Rate-Determining Step
Definition and meaning of Rate-Determining Step in chemistry.
The rate-determining step is the slowest elementary step in a multi-step reaction mechanism. It controls the overall reaction rate because the reaction as a whole cannot proceed any faster than this bottleneck step, regardless of how quickly the other steps occur.
In more detail
In a multi-step reaction mechanism, the rate-determining step acts as a bottleneck that limits the overall rate, even if all other steps occur very rapidly. The experimental rate law can be derived from the rate-determining step and any fast equilibrium steps that precede it, using the pre-equilibrium approximation to express the concentration of any intermediates involved. Identifying the rate-determining step requires comparing the rate law predicted by a proposed mechanism with the experimentally observed rate law. Once identified, it reveals critical information about reaction kinetics and molecular interactions.
Key facts
| Also called | rate-limiting step |
|---|---|
| Field | Physical Chemistry |
| Determines | the form of the experimental rate law |
| Classic example | 2NO + Cl2 → 2NOCl |
In the reaction 2NO + Cl2 → 2NOCl, the mechanism consists of a fast equilibrium step (NO + Cl2 ⇌ NOCl2) followed by a slow step (NOCl2 + NO → 2NOCl). The slow second step is the rate-determining step and controls the overall reaction rate, producing the experimentally observed rate law: rate = k[NO]2[Cl2].
Frequently asked questions
How do you identify the rate-determining step?
Compare the rate law predicted by a proposed reaction mechanism (derived from the slow step) with the experimentally measured rate law. When they match, the slow step in that mechanism is the rate-determining step.
Can a reaction have multiple rate-determining steps?
Strictly, no. A mechanism has only one rate-determining step that is slowest. However, if two steps have similar rates, they may both substantially influence the overall rate and be difficult to distinguish experimentally.