Rate-Determining Steps

Most chemical reactions proceed in a multistep reaction mechanism. But how is a reaction mechanism determined?

Reaction mechanisms are hypothesized based on their balanced chemical equations and experimentally determined rate laws of each elementary step.

Each step has a specific reaction rate, rate constant, and activation energy. The slowest step is called the rate-determining step and influences the net reaction rate. It can be used to verify the rate law for the overall reaction and to validate a proposed reaction mechanism.

Consider the decomposition of nitrous oxide to nitrogen and oxygen. The experimentally determined rate law does not correspond to the rate expression of a single-step reaction, which is corroborated by the observed presence of oxygen atoms—a reaction intermediate.

Hence, a reaction mechanism is proposed where all steps cumulate to give the overall reaction.

First, the rate constants indicate that the first step is the rate-limiting step. It is the slowest, and thus influences the overall reaction rate. A rate law proposed from this step can be set equal to the overall rate law.

This proposed rate-law, directly derived from the molecular concentration of the