Collision Theory and Activation Energy — AQA GCSE Chemistry
Collision theory explains why the rate of a reaction changes when conditions change.
Collision theory
For a reaction to happen, particles must collide with each other:
- with enough energy (at least the activation energy), and
- in the correct orientation.
The rate of reaction depends on the frequency of collisions and the energy of those collisions. Anything that increases the number of successful collisions per second increases the rate.
Activation energy
The activation energy is the minimum amount of energy that colliding particles must have for a reaction to occur. Only collisions with energy equal to or greater than the activation energy are successful.
Explaining the factors using collision theory
- Higher concentration / pressure — more particles in a given volume → more frequent collisions → faster.
- Higher temperature — particles move faster, so they collide more often and with more energy → much faster.
- Larger surface area (smaller pieces) → more particles exposed → more frequent collisions → faster.
- Catalyst — provides a pathway with a lower activation energy, so more collisions are successful.
Exam tips
- A successful collision needs enough energy (activation energy) and the right orientation.
- Rate depends on the frequency and energy of collisions.
- Always explain rate factors using collision theory (more frequent/more energetic collisions).
- Define activation energy precisely.