AP Chemistry Flashcards: Pre-Equilibrium Approximation
Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026
Review key ideas with interactive flashcards. This set includes 10 cards to help you master important concepts.
What fundamental challenge does a fast initial step present when trying to write a rate law for a reaction mechanism?
A fast initial step often produces an intermediate whose concentration cannot appear in the final rate law, requiring an approximation to solve for it.
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What fundamental challenge does a fast initial step present when trying to write a rate law for a reaction mechanism?
A fast initial step often produces an intermediate whose concentration cannot appear in the final rate law, requiring an approximation to solve for it.
Define the scenario that necessitates using the pre-equilibrium approximation.
This approximation is used for a multi-step reaction mechanism in which a fast, reversible equilibrium step precedes the slow, rate-determining step.
Why can't the rate law be determined from the slow step alone if it's not the first step?
Because the rate law for the slow step will contain an intermediate species, an approximation is needed to express this intermediate's concentration in terms of reactants.
What is the purpose of using an approximation, like pre-equilibrium, in chemical kinetics?
Its purpose is to allow for the determination of the rate law expression for a reaction mechanism where the initial step is fast, not rate-limiting.
A proposed two-step mechanism has a fast, reversible first step and a slow second step. What general method must be used to derive the overall rate law?
An approximation, such as the pre-equilibrium approximation, must be used because the first step is not the rate-limiting step.
A student attempts to write a rate law based on the first step of a mechanism, but it doesn't match the experimental data. What is a likely reason related to the reaction steps?
It is likely that the first step is not rate-limiting, and therefore an approximation is needed to correctly derive the rate law.
What is the primary goal when analyzing a mechanism where the first step is not the slow step?
The primary goal is to identify the correct overall rate law by using an approximation to relate the concentration of intermediates to the reactants.
Under what condition is the pre-equilibrium approximation necessary to determine a reaction's rate law from its mechanism?
The pre-equilibrium approximation is needed when the first elementary step in the reaction mechanism is not the rate-limiting (slow) step.
Term: Non-rate-limiting first step
This refers to a scenario in a multi-step reaction mechanism where the initial step proceeds much faster than a subsequent step.
If the first step of a mechanism is rate-limiting, is the pre-equilibrium approximation needed?
No, if the first step is rate-limiting, the rate law can be determined directly from that step without the need for an approximation.