The activation energy (Ea) is the minimum amount of the extra energy absorbed by the reactant molecules from an energy source such as heat, light, etc., to attain the threshold value for effective collisions to occur and thereby to form an intermediate complex (active state) that finally results in product formation.
It can also be expressed as the difference between the threshold energy and the average kinetic energy possessed by the reactant molecules at the start of a chemical reaction.
Therefore,
Activation Energy = Threshold Energy – Average Kinetic Energy of the reactant molecules