Series of elementary steps that describe how a chemical reaction occurs.
These steps add together to be equivalent to the balanced chemical equation (Hess’ Law).
Most elementary steps involve only 1 or 2 reactants.
The slowest step is called the rate-determining step. A reaction can only proceed as fast as the slowest step. The rate law equation can be determined from this step.
Entities that are produced in one step then use in another step and do not appear in overall balanced chemical equation are called intermediates.
Examples of elementary steps
A → products
rate: k[A]
example: CO2 → C + O2
A + A → products
rate: k[A]2
example: N + N → N2
A + B→ products
rate: k[A][B]
example: NO + O2 → NO2 + O
Requirements
Sum of elementary steps must give the overall balanced chemical equation (Hess’ Law).
Reaction mechanism must agree with experimentally determined rate law (rate law from rate determining step must equal experimental rate law).