X⟶Y, it is an exothermic reaction whose ∆E or ∆H=−20kJmol−1
Given, activation energy of the forward reaction (X→Y), Eaf=30kJmol−1 Activation energy of the backward or reverse reaction (Y→X), Eab can be calculated as, ∆H=Eaf−Eab ⇒Eab=Eaf−∆H=30−(−20)=50kJmol−1