Collision Theory

Collision theory is the model that chemists use to explain why reactions happen and why changing conditions can make them faster or slower. This theory underpins every rate-of-reaction topic in the Edexcel GCSE Chemistry (1CH0) specification. In this lesson you will learn the conditions needed for a successful collision, what activation energy means, and how collision theory explains the effects of different factors on the rate of reaction.

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The Basic Idea

For a chemical reaction to take place, the reacting particles must collide with each other. However, not every collision leads to a reaction. Two conditions must be met for a collision to be successful (i.e. for it to result in a reaction):

1. The particles must collide with sufficient energy — this minimum energy is called the activation energy.
2. The particles must collide with the correct orientation — the reactive parts of the molecules must be facing each other.

If either condition is not met, the particles simply bounce apart and no reaction occurs.

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Activation Energy

The activation energy (symbol Eₐ) is the minimum amount of energy that colliding particles must have in order for a reaction to occur.