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Reaction Kinetics: Factors Affecting Rates and Rate-Limiting Steps

This lecture covers reaction rates and factors that affect them, including concentration, temperature, and catalysts. It also discusses reaction mechanisms, including elementary and multistep reactions, and rate-limiting steps. Examples of rate laws and overall reaction equations are provided.

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Reaction Kinetics: Factors Affecting Rates and Rate-Limiting Steps

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  1. CHE1031Lecture 10: Reaction kinetics • Lecture 10 topics Brown chapter 14 • 1. Reaction rates • Factors that effect reaction rates 14.1 • Visualizing rates & units 14.2 • Average reaction rates • Instantaneous reaction rates • Stoichiometry & reaction rates • 2. Concentration & reaction rates 14.3 • Rate laws • Reaction orders • 3. Change in concentration with time 14.4 • First- & second-order reactions • Half-life • 4. Temperature & reaction rate 14.5 • Collision, orientation & Ea • 5. Reaction mechanisms 14.6 • Elementary • Multistep • 6. Catalysis` 14.7

  2. Reaction mechanisms Elementary vs. multistep reactions Rate-limiting steps

  3. Reaction mechanisms Reaction mechanism – describe the details by which a chemical rxntakes place. There are a series of basic & common mechanisms. Elementary mechanisms: single-step reactions Multistep reactions: require several steps, or reactions to reach completion Molecularity? Uni-, bi-, or termolecular describe reactions of 1, 2 or 3 molecules. = = Unimolecular: H3C – N = C  H3C – C = N Bimolecular: NO + O3  NO2 + O2 Two-step reaction: (1) NO2 + NO2  NO3 + NO (2) NO3 + CO  NO2 + CO2 sum NO2 + CO  NO + CO2 Molecules that don’t appear in thesummed (overall) reaction areintermediates. p. 581 - 2

  4. Elementary reactions & rate laws The relationship is quite simple as you can see here: p. 583 - 4

  5. Rate-limiting steps When chemical reactions require more than one step, their overallrate is often limited by the slowest of the steps. So this slowest stop is called the rate-limiting step because it limits the overall rate of reaction. Step 1: NO2 + NO2  NO3 + NO (slow) Step 2: NO3 + CO  NO2 + CO2 (fast) Overall: NO2 + CO  NO + CO2 k1 k2 >> k1 k2 What is the rate law of the overall reaction? • Because step 1 is much slower than step 2, it is rate-limiting. • The rate of the overall reaction is equal to the rate of the slow step (1). • Step 1 is bimolecular, so • rate = k1[NO2]2 p. 584 - 5

  6. Rate-limiting examples Nitrous oxide decomposes by a two-step mechanism. N2O  N2 + O (slow) N2O + O  N2 + O2 (fast) Write the equation for the overall reaction. Write the rate law for the overall reaction. a) 2N2O  2N2 + O2 b) Rate = k[N2O]2 Ozone reacts with nitrogen dioxide by a two-step mechanism: Step 1: O3 + NO2  NO3 + O2 Step 2: NO3 + NO2  N2O5 Overall: O3 + 2NO2  N2O5 + O2 Overall experimental rate law is: rate = k[O3][NO2]. Which step is slower? Step 1 is the slow step, since it is used in the rate law for the overall reaction. p. 584 - 5

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