problem Hydrogen peroxide, H 2 O 2 decomposes by a first order decomposition to water and oxygen and has a rate constant of 0.015 1/min at 200 o C. Suppose you start with a 0.500 M solution of
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230_Sp10_L31_post - CHEM 230 Sp10 Lecture 31 Chapter 13:...

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