E

Elementary Step

Rate = k[A]^m[B]^n; molecularity = m+n for elementary step
Quick Reference
Formula / NotationRate = k[A]^m[B]^n; molecularity = m+n for elementary step
Also Known AsElementary reaction, mechanistic step, rate-determining step (specific type)

What is Elementary Step?

An individual step in a reaction mechanism that occurs in a single event at the molecular level. Elementary steps involve one, two, or rarely three molecules colliding simultaneously. The molecularity of an elementary step determines its rate law directly from stoichiometry.

Formula & Notation

Rate = k[A]^m[B]^n; molecularity = m+n for elementary step

Other Names / Synonyms: Elementary reaction, mechanistic step, rate-determining step (specific type)

Properties & Characteristics

An elementary step is a single molecular event in a reaction mechanism that cannot be broken down into simpler steps. Elementary steps are characterised by their molecularity: unimolecular (A → products), bimolecular (A + B → products), or termolecular (A + B + C → products, rare). Unlike overall rate laws, the rate law for an elementary step can be directly written from its stoichiometry. The rate-determining step (slowest elementary step) governs the overall rate.

Uses & Applications

Elementary steps are the building blocks of reaction mechanisms in kinetics. Understanding mechanisms at the level of elementary steps allows chemists to design better catalysts, predict selectivity, and propose experimental tests (isotopic labelling, stereochemistry probes).

Safety Information

Some elementary steps generate highly reactive, toxic, or energetic intermediates (radicals, carbenes, oxene). Understanding these helps design reactions that suppress hazardous intermediates or quickly convert them to stable products.

Always consult the SDS/MSDS before handling any chemical. This information is for educational purposes only.

Key Facts

Term Elementary Step
Formula Rate = k[A]^m[B]^n; molecularity = m+n for elementary step
Synonyms Elementary reaction, mechanistic step, rate-determining step (specific type)

Frequently Asked Questions

An individual step in a reaction mechanism that occurs in a single event at the molecular level. Elementary steps involve one, two, or rarely three molecules colliding simultaneously. The molecularity of an elementary step determines its rate law directly from stoichiometry.

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