5.4 Elementary Reactions

What makes a reaction elementary

An elementary reaction is a single step that occurs exactly as written, representing one molecular collision or event. Unlike overall reactions, its rate law can be derived directly from stoichiometry using the molecularity — the number of reactant particles colliding. Most reactions occur as a series of elementary steps, called a reaction mechanism.

Molecularity and rate law

Unimolecular steps (one molecule) are first order; bimolecular steps (two molecules) are second order overall (first order in each reactant); termolecular steps (three molecules) are rare due to low collision probability. For an elementary step, the rate law exponents equal the coefficients in that step — this only applies to elementary steps, not overall reactions.

Multistep mechanisms

A mechanism's steps must sum to the overall balanced equation. Species produced in one step and consumed in another are intermediates, not seen in the overall equation. The mechanism must be consistent with the experimentally determined rate law for the overall reaction to be considered valid.

Key terms

Elementary reaction

A single-step reaction occurring exactly as written, representing one molecular collision or event.

Molecularity

The number of reactant particles involved in an elementary step (unimolecular, bimolecular, termolecular).

Rate law for elementary steps

Exponents equal the coefficients of reactants in that step (only true for elementary steps).

Intermediate

A species produced in one mechanism step and consumed in another; does not appear in the overall equation.

Valid mechanism criteria

Steps must sum to the overall balanced equation, and the mechanism must agree with the experimental rate law.

Try an AP-style question

2NO₂ → N₂O₄ is an elementary step. What is its rate law?

1. rate = k[N₂O₄]
2. It can't be determined
3. rate = k[NO₂]
4. rate = k[NO₂]²