20080213 Kinetics 4 - Intro Chemistry II 030.102 - Chapter...

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Intro Chemistry II – 030.102 - Chapter 13 Spring, 2008 1 Problem 13.21 Consider the following reaction mechanism: H 2 O 2 H 2 O + O O + CF 2 Cl 2 ClO + CF 2 Cl ClO + O 3 Cl + 2 O 2 Questions: Molecularity of each step? Overall reaction? Identify reaction intermediates Cl + CF 2 Cl CF 2 Cl 2 Kinetics and Chemical Equilibrium A direct connection between rates for elementary steps in a chemical reaction mechanism and the overall equilibrium constant K . Consider the reaction N 2 O 2 k –2 k 2 ! N 2 O + H 2 O N 2 O + H 2 k –3 k 3 ! N 2 + H 2 O Elementary reactions shown as equilibria, with forward and reverse rate constants NO + NO k –1 k 1 ! N 2 O 2 2 NO( g ) + 2 H 2 ( g ) ! N 2 ( g ) + 2 H 2 O( g ) Reaction believed to involve 3 steps, with N 2 O 2 and N 2 O intermediates:
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Intro Chemistry II – 030.102 - Chapter 13 Spring, 2008 2 Kinetics and Chemical Equilibrium We invoke principle of detailed balance : At equilibrium, rate of each elementary process is equal to rate of reverse process. We have here Equilibrium constants for the elementary reactions: k 1 [NO] eq 2 = k –1 [N 2 O 2 ] eq k 2 [N 2 O 2 ] eq [H 2 ] eq = k –2 [N 2 O] eq [H 2 O] eq k 3 [N 2 O] eq [H 2 ] eq = k –3 [N 2 ] eq [H 2 O] eq K 1 = [N 2 O 2 ] eq [NO] eq 2 = k 1 k –1 K 2 = [N 2 O] eq [H 2 O] eq [N 2 O 2 ] eq [H 2 ] eq = k 2 k –2 K 3 = [N 2 ] eq [H 2 O] eq [N 2 O] eq [H 2 ] eq = k 3 k –3 Kinetics and Chemical Equilibrium Add steps of mechanism together to obtain overall reaction: Recall equilibrium constant for sum of reactions is product of individual equilibrium constants. K = K 1 K 2 K 3 = [N 2 O 2 ] eq [N 2 O] eq [H 2 O] eq [N 2 ] eq [H 2 O] eq [NO] eq 2 [N 2 O 2 ] eq [H 2 ] eq [N 2 O] eq [H 2 ] eq = k 1 k 2 k 3 k –1 k –2 k –3 2 NO + 2 H 2 + N 2 O 2 + N 2 O ! N 2 + 2 H 2 O + N 2 O 2 + N 2 O OR 2 NO + 2 H 2 ! N 2 + 2 H 2 O = [H 2 O] eq 2 [N 2 ] eq [NO] eq 2 [H 2 ] eq 2 Result can be generalized to any reaction mechanism. Intermediates cancel in K eq
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Intro Chemistry II – 030.102 - Chapter 13 Spring, 2008 3 13.4. Reaction Mechanisms and Rate In many mechanisms, one step (called the rate-determining step ) is significantly slower than all others. Overall reaction can only proceed
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This note was uploaded on 04/14/2008 for the course CHEMISTRY 102 taught by Professor Dagdigian during the Spring '08 term at Johns Hopkins.

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20080213 Kinetics 4 - Intro Chemistry II 030.102 - Chapter...

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