Rate-Determining Step
Definition
The slowest step in a reaction mechanism: it acts like a bottleneck and sets the speed of the whole reaction.
Technical definition
The rate-determining step is the slowest elementary step in a reaction mechanism, with the highest activation barrier. It dominates the observed rate law, so the overall rate is approximately the rate of this step.
Example
If a three-step mechanism has steps taking 1 ms, 1 s, and 1 ms, the 1-second step is rate-determining: speeding up the others changes nothing.
Common confusion
The rate-determining step is not necessarily the first step. It is simply whichever is slowest, and identifying it requires kinetic experiments, not intuition.