Rates of reactionActivation energy [Higher tier only]

Chemical reactions vary in speed. The rate of reaction measures how much product is made in a given time. For reactions to occur, reactant particles must collide.

Part ofChemistry (Single Science)Rate of chemical change

Activation energy [Higher tier only]

Activation energy is the minimum energy needed for ‘successful collisions’ where atoms within the reactants rearrange to form products. It can be represented on an energy level diagram.

Graph of energy against reaction progress for a catalysed reaction. A reduced amount of energy is required to start the reaction, but the overall net loss remains constant.

The diagram shows that when a catalyst is used, the activation energy is reduced. This makes more of the collisions successful at a given temperature. So a catalyst increases the rate of reaction by lowering the minimum energy required for ‘successful collisions’.