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Pre-Equilibrium Approximation

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The pre-equilibrium approximation applies when a fast, reversible step comes before the slow rate-determining step. Because the fast step reaches equilibrium quickly, you can use its equilibrium relationship to express the intermediate in terms of measurable reactants, then substitute it into the slow step's rate law.

UNIT 5 TOPIC 5.9 • PRE-EQUILIBRIUM APPROXIMATION PRE-EQUILIBRIUM APPROXIMATION TWO-STEP MECHANISM A B I P + FAST EQUILIBRIUM A + B ⇌ I SLOW (rate-determining) I → P STEP 1 — FAST EQUILIBRIUM A + B ⇌ I K = [I] / ([A][B]) [I] = K [A][B] STEP 2 — SLOW (RDS) I → P The slow step sets the rate: rate = k₂ [I] SUBSTITUTE [I] rate = k₂ (K [A][B]) Combine constants k = k₂K : rate = k [A][B] THE INTERMEDIATE I • Forms in a fast, reversible step • Consumed in the slower, later step • Present in small concentration • Not measured directly — solved via K USE IT WHEN A fast, reversible first step reaches equilibrium BEFORE the slow, rate-determining step — so the pre-equilibrium fixes [I] and the rate law follows from K. KEY IDEA The intermediate is made quickly in tiny amounts, then consumed slowly. Its concentration is locked by the fast equilibrium ([I] = K[A][B]), letting us write the overall rate law in measurable [A] and [B]. AP Chemistry · Unit 5 · Kinetics
The pre-equilibrium approximation handles a fast reversible step before a slow one. Because the fast step reaches equilibrium quickly, its equilibrium expression lets you replace an intermediate in the slow step's rate law with measurable reactant concentrations.
Pre-Equilibrium · Open the sandbox →

The traps mirror mechanism rate laws: mishandling the intermediate substitution, or misapplying the fast equilibrium. Use the fast step's equilibrium to eliminate the intermediate, giving a rate law in reactant concentrations only.

The work

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Lesson
Pre-Equilibrium ›

A fast equilibrium before the slow step lets you substitute out an intermediate. The lesson applies the pre-equilibrium approximation to derive a rate law, then closes with a ten-scenario check.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items spanning the Topic 5.9 misconceptions: mishandling the intermediate substitution, orders copied from coefficients, and Eₐ/ΔH confusion in the mechanism's energetics.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions
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