A catalyst is a substance that:

  • speeds up the rate of a reaction
  • does not alter the products of the reaction
  • is not chemically changed or used up at the end of the reaction

Only a very small mass of catalyst is needed to increase the rate of a reaction. However, not all reactions have suitable catalysts. Different substances catalyse different reactions.

Catalysts in biological reactions are called enzymes. Catalysts do not appear in the overall chemical equation for a reaction.

How catalysts work

A catalyst provides an alternative reaction pathway that has a lower activation energy than the uncatalysed reaction. This does not change the frequency of collisions. However, it does increase the frequency of successful collisions because more particles have energy greater than the activation energy, therefore there are more successful collisions.

The effect of a catalyst on the activation energy is shown on a chart called a reaction profile. This shows how the energy of the reactants and products change during a reaction.

A reaction profile for a reaction with and without a catalystA reaction profile for a reaction with and without a catalyst