A reaction mechanism is an educated guess about the step-by-step sequence of elementary reactions by which an overall chemical reaction occurs.
An overall reaction and a reaction mechanism consist of separate elementary processes.
Each step is called an elementary step, and each has its own rate law and molecularity.
A mechanism describes in detail exactly what takes place during each step of an overall reaction.
It describes each intermediate, activated complex, and transition state, which bonds are broken and in what order, and which bonds are formed and in what order.
A complete mechanism must account for all reactants used, the function of a catalyst, stereochemistry, all products formed, and the amounts of each.
It must also describe the relative rates of the reaction steps and the rate equation for the overall reaction.
For example, consider the following reaction:
CO + NO₂ → CO₂ + NO
The rate law for the reaction is
This suggests that the rate-determining step is a reaction between two molecules of NO₂.
A possible mechanism is:
NO₂ + NO₂ → NO₃ + NO (slow)
NO₃ + CO → NO₂ + CO₂ (fast)