# Reaction rate

Reaction rate is how fast a reactant is used up or a product appears: change in concentration divided by time. Units are usually mol/(L·s). Rates are positive, and they slow as reactants are used up.

Chemistry (university) · Part 06: Chemical kinetics · Card 1 of 9 · Last updated 2026-10-19

## Analogy

Like a car's speed: it tells how quickly the concentration 'distance' is covered each second.

## Example

**Rate from data**

[A] falls from 0.80 M to 0.60 M in 20 s
Rate = 0.20 M / 20 s
Rate = 0.010 M/s

## In practice

Food goes off more slowly in a fridge because the rate of the spoiling reactions falls at lower temperature.

## Practice

### Flashcards

- **What are the usual units of rate?** mol/(L·s), or M/s.

## Questions people ask

### Why does the rate change during a reaction?

Reactants collide less often as they are used up, so the rate drops over time. The rate at one moment is the slope of the concentration against time curve at that point. The start of a reaction is therefore usually the fastest part.

### How do coefficients affect rate expressions?

For aA → bB, the rate of reactant loss and product gain differ by the coefficients. The rate of reaction is defined as −(1/a)Δ[A]/Δt = (1/b)Δ[B]/Δt. In 2NO₂ → 2NO + O₂, oxygen forms at half the rate NO₂ disappears.

## Related concepts

- [Rate law and reaction order](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-6/chemistry-university-part-6-concept-2)
- [Method of initial rates](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-6/chemistry-university-part-6-concept-3)

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