Lecture 06_Ch15b

# Lecture 06_Ch15b - Summary of Rate Laws (Table 15.6) Key...

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Summary of Rate Laws Key assumption so far: forward reactions only . This assumption is generally good over short times. (Table 15.6)

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Click to edit Master subtitle style Crystal Violet Demo (CH 3 ) 2 N C N(CH 3 ) 2 N(CH 3 ) 2 (CH 3 ) 2 N C N(CH 3 ) 2 N(CH 3 ) 2 OH + + OH -
Integrated Rate Law for Reactions with More than 1 Reactant Example: aA + bB + cC products Overall reaction order = n + m + p How do we solve for all of these unknowns? rxn order w.r.t. A = n w.r.t. B = m w.r.t. C = p Rate = k [A]n [B]m [C]p

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Method of Initial Rates Rate1 = k [A]1n [B]0m [C]0p Rate2 = k [A]2n [B]0m [C]0p Rate2 [A]2 Rate1 [A]1 = n Solve for n: Rate2 [A]2 Rate1 [A]1 = n ln ln Rate2 [A]2 Rate1 [A]1 ln ln n =
Suppose we used a large excess of [B]0 and [C]0 i.e. [B]0 >> [A]0 and [C]0 >> [A]0 This simplifies the rate law, which becomes pseudo-first order : Rate = k ' [A]n where k ' = k [B]0 [C]0 We can then solve for k ' and n in the usual way Determination of k ' at several different [B]0 and [C]0 allows determination of m , p , and k

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What have we learned about kinetics so far? (2) Reaction rates are expressed quantitatively by rate constants and rate orders , both of which must be determined experimentally. (1) Reaction kinetics are described by rate laws (4) Second-order reactions show a curved decrease of the reactant concentration with time; the half-life is inversely related to the initial reactant concentration (5) Zero-order reactions show a linear decrease of the reactant concentration with time; the half-life is directly related to the initial reactant concentration (3) First order reactions show an exponential decrease of the reactant concentration with time; the half-life is independent of the initial reactant concentration
What have we learned about kinetics? (1) Rate: change in concentration per unit of time (2) Rate expression: for aA + bB cC + dD rate = -d [A] = -d [B] = d [C] = d [D] a*dt b*dt c*dt d*dt (3) Differential rate law: rate=k[A]n[B]m , where k, m, and n have to be experimentally determined (4) Order of reaction w.r.t. A: 1st: rate = k[A]; 2nd: rate = k[A]2; zero: rate = k (5) Integrated rate law: 1st: plot of ln[A] vs t is linear ln[A] = -kt + ln[A]o 2nd: plot of 1/[A] vs t is linear 1/[A] = kt + 1/[A] o zero: plot of [A] vs t is linear [A] = -kt + [A] o (6) Integrated rate law for reactions with more than 1 reactant: Pick one reactant to change and hold the others constant (use large excess). Combine rate constant and constant concentrations into k’, the pseudo-rate constant, simplifying the rate law with respect to the chosen reactant.

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Chapter #15 – Chemical Kinetics 15.1) Reaction Rates 15.2) Rate Laws: Introduction 15.3) Determining the Form of the Rate Law 15.4) Integrated Rate Law 15.5) Rate Laws: Summary 15.6) Reaction Mechanisms 15.7) The Steady-State Approximation 15.8) A Model for Chemical Kinetics 15.9) Catalysis
Why do we study reaction kinetics? To fully understand or apply any chemical reaction,

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## This note was uploaded on 04/11/2011 for the course CHEM 162 taught by Professor N. during the Spring '08 term at University of Washington.

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Lecture 06_Ch15b - Summary of Rate Laws (Table 15.6) Key...

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