A catalyst reduces the activation energy for both forward and back reactions, leaving the equilibrium position unchanged.

For example when the reaction

2HI(g) H₂(g) + I₂(g)

is uncatalysed, its activation energy is 185 kJ mol^-1 in the forward direction and 164 kJ mol^-1 in the reverse direction.

When a platinum catalyst is used, the activation energy in the forward direction is reduced to 59 kJ mol-1.

Each catalyst has a different, individual effect on a reaction. The effect of two metallic catalysts on the activation energy for the decomposition of ammonia is shown in the table

catalyst	Ea/ kJmol-1
none	73
iodide ion	54
Pt surface	46
iron(III)	40
catalase	4

How does a catalyst change the activation energy?

In two ways:

• It forms bonds with one or more of the reactants and so reduces the energy needed by the reactant molecules in order to complete the reaction

• It brings the reactants together and holds them in a way that makes reaction more likely. When molecules come together in the correct orientation for reaction there is a big reduction in entropy. If a catalyst already holds the molecules next to each other then the entropy change for the reaction step will be far less negative than it would be without the catalyst, and the reaction is more likely. Note that the catalyst does not affect the overall entropy change for the reaction.

In other words catalysts change the path of a reaction; they change its mechanism.