Reaction Mechanism
Definition
The step-by-step story of how a reaction actually happens: which bonds break when, in what order, through which intermediates.
Technical definition
A reaction mechanism is the detailed sequence of elementary steps by which reactants convert to products, including intermediates and transition states. The slowest, rate-determining step governs the observed rate law, and mechanisms are proposed from kinetic and spectroscopic evidence.
Example
The reaction H2 + I2 -> 2HI was long thought to be a single collision, but evidence shows it actually proceeds through iodine atoms as intermediates.
Common confusion
The balanced equation never reveals the mechanism. Two reactions with identical overall equations can have completely different step-by-step pathways.