8.2.1—Activation energy, EA

Activation energy, Eₐ, is the minimum energy particles must have for a reaction to occur when they collide in a suitable orientation.

A catalyst provides an alternative pathway with a lower Eₐ. It does not change the reactants’ or products’ enthalpy, nor does it alter the equilibrium constant.

If an uncatalysed pathway has Eₐ=80 kJ mol⁻¹ and a catalyst pathway has Eₐ=45 kJ mol⁻¹, more particles exceed the threshold at the same temperature, so the rate increases.

Activation energy is not the overall ΔH of the reaction. An exothermic reaction can still have a large activation barrier.