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Ch15_Part2_062811

# Ch15_Part2_062811 - What have we learned about kinetics so...

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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

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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: 1 st : rate = k[A]; 2 nd : rate = k[A] 2 ; zero: rate = k (5) Integrated rate law: 1 st : plot of ln[A] vs t is linear ln[A] = -kt + ln[A] o 2 nd : 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.
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

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Why do we study reaction kinetics? To fully understand or apply any chemical reaction, we must know more than just the identities of the reactants and the products. We must know: if the reaction will occur (thermodynamically favorable?) how long it will take to occur (kinetically feasible?) For a complete understanding, we should also know how it occurs…reaction mechanisms and a model of chemical kinetics (collisions, effect of temperature…) .
When chemists asks: " how does this reaction occur ?" they are asking for a reaction mechanism . A reaction mechanism is the series of bond- breaking and bond-forming steps that occur during the conversion of reactants to products. Chemical reactions are often represented by a single balanced equation, but that does not mean that the reaction will take place in only one step. Most reactions occur in more than one step . Consider for example the reaction: NO 2 (g) + CO(g) NO(g) + CO 2 (g) How does this reaction occur? Kinetics measurements are key experiments used to elucidate reaction mechanisms.

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Experimental results show: Rate = k [NO 2 ] 2 An elementary step is a reaction for which the rate law can be written from the molecularity. ..
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Ch15_Part2_062811 - What have we learned about kinetics so...

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