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AP Chemistry Practice Quiz: Pre-Equilibrium Approximation

Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026

Test your understanding with short quizzes. This quiz has 7 questions to check your progress.

Question 1 of 7

According to the provided information, the pre-equilibrium approximation is a method used primarily to achieve which of the following goals in chemical kinetics?

All Questions (7)

According to the provided information, the pre-equilibrium approximation is a method used primarily to achieve which of the following goals in chemical kinetics?

A) To calculate the activation energy of the rate-limiting step.

B) To determine the overall enthalpy change of a reaction.

C) To identify the rate law for a reaction from its mechanism.

D) To prove that the first step of a mechanism is always rate-limiting.

Correct Answer: C

The provided content explicitly states that the goal is to 'Identify the rate law for a reaction from a mechanism in which the first step is not rate limiting.' This directly corresponds to option C.

Under which specific condition is it necessary to use an approximation, such as the pre-equilibrium approximation, to determine a rate law from a reaction mechanism?

A) When the first elementary step is the rate-limiting step.

B) When the overall reaction is at equilibrium.

C) When the first elementary step is not the rate-limiting step.

D) When the reaction involves only a single elementary step.

Correct Answer: C

The text states, 'If the first elementary step is not rate-limiting, approximations (such as pre-equilibrium) are needed to determine the rate law expression.' This directly indicates that the condition requiring this approximation is when the first step is not the slow, rate-determining step.

A proposed two-step reaction mechanism involves a fast, reversible first step followed by a slow second step. Which statement accurately describes the process of determining the overall rate law for this mechanism?

A) The rate law can be determined directly and solely from the stoichiometry of the slow second step.

B) An approximation method is required because the initial step is not rate-limiting.

C) The rate law is determined by the stoichiometry of the fast, reversible first step.

D) No rate law can be determined for this type of mechanism.

Correct Answer: B

The scenario describes a mechanism where the first step is not rate-limiting (the second step is). According to the provided content, this exact situation requires the use of approximations like pre-equilibrium to determine the rate law expression.

The use of the pre-equilibrium approximation is specifically intended for reaction mechanisms where:

A) the rate-limiting step is the very first step.

B) all elementary steps proceed at identical rates.

C) the determination of the rate law is complicated by a non-rate-limiting first step.

D) the reaction produces gaseous products.

Correct Answer: C

The provided text establishes a clear link between a non-rate-limiting first step and the need for approximations. This implies that such a step complicates the direct determination of the rate law, which is the problem the pre-equilibrium approximation is designed to solve.

Why is it not always possible to write the rate law for a multi-step reaction based solely on the molecularity of the designated slow step?

A) Because the slow step may be reversible.

B) Because the temperature of the reaction may change.

C) Because if the slow step is not the first step, an approximation is needed to formulate the final rate law expression.

D) Because the rate law is always determined by the overall balanced chemical equation, not any single step.

Correct Answer: C

The provided content states that if the first step is not rate-limiting, an approximation is needed to find the rate law. This implies that simply using the slow step is insufficient in this case, which is the core of the problem addressed by the pre-equilibrium approximation.

A key principle for applying the pre-equilibrium approximation is that the rate of the overall reaction is governed by the slow step, but the final rate law expression is derived using an approximation. This apparent contradiction is resolved by understanding that the approximation is needed when:

A) the slow step occurs after an initial, non-rate-limiting step.

B) the slow step is the only step in the mechanism.

C) the reaction has a very high activation energy.

D) the reactants are in the solid phase.

Correct Answer: A

The content explicitly states that approximations are needed 'If the first elementary step is not rate-limiting'. This directly supports the idea that the approximation is used when a slow step is preceded by a faster, initial step.

An experimentally determined rate law for a reaction is found to be second order with respect to reactant A. A proposed mechanism has a slow, rate-determining step that is first order in A and first order in an intermediate, B. For this mechanism to be valid, what must be true about the step that produces intermediate B?

A) The step producing B must be the rate-limiting step.

B) The step producing B must be a non-rate-limiting first step, requiring an approximation to relate B to A.

C) The step producing B must be irreversible and very slow.

D) The mechanism is invalid because the molecularity of the slow step does not match the overall order.

Correct Answer: B

This question requires synthesizing the information. If the slow step involves an intermediate, its concentration must be expressed in terms of reactants to get the final rate law. The provided text states that approximations are used to find the rate law when the first step is not rate-limiting. This implies that the step producing the intermediate (B) is a fast, non-rate-limiting first step, and the pre-equilibrium approximation would be used to express [B] in terms of [A], potentially leading to the observed second-order dependence on A.