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AP Chemistry · Unit 5: Kinetics

5.8 Reaction Mechanism and Rate Law

Mechanisms are made of elementary steps

A reaction mechanism is a series of elementary steps that add up to the overall reaction. Each elementary step's rate law can be written directly from its stoichiometric coefficients (unlike the overall reaction). The molecularity of a step (unimolecular, bimolecular) describes how many particles collide in that step.

The rate-determining step controls the rate

The slowest elementary step is the rate-determining step (RDS), and it limits the overall reaction rate. The rate law of the overall reaction must match the rate law derived from the RDS. If the RDS comes after a fast, reversible step, an intermediate in the RDS rate law must be replaced using the fast equilibrium step.

Validating a proposed mechanism

A mechanism is considered plausible if: (1) the elementary steps sum to the overall balanced equation, and (2) the rate law predicted from the RDS matches the experimentally determined rate law. Any species that appears as a reactant in one step and product in an earlier step is a reaction intermediate, not part of the final rate law.

Key terms

Elementary step
A single molecular event in a mechanism whose rate law can be written directly from its coefficients.
Rate-determining step (RDS)
The slowest step in a mechanism; it controls the overall reaction rate and rate law.
Reaction intermediate
A species produced in one step and consumed in a later step; not present in the overall equation or final rate law.
Molecularity
The number of reactant particles involved in an elementary step (e.g., unimolecular = 1, bimolecular = 2).
Validating a mechanism
Steps must sum to the overall balanced equation, and the rate law from the RDS must match the experimental rate law.

Try an AP-style question

Data

Mechanism — Step 1 (slow): NO₂ + NO₂ → NO₃ + NO. Step 2 (fast): NO₃ + CO → NO₂ + CO₂.

What rate law does this mechanism predict?

  1. rate = k[NO₂]²
  2. rate = k[NO₃][CO]
  3. rate = k[CO]
  4. rate = k[NO₂][CO]
Show the answer

A. The slow first step controls the rate.

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