The activation energy for a simple chemical reaction $\mathrm{A} \rightarrow \mathrm{B}$ is $\mathrm{E}_a$ in forward direction. The activation energy for reverse reaction:

The activation energy for the reverse reaction, $\mathrm{E}_{\mathrm{a}(\mathrm{rev})}$, is the difference between the product energy and transition state at the peak of the diagram. $\mathrm{H}_r$ is the difference between the potential energy of the reactant and the potential energy of the product. Related Theory In a reversible reaction of bi-molecular type the activation energy for forward reaction is positive. Whereas, for the backward reaction activation energy is negative.