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Ch.14 - Chemical Kinetics
All textbooksBrown 15th EditionCh.14 - Chemical KineticsProblem 73a
Chapter 14, Problem 73a

The reaction 2 NO1g2 + Cl21g2¡2 NOCl1g2 was performed and the following data were obtained under conditions of constant 3Cl24:
Graph showing the linear relationship of ln[H2] over time in chemical kinetics.
(a) Is the following mechanism consistent with the data? NO1g2 + Cl21g2ΔNOCl21g2 1fast2 NOCl21g2 + NO1g2¡2 NOCl1g2 1slow2

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1
Step 1: Identify the given reaction and proposed mechanism. The reaction is 2 NO(g) + Cl2(g) -> 2 NOCl(g). The proposed mechanism is: Step 1: NO(g) + Cl2(g) -> NOCl2(g) (fast), Step 2: NOCl2(g) + NO(g) -> 2 NOCl(g) (slow).
Step 2: Analyze the graph provided. The graph shows a linear relationship between ln[H2] and time, indicating a first-order reaction with respect to H2.
Step 3: Determine the rate-determining step (RDS) in the proposed mechanism. The slow step (Step 2) is the RDS: NOCl2(g) + NO(g) -> 2 NOCl(g).
Step 4: Write the rate law for the RDS. Since the RDS involves NOCl2 and NO, the rate law is: rate = k[NOCl2][NO].
Step 5: Relate the intermediate concentration to the reactants. From the fast equilibrium step (Step 1), NOCl2 can be expressed in terms of NO and Cl2: [NOCl2] = K[NO][Cl2]. Substitute this into the rate law to get: rate = k' [NO]^2 [Cl2].

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Reaction Mechanism

A reaction mechanism is a step-by-step description of the pathway by which reactants are converted into products. It includes elementary steps, which can be classified as either fast or slow, and helps to explain the overall rate of the reaction. Understanding the mechanism is crucial for determining if it aligns with experimental data, such as reaction rates and concentration changes.
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Rate Law

The rate law expresses the relationship between the rate of a chemical reaction and the concentration of its reactants. It is determined experimentally and can be derived from the reaction mechanism. For the given reaction, the rate law will help assess whether the proposed mechanism is consistent with the observed data, particularly the linear relationship shown in the graph.
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Integrated Rate Laws

Integrated rate laws relate the concentration of reactants or products to time, allowing for the analysis of concentration changes over the course of a reaction. The graph provided indicates a linear relationship of ln[H2] over time, suggesting a first-order reaction. This information is essential for evaluating the proposed mechanism and determining if it accurately reflects the kinetics of the reaction.
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Related Practice
Textbook Question

The decomposition of hydrogen peroxide is catalyzed by iodide ion. The catalyzed reaction is thought to proceed by a two-step mechanism:

H2O2(aq) + I-(aq) → H2O(l) + IO-(aq) (slow)

IO-(aq) + H2O2(aq) → H2O(l) + O2(g) + I-(aq) (fast)

(a) Write the chemical equation for the overall process.

Textbook Question

The decomposition of hydrogen peroxide is catalyzed by iodide ion. The catalyzed reaction is thought to proceed by a two-step mechanism:

H2O2(aq) + I-(aq) → H2O(l) + IO-(aq) (slow)

IO-(aq) + H2O2(aq) → H2O(l) + O2(g) + I-(aq) (fast)

(b) Identify the intermediate, if any, in the mechanism.

Textbook Question

The decomposition of hydrogen peroxide is catalyzed by iodide ion. The catalyzed reaction is thought to proceed by a two-step mechanism:

H2O2(aq) + I-(aq) → H2O(l) + IO-(aq) (slow)

IO-(aq) + H2O2(aq) → H2O(l) + O2(g) + I-(aq) (fast)

(c) Assuming that the first step of the mechanism is rate determining, predict the rate law for the overall process.

Textbook Question

You have studied the gas-phase oxidation of HBr by O2: 4 HBr(g) + O2(g) → 2 H2O(g) + 2 Br2(g)

You find the reaction to be first order with respect to HBr and first order with respect to O2. You propose the following mechanism:

HBr(g) + O2(g) → HOOBr(g)

HOOBr(g) + HBr(g) → 2 HOBr(g)

HOBr(g) + HBr(g) → H2O(g) + Br2(g)

(a) Confirm that the elementary reactions add to give the overall reaction.

Textbook Question

You have studied the gas-phase oxidation of HBr by O2: 4 HBr(g) + O2(g) → 2 H2O(g) + 2 Br2(g)

You find the reaction to be first order with respect to HBr and first order with respect to O2. You propose the following mechanism:

HBr(g) + O2(g) → HOOBr(g)

HOOBr(g) + HBr(g) → 2 HOBr(g)

HOBr(g) + HBr(g) → H2O(g) + Br2(g)

(b) Based on the experimentally determined rate law, which step is rate determining?

Textbook Question

(a) What is a catalyst? (b) What is the difference between a homogeneous and a heterogeneous catalyst?