Textbook Question
Consider the following energy profile.
(a) How many elementary reactions are in the reaction mechanism?
Ch.14 - Chemical Kinetics
Chapter 14, Problem 71
The following mechanism has been proposed for the gas-phase reaction of H2 with ICl: H2(g) + ICl(g) → HI(g) + HCl(g), HI(g) + ICl(g) → I2(g) + HCl(g). (c) If the first step is slow and the second one is fast, which rate law do you expect to be observed for the overall reaction?
Verified step by step guidance1
Identify the elementary steps in the proposed mechanism: Step 1: H2(g) + ICl(g) → HI(g) + HCl(g) (slow), Step 2: HI(g) + ICl(g) → I2(g) + HCl(g) (fast).
Recognize that the rate-determining step is the slowest step in the mechanism, which is Step 1 in this case.
Write the rate law based on the rate-determining step. For an elementary reaction, the rate law is determined by the stoichiometry of the reactants in that step. Therefore, the rate law for Step 1 is: rate = k[H2][ICl].
Since Step 1 is the rate-determining step, the overall rate law for the reaction is the same as the rate law for Step 1.
Conclude that the observed rate law for the overall reaction is rate = k[H2][ICl], where k is the rate constant for the slow step.
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Rate Law
The rate law of a reaction expresses the relationship between the rate of a chemical reaction and the concentration of its reactants. It is typically formulated as rate = k[A]^m[B]^n, where k is the rate constant, and m and n are the orders of the reaction with respect to reactants A and B. Understanding the rate law is crucial for predicting how changes in concentration affect the reaction rate.
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Rate Law Fundamentals
Elementary Steps
Elementary steps are the individual reactions that make up a complex reaction mechanism. Each step represents a single molecular event, and the rate of the overall reaction is determined by the slowest step, known as the rate-determining step. In this case, since the first step is slow, it will dictate the rate law for the overall reaction.
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Mechanism of Reaction
The mechanism of a reaction describes the sequence of elementary steps that lead to the overall transformation of reactants into products. It provides insight into how reactants interact at the molecular level. In the given reaction, understanding the mechanism helps in determining which step influences the rate law, particularly when one step is significantly slower than the others.
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Related Practice
Textbook Question
Consider the following energy profile.
(b) How many intermediates are formed in the reaction?
Textbook Question
Consider the following energy profile.
(c) Which step is rate limiting?
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.