The energy required to form the intermediate, called activated complex `(C )` is known as activation energy `(E_(a))`. The diagram is obtained by plotting potential energy vs. reaction coordinate. Reaction coordinate represents the profile of energy change when reactants change into products.
Some energy is released when the complex decomposes to form products. So, the heat of the reaction depends upon the nature of reactants and products.

If a reaction `A+Brarr C`, is exothermic to the extent of `30 kJ mol^(-1)` and the forward reaction has an activation energy of `249 kJ mol^(-1)`, the activation energy for reverse reaction in `kJ mol^(-1)` is: