A balanced equation gives the reactants, products, and stoichiometry but does not provide any detail on how the overall process is accomplished. As a result, the rate of the reaction cannot be predicted from the balanced equation alone.

A reaction mechanism is a sequence of elementary steps or reactions that add up to give the balanced equation. These can include chemical intermediates which are formed and consumed in the elementary steps and so never appear in the overall balanced equation. An elementary step is not made up of simpler steps.

The following example involves two elementary steps and a balanced or overall equation. Note that even though C does not appear in the overall equation, its role is critical: