Collision theory [Higher tier only]
A collision with too little energy will not produce a reaction.
Image caption, 1. Two pairs of particles move towards each other
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The colliding particles must have enough energy for the collision to be successful or effective in producing a reaction.
The minimum amount of energy for a collision to be successful is called the activation energy.
The rate of a reaction depends on the rate of successful collisions between reactant particles. The more successful collisions there are, the faster the rate of reaction.