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Kinetics 5 Reaction Mechanisms Reaction Mechanisms Series of Steps Resulting in a Reaction

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Kinetics 5 Reaction Mechanisms Reaction Mechanisms Series of Steps Resulting in a Reaction Kinetics 5 Reaction Mechanisms Reaction Mechanisms Series of steps resulting in a reaction NO2(g) + CO(g) →NO(g) + CO2(g) From experiments we know 2 Rate = k[NO2] Mechanism involves following steps: k1 k2 Mechanism involves following steps: intermediate k1 k2 Elementary Steps in Reaction Mechanisms Elementary step = a reaction whose rate law can be written directly from its molecularity Molecularity is defined as the number of species that have to collide to produce the reaction indicated by that step Unimolecular, bimolecular, termolecular (unusual) Mechanism involves following steps: 2 Bimolecular step, rate = k1[NO2] intermediate Bimolecular step, rate = k2[NO2][CO] k2 Reaction Mechanisms A series of elementary steps that must satisfy two requirements: 1) The sum of the elementary steps must give the overall balanced equation for the reaction 2) The mechanism must agree with the experimentally determined rate law. Reaction: NO2(g) + CO(g) →NO(g) + CO2(g) NO2(g) + NO2(g) →NO3(g) + NO(g) ES 1 NO3 + CO(g) →NO2(g) + CO2(g) ES 2 NO2(g)+ NO2(g) +NO3 + CO(g) →NO3(g)+NO(g)+NO2(g) +CO2(g) Overall reaction: Reaction: NO2(g) + CO(g) →NO(g) + CO2(g) NO2(g) + NO2(g) →NO3(g) + NO(g) ES 1 NO3 + CO(g) →NO2(g) + CO2(g) ES 2 NO2(g)+ NO2(g) +NO3 + CO(g) →NO3(g)+NO(g)+NO2(g) +CO2(g) Overall reaction: NO2(g) + CO(g) → NO(g) + CO2(g) Reaction Mechanisms A series of elementary steps that must satisfy two requirements: 1) The sum of the elementary steps must give the overall balanced equation for the reaction 2) The mechanism must agree with the experimentally determined rate law. Reaction: NO2(g) + CO(g) →NO(g) + CO2(g) NO2(g) + NO2(g) →NO3(g) + NO(g) IF Slow NO3 + CO(g) →NO2(g) + CO2(g) Fast Assuming formation of NO3 occurs more slowly than its reaction with NO, the rate of CO2 production is controlled by the rate of formation of NO3 in this first slow step. Reaction: NO2(g) + CO(g) →NO(g) + CO2(g) NO2(g) + NO2(g) →NO3(g) + NO(g) IF Slow NO3 + CO(g) →NO2(g) + CO2(g) Fast For elementary steps the rate law can be written from the molecularity, so 2 Rate = Δ[NO3] = k[NO2] Δt Reaction: NO2(g) + CO(g) →NO(g) + CO2(g) NO2(g) + NO2(g) →NO3(g) + NO(g) IF Slow NO3 + CO(g) →NO2(g) + CO2(g) Fast Overall reaction cannot go faster than the slowest step so 2 overall rate = k1[NO2] Agrees with experimentally determined rate law. Reaction Mechanisms A series of elementary steps that must satisfy two requirements: 1) The sum of the elementary steps must give the overall balanced equation for the reaction 2) The mechanism must agree with the experimentally determined rate law. Reaction:NO2(g) + CO(g) →NO(g) + CO2(g) NO2(g) + NO2(g) →NO3(g) + NO(g) IF Slow NO3 + CO(g) →NO2(g) + CO2(g) Fast The mechanism satisfies the 2 requirements and may be the correct mechanism for the reaction. Slow step is called the “rate determining step” Homogeneous Catalysis - related to mechanism Many examples in liquid and gas phase reactions Atmospheric Chemistry In lower atmosphere, anytime high temperature combustion goes on we can get N2(g) + O2(g) → 2NO (g) Homogeneous Catalysis Then 2NO(g) + O2(g) → 2NO2(g) light NO2(g) → NO(g) + O(g) and then O(g) + O2(g) → O3(g) Ozone is a powerful oxidizing agent - reacts with air pollutants forming irritants to lungs and eyes, as well as being toxic itself. Homogeneous Catalysis Clarifying how NO is a true catalyst here: NO(g) + ½ O2(g) → NO2(g) light NO2(g) → NO(g) + O(g) O(g) + O2(g) → O3(g) 3/2 O2(g) → O3(g) In the upper atmosphere, NO, leads to depletion of ozone NO(g) + O3(g) → NO2(g) + O2(g) O(g) + NO2(g) → NO(g) + O2(g) O(g) + O3(g) → 2 O2(g) NO is again catalytic here. In the upper atmosphere this is a problem because In the upper atmosphere, NO, leads to depletion of ozone In the upper atmosphere this is a problem because ozone absorbs ultraviolet light and is necessary to protect those on Earth from the harmful effects of this high energy radiation. Look at multiple choice questions you’ve highlighted When free Cl(g) atoms encounter O3(g) molecules in the upper atmosphere, the following reaction mechanism is proposed to occur. Cl(g) + O3(g) → ClO(g) + O2(g) slow step ClO(g) + O3(g) → Cl(g) + 2 O2(g) fast step ________________________________________________ 2 O3(g) → 3 O2(g) overall reaction ΔH = −285 kJ/molrxn 15.Which of the following rate laws for the overall reaction corresponds to the proposed mechanism? Cl(g) + O3(g) → ClO(g) + O2(g) slow step ClO(g) + O3(g) → Cl(g) + 2 O2(g) fast step ________________________________________________ 2 O3(g) → 3 O2(g) overall reaction ΔH = −285 kJ/molrxn 15.Which of the following rate laws for the overall reaction corresponds to the proposed mechanism? Cl(g) + O3(g) → ClO(g) + O2(g) slow step RDS ClO(g) + O3(g) → Cl(g) + 2 O2(g) fast step ________________________________________________ 2 O3(g) → 3 O2(g) overall reaction ΔH = −285 kJ/molrxn Molecularity of RDS gives (B) Rate = k[Cl][O3] 16. Which of the following reaction energy profiles best corresponds to the proposed mechanism? Cl(g) + O3(g) → ClO(g) + O2(g) slow step RDS ClO(g) + O3(g) → Cl(g) + 2 O2(g) fast step ________________________________________________ 2 O3(g) → 3 O2(g) overall reaction ΔH = −285 kJ/molrxn What are we looking for? 16. Which of the following reaction energy profiles best corresponds to the proposed mechanism? −285 kJ/molrxn −285 kJ/molrxn 16. Which of the following reaction energy profiles best corresponds to the proposed mechanism? 2 intermediates, 1st one with highest Ea −285 kJ/molrxn 17. The proposed mechanism can be written in a more general form, as shown above. Species other than Cl can also decompose O3 through the same mechanism. Which of the following chemical species is most likely to decompose O3 in the upper atmosphere through the above mechanism? X + O3 → XO + O2 XO+O3 → X + 2O2 _____________________________ 2 O3 → 3 O2 17. The proposed mechanism can be written in a more general form, as shown above. Species other than Cl can also decompose O3 through the same mechanism. Which of the following chemical species is most likely to decompose O3 in the upper atmosphere through the above mechanism? X + O3 → XO + O2 XO+O3 → X + 2O2 b) Br ______________________ Same grp as Cl 2 O3 → 3 O2 likely similar chemistry 18. 2 NO2(g) + F2(g) → 2 NO2,F(g) The rate law for the reaction represented by the equation above is rate = k[NO2][F2]. Which of the following could be the first elementary step of a two-step mechanism for the reaction if the first step is slow and the second step is fast? Rate determining step gives us molecularity of rate law - bimolecular step 18. 2 NO2(g) + F2(g) → 2 NO2,F(g) The rate law for the reaction represented by the equation above is rate = k[NO2][F2]. Which of the following could be the first elementary step of a two-step mechanism for the reaction if the first step is slow and the second step is fast? NO2(g) + F2(g) → NO2F + F slow step 2 NO2(g) + CO(g) → NO (g) + CO2(g) 19. The reaction between NO2(g) and CO(g) is represented above. The elementary steps of a proposed reaction mechanism are represented below. Step 1: 2 NO2(g) → NO(g) + NO3(g) (slow) Step 2: NO3(g) + CO(g) → NO2(g) + CO2(g) (fast) Which of the following is the rate law for the overall reaction that is consistent with the proposed mechanism? 2 NO2(g) + CO(g) → NO (g) + CO2(g) 19. The reaction between NO2(g) and CO(g) is represented above. The elementary steps of a proposed reaction mechanism are represented below. Step 1: 2 NO2(g) → NO(g) + NO3(g) (slow) Step 2: NO3(g) + CO(g) → NO2(g) + CO2(g) (fast) 2 (B) Rate = k [NO2] 2 NO(g) + O2(g) → 2 NO2(g) 24. Consider the following mechanism for the reaction represented above. ⇄ Step 1. 2NO N2O2 (fast, reversible) Step 2. N2O2 + O2 → 2 NO2 (slow) Which of the following statements is true? (A) Step 1 represents a unimolecular reaction. (B) Increasing the concentration of NO will decrease the overall rate of the reaction. 2 NO(g) + O2(g) → 2 NO2(g) 24. Consider the following mechanism for the reaction represented above. ⇄ Step 1. 2NO N2O2 (fast, reversible) Step 2. N2O2 + O2 → 2 NO2 (slow) Which of the following statements is true? (A) Step 1 represents a unimolecular reaction. (B) Increasing the concentration of NO will decrease the overall rate of the reaction. 2 NO(g) + O2(g) → 2 NO2(g) 24. Consider the following mechanism for the reaction represented above. ⇄ Step 1. 2NO N2O2 (fast, reversible) Step 2. N2O2 + O2 → 2 NO2 (slow) Which of the following statements is true? (C) Raising the temperature will have no effect on the numerical value of the rate constant. (D) The rate law that is consistent with the mechanism is rate 2 = k[NO] [O2] 2 NO(g) + O2(g) → 2 NO2(g) 24. Consider the following mechanism for the reaction represented above. What can we learn about mechanisms from this question then? ⇄ Step 1.
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