A collision between high energy molecules overcomes the forces of repulsion and brings the formation of an unstable molecule cluster, called activated complex. The life span of an activatd complex is very small. Thus, the activated complex breaks either into reactants again or new substances, i.e. products. The activation energy (E_(a)) depends upon the nature of chemical bonds undergoing rupture and is independent of enthalpies of reactants and products. The energy changes during exothermic and endothermic versus the progress of the reaction are as shown below : When temperature is increased, the number of active molecule increases, i.e. the number of effective collisions will increase and the rate of reaction will increase. Activation energy (E_(a))=E_(("activated complex"))-E_(("ground state")) DeltaH=E_(a)(f)-E_(a)(b) The activation energy for the forward reaction ,X to Y, is 60kJ mol^(-1) and DeltaH is -20kJ//mol^(-1), the activation energy for the reverse reaction is