For the reaction $\mathrm{A}+\mathrm{B}\iff \mathrm{C}+\mathrm{D}$, the forward reaction is exothermic. The activation energy of formation of $\mathrm{A}+\mathrm{B}$ _________is that for the formation of $\mathrm{C}+\mathrm{D}$

${\mathrm{E}}_{\mathrm{a}}$(backward reaction) $={\mathrm{E}}_{\mathrm{a}}$ (forward reaction) $+\mathrm{\Delta H}$, for exothermic reaction.

so, the backward reaction will take place when the activation energy of the reactant is more than the product.