For an endothermic reaction, energy of activation is \(E_a\) and enthalpy of reaction is \(\Delta H\) (both of these in kJ mol−1). The minimum value of \(E_a\) will be:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → More than \(\Delta H\)
On the potential energy diagram, the activation energy Ea is the height of the energy barrier above the reactants, and ΔH is the difference between the energy of the products and the reactants. For an endothermic reaction the products lie above the reactants by ΔH, and the activated complex must lie higher still (at least as high as the products):
Ea(forward) = ΔH + Ea(backward), so Ea ≥ ΔH
Since the backward reaction also has some activation energy, Ea of the forward reaction is greater than ΔH.
Why the other options are incorrect
Option (1) Less than \(\Delta H\) — Incorrect. The barrier cannot be lower than the products, which already lie ΔH above the reactants.
Option (2) Equal to \(\Delta H\) — Incorrect. This would require zero activation energy for the backward reaction, which is not the normal case; it is only the theoretical lower limit.
Option (4) Equal to zero — Incorrect. A reaction that absorbs ΔH of energy must have Ea at least ΔH; it cannot be zero.
NCERT (2026-27 edition):
Temperature Dependence of the Rate of a Reaction – “… The energy required to form this intermediate, called activated complex (C), is known as activation energy (Ea). Fig. 3.7 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 final enthalpy of the reaction depends upon the nature of reactants and products.”
