5.7 Introduction to Reaction Mechanisms

Mechanisms are step-by-step pathways

A reaction mechanism is a series of elementary steps that describe how a reaction actually occurs at the molecular level. Each elementary step shows a single molecular event (collision or breakup). The sum of all elementary steps must add up to the overall balanced equation, and species that appear in steps but cancel out overall are intermediates.

Molecularity and rate laws of elementary steps

For an elementary step (only), the rate law can be written directly from its stoichiometry: a unimolecular step is first order, a bimolecular step is second order overall (first order in each reactant). This is different from the overall reaction, whose rate law must be determined experimentally and reflects the mechanism's rate-determining step (slow step).

Validating a mechanism

A proposed mechanism is considered plausible if (1) the elementary steps sum to the overall equation and (2) the rate law derived from the slow step (using intermediates from fast pre-equilibrium steps if needed) matches the experimentally determined rate law. If it doesn't match, the mechanism must be rejected or revised.

Key terms

Reaction mechanism

A sequence of elementary steps that describes the molecular pathway of an overall reaction.

Elementary step

A single molecular event (e.g., collision or decomposition) in a mechanism, whose rate law can be written from its stoichiometry.

Intermediate

A species produced in one step and consumed in a later step; does not appear in the overall balanced equation.

Rate-determining step

The slowest elementary step in a mechanism; it controls the overall reaction rate and rate law.

Validating a mechanism

Steps must sum to the overall equation, and the rate law derived from the mechanism must match the experimentally determined rate law.

Try an AP-style question

Which species is an intermediate?

1. NO₂
2. NO₃
3. CO₂
4. CO