Very few reactions occur as simply as the final balanced reaction implies. Most reactions occur through a series of simple steps or elementary reactions. An elementary reaction is a reaction which occurs in a single step and cannot be subdivided into simpler steps. The series of elementary reactions is known as the reaction mechanism. Reaction mechanisms can only be proven with experimental evidence. Reaction mechanisms describing each step of a reaction allow precise rate laws to be derived for any reaction.
In one or more of the elementary reactions, a product may be produced that does not appear in the final equation. The absence of this product from the final equation implies that this product must be completely used as a reactant in another elementary reaction. These products are known as intermediates.
Generally, one of the elementary reactions proceeds at a much slower rate than the other elementary reactions. The overall reaction can only proceed as fast as its slowest step, so this slow elementary reaction is the rate-determining step of the reaction.
In using mechanisms to derive rate laws, the first task is to identify the rate determining slow step. The rate law can be derived directly from the stochiometric coefficients of the most elemental slow step. DO NOT USE THE STOCHIOMETRIC COEFFICIENTS OF THE OVERALL REACTION TO DERIVE THE RATE LAW FOR A REACTION.
