# Reaction mechanism

A mechanism is the series of simple steps (elementary steps) that make up an overall reaction. Intermediates form in one step and are used in another. The slowest step, the rate-determining step, controls the overall rate.

Chemistry (university) · Part 06: Chemical kinetics · Card 8 of 9 · Last updated 2026-10-19

## Analogy

A relay race: the slowest runner decides the team's finishing time, however fast the others are.

## Example

**Elementary steps**

Step 1 (slow): NO₂ + F₂ → NO₂F + F
Step 2 (fast): NO₂ + F → NO₂F
Rate = k[NO₂][F₂]
Intermediate: F

## In practice

For 2NO₂ + F₂ → 2NO₂F, the observed rate law is first order in each reactant, which points to a slow first step involving one of each.

## Practice

### Flashcards

- **What is the rate-determining step?** The slowest elementary step in a mechanism.

### Interview questions

**Describe how a rate-determining step relates to the rate law of a multi-step reaction.**

- Overall rate set by the slowest step
- Rate law follows the slow step's reactants
- Intermediates are replaced using earlier fast steps
- Mechanism must match the experiment

## Questions people ask

### What is the molecularity of an elementary step?

Molecularity is the number of molecules colliding in a single elementary step. One molecule is unimolecular, two is bimolecular, and three is termolecular. For an elementary step only, the rate law can be written directly from the coefficients. For a multi-step reaction, it cannot.

### What is an intermediate and how is it different from a catalyst?

An intermediate is made in one step and used up in a later step, so it does not appear in the overall equation. A catalyst is used in an early step and then regenerated, and is present at the start. Both are missing from the overall equation, but a catalyst enters first.

## Related concepts

- [Nucleophiles, electrophiles and curved arrows](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-10/chemistry-university-part-10-concept-1)
- [Carbocation stability](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-10/chemistry-university-part-10-concept-4)

Web page: https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-6/chemistry-university-part-6-concept-8
Study this card in the app: https://studyvisually.com/app/learn?category=chemistry-university&card=chemistry-university-part-6-concept-8