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Ch.15 - Chemical Kinetics
Tro - Chemistry: A Molecular Approach 6th Edition
Tro6th EditionChemistry: A Molecular ApproachISBN: 9780137832217Not the one you use?Change textbook
All textbooksTro 6th EditionCh.15 - Chemical KineticsProblem 83c
Chapter 15, Problem 83c

Consider this three-step mechanism for a reaction:
Cl2 (g) k1⇌k2 2 Cl (g) Fast
Cl (g) + CHCl3 (g) →k3 HCl (g) + CCl3 (g) Slow
Cl (g) + CCl3 (g) →k4 CCl4 (g) Fast
c. What is the predicted rate law?

Verified step by step guidance
1
Identify the slow step in the mechanism, as it is the rate-determining step. In this case, the slow step is: Cl (g) + CHCl_3 (g) → HCl (g) + CCl_3 (g).
Write the rate law based on the slow step. The rate law is determined by the reactants involved in this step: rate = k_3[Cl][CHCl_3].
Since Cl is an intermediate, express its concentration in terms of the reactants and products of the fast equilibrium step: Cl_2 (g) ⇌ 2 Cl (g).
Use the equilibrium expression for the fast step to express [Cl] in terms of [Cl_2]: K_eq = [Cl]^2 / [Cl_2], where K_eq is the equilibrium constant for the fast step.
Substitute the expression for [Cl] from the equilibrium step into the rate law from the slow step to obtain the overall rate law in terms of the initial reactants.

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 how a chemical reaction occurs at the molecular level. It outlines the individual elementary steps that lead to the overall reaction, including the formation and consumption of intermediates. Understanding the mechanism is crucial for predicting the rate law, as it reveals which steps are rate-determining and how reactants are transformed into products.
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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 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. The rate law can be derived from the slowest step in the reaction mechanism, which dictates the overall reaction rate.
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Elementary Steps

Elementary steps are the individual reactions that occur in a reaction mechanism. Each step represents a single molecular event, such as the collision of molecules or the breaking/forming of bonds. The rate of the overall reaction is influenced by the slowest elementary step, known as the rate-determining step, which controls how quickly the products are formed from the reactants.
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Related Practice
Textbook Question

Which of these two reactions would you expect to have the smaller orientation factor? Explain. a. O(g) + N2(g) → NO( g) + N(g) b. NO(g) + Cl2(g) → NOCl( g) + Cl(g)

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Textbook Question

Consider this three-step mechanism for a reaction:

Cl2 (g) k1⇌k2 2 Cl (g) Fast

Cl (g) + CHCl3 (g) →k3 HCl (g) + CCl3 (g) Slow

Cl (g) + CCl3 (g) →k4 CCl4 (g) Fast

a. What is the overall reaction?

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Textbook Question

Many heterogeneous catalysts are deposited on high-surfacearea supports. Why?

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Textbook Question

Suppose that the reaction A¡products is exothermic and has an activation barrier of 75 kJ/mol. Sketch an energy diagram showing the energy of the reaction as a function of the progress of the reaction. Draw a second energy curve showing the effect of a catalyst.

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