Mechanism
Definition and meaning of Mechanism in chemistry.
A mechanism is the sequence of elementary steps that describes how a chemical reaction occurs at the molecular level, showing the specific interactions and pathways from reactants to products.
In more detail
Each elementary step in a mechanism involves molecules colliding and reacting in a single event, with a defined molecularity (number of molecules that must collide). When all elementary steps are added together, intermediate species (produced in one step and consumed in another) cancel out, leaving the overall balanced chemical equation. The slowest elementary step, called the rate-determining step, controls the overall reaction rate and determines the reaction's rate law. Mechanisms explain why reactions occur at different speeds and how changing conditions like temperature or concentration affects the reaction.
Key facts
| Field | General Chemistry |
|---|---|
| Elementary step | An individual reaction in a mechanism occurring in a single molecular collision |
| Intermediate | Species formed in one step and completely consumed in a later step; does not appear in the overall equation |
| Rate-determining step | The slowest elementary step that controls the overall reaction rate |
For the overall reaction 2NO2 + F2 → 2NO2F, the mechanism consists of two steps: Step 1 (slow): NO2 + F2 → NO2F + F; Step 2 (fast): NO2 + F → NO2F. The F atom is an intermediate, and the rate law depends on the slow step.
Frequently asked questions
Why is determining a reaction mechanism important?
The mechanism reveals how a reaction actually occurs at the atomic level, predicts the rate law, explains why certain factors affect reaction speed, and allows chemists to control or improve reactions.
Can different mechanisms produce the same overall reaction?
Yes. Multiple mechanisms can yield the same overall balanced equation, which is why experimental determination of rate laws is necessary to identify the actual mechanism.