Reaction Mechanisms, the Rate-Determining Step & the Arrhenius Equation

(Higher Level)

Most reactions do not occur in a single step in which all reactant molecules collide simultaneously; such an event would be improbable. Instead, a reaction proceeds through a mechanism, a sequence of simpler elementary steps, each involving the collision or breakdown of just one or two species. Adding the elementary steps together, and canceling any species that are produced and then consumed, must reproduce the overall balanced equation. Species that appear in the mechanism but not in the overall equation, being formed in one step and used up in a later one, are called reaction intermediates.

The number of species that come together in an elementary step is its molecularity. A unimolecular step involves a single molecule breaking apart, a bimolecular step involves two speci