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Elementary Reactions

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An elementary reaction is a single molecular event — one step, exactly as written. Its molecularity counts the particles involved: unimolecular (one), bimolecular (two colliding), or rarely termolecular (three). For an elementary step only, the rate law can be written directly from the molecularity.

UNIT 5 TOPIC 5.4 • ELEMENTARY REACTIONS ELEMENTARY STEPS An elementary reaction occurs in a single step, so its molecularity gives its rate law directly. UNIMOLECULAR BIMOLECULAR TERMOLECULAR A A → products rate = k[A] one molecule reacts on its own A B A + B → products rate = k[A][B] or 2A → products, rate = k[A]2 A B C new A + B + C → products rate = k[A][B][C] three-body collision — rare CED ANCHOR · reading a rate law from molecularity You may read the rate law directly from the molecularity ONLY for an elementary step. NEVER do this for an overall (non-elementary) reaction — its rate law must be found by experiment. Molecularity (uni / bi / ter) counts the particles that collide in that single step. AP Chemistry · Unit 5 · Kinetics
An elementary reaction is a single molecular event, classified by molecularity: unimolecular (one particle), bimolecular (two colliding), or the rare termolecular (three). For an elementary step only, the rate law comes directly from the molecularity — the reactants and their counts.
Molecularity Theater · Open the sandbox →

The traps: treating an overall (multistep) equation as if it were elementary, and forgetting that molecularity is a whole-number count of particles in a single step. Only for a genuine elementary step do the coefficients become the rate-law orders.

The work

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Lesson
Elementary Reactions ›

An elementary step is one molecular event whose molecularity gives its rate law directly — unlike an overall equation. The lesson tells them apart, 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 5.4 misconceptions: an overall equation treated as elementary, molecularity mistaken for something other than a whole-number particle count, and orders taken from a non-elementary equation.

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
Diagnostic first