Explanation:

The minimum amount of energy a reactant must have to take part in a reaction is called activation energy.

It is given by Arrhenius equation-

$\mathrm{K}=\mathrm{A}·{\mathrm{e}}^{\frac{-{\mathrm{E}}_{\mathrm{a}}}{\mathrm{RT}}}$ 

Where, K is rate constant
            A is Arrhenius constant
            E_a is activation energy
            T is temperature
            R is Rate constant
Activation energy is the energy difference between reactant and activation energy.
Diagrammatically activation energy can be shown as;