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Ch.14 - Chemical Kinetics
All textbooksBrown 15th EditionCh.14 - Chemical KineticsProblem 89b
Chapter 14, Problem 89b

Consider the reaction A + B → C + D. Is each of the following statements true or false? (b) If the reaction is an elementary reaction, the rate law is second order.

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Identify the type of reaction given. In this case, it is an elementary reaction involving reactants A and B.
Understand that for elementary reactions, the rate law can be directly written from the stoichiometry of the reaction as it appears in the balanced equation.
Recognize that each reactant in an elementary reaction contributes to the rate law according to its stoichiometric coefficient in the balanced equation.
Since the reaction is A + B → C + D, both A and B have a stoichiometric coefficient of 1.
Write the rate law based on the stoichiometry: Rate = k[A][B]. This indicates that the reaction is second order (first order in A and first order in B).

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Key Concepts

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

Elementary Reactions

An elementary reaction is a single step process in which reactants are converted to products in a single transition state. The rate of an elementary reaction is directly related to the molecularity, which is the number of reactant molecules involved. For example, a reaction involving two reactant molecules is bimolecular and typically has a rate law that reflects this molecularity.
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Rate Law

The rate law of a chemical reaction expresses the relationship between the rate of the reaction and the concentration of its reactants. It is determined experimentally and can vary depending on the reaction mechanism. For elementary reactions, the rate law can be directly derived from the stoichiometry of the reaction, meaning that the exponents in the rate law correspond to the coefficients in the balanced equation.
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Order of Reaction

The order of a reaction is the sum of the powers of the concentration terms in the rate law. It indicates how the rate of reaction is affected by the concentration of reactants. A second-order reaction can arise from either a single elementary reaction involving two reactant molecules or from two first-order reactions, thus the statement in the question can be true if the reaction is indeed second order.
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Related Practice
Textbook Question

The activation energy of an uncatalyzed reaction is 95 kJ/mol. The addition of a catalyst lowers the activation energy to 55 kJ/mol. Assuming that the collision factor remains the same, by what factor will the catalyst increase the rate of the reaction at (a) 25 C

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Textbook Question

The activation energy of an uncatalyzed reaction is 95 kJ/mol. The addition of a catalyst lowers the activation energy to 55 kJ/mol. Assuming that the collision factor remains the same, by what factor will the catalyst increase the rate of the reaction at (b) 125 °C?

Textbook Question

Consider the reaction A + B → C + D. Is each of the following statements true or false? (c) If the reaction is an elementary reaction, the rate law of the reverse reaction is first order.

Textbook Question

The reaction 2 NO(g) + O2(g) → 2 NO2 (g) is second order in NO and first order in O2. When [NO] = 0.040 M, and [O2] = 0.035 M, the observed rate of disappearance of NO is 9.3⨉10-5 M/s. (b) What is the value of the rate constant?

Textbook Question

The reaction 2 NO(g) + O2(g) → 2 NO2 (g) is second order in NO and first order in O2. When [NO] = 0.040 M, and [O2] = 0.035 M, the observed rate of disappearance of NO is 9.3⨉10-5 M/s. (c) What are the units of the rate constant?