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Reaction Mechanism and Rate Law

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When a reaction has a mechanism, the overall rate law is set by the rate-determining step — the slowest elementary step, the bottleneck. Its molecularity gives the rate law's orders. If that step involves an intermediate, the intermediate must be re-expressed using an earlier fast step so the rate law is in terms of measurable reactants.

UNIT 5 TOPIC 5.8 • REACTION MECHANISM AND RATE LAW RATE-LIMITING STEP THE MECHANISM (two elementary steps) Step 1 fast equilibrium NO + Br₂ ⇌ NOBr₂ Step 2 slow • rate-determining NOBr₂ + NO → 2 NOBr Overall (add the steps, cancel intermediate): 2 NO + Br₂ → 2 NOBr NOBr₂ cancels — it is an intermediate, not a reactant or product. RATE = SLOW STEP The rate-limiting step sets the overall rate law: rate = k[NOBr₂][NO] Problem: NOBr₂ is an intermediate. A valid rate law can contain only reactants (and catalysts) — never an intermediate. Fix: use Step 1's fast equilibrium to replace [NOBr₂]. SUBSTITUTE THE INTERMEDIATE Fast equilibrium → forward rate = reverse rate: k₁[NO][Br₂] = k₋₁[NOBr₂] → [NOBr₂] = (k₁/k₋₁)[NO][Br₂] Put this into rate = k[NOBr₂][NO]: rate = k′[NO]²[Br₂] (k′ = k·k₁/k₋₁) CED ANCHOR Rate law from mechanism Do NOT read the rate law off the overall coefficients — unless the reaction is elementary. Overall is 2 NO + Br₂ → 2 NOBr, yet the rate law is fixed by the slow step + equilibrium, then checked against experiment. AP Chemistry · Unit 5 · Kinetics
The slowest elementary step — the rate-determining step — sets the overall rate law. From a mechanism, the rate law comes from that slow step's reactants, not from the overall balanced equation, and intermediates must be expressed via prior fast steps.

The mistakes come from taking the rate law from the overall equation instead of the slow step, or mishandling intermediates. The bottleneck step governs the rate; everything else is either fast or must be substituted out.

The work

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Lesson
Mechanism & Rate Law ›

The rate-determining (slowest) step sets the rate law, with intermediates substituted via fast steps. The lesson derives the rate law from a mechanism, 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 5.8 misconceptions: rate law taken from the overall equation, orders copied from coefficients, and intermediates in the rate law mishandled.

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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