The figure shows the potential energy of a reacting system plotted against the reaction coordinate, for the reaction with and without a catalyst.
The activation energy of the reverse reaction (products → reactants) is the energy gap from the products up to the top of the curve. According to the figure, how does the activation energy of the reverse reaction change when the catalyst is used?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → It decreases by the same amount as the activation energy of the forward reaction.
A catalyst gives the reaction an alternate pathway with a lower energy barrier. In the figure the reactants start at the same level and the products end at the same level on both curves; only the top of the hump is lower on the dashed curve.
Forward activation energy = top of the hump − energy of the reactants. The top comes down, so it decreases.
Reverse activation energy = top of the hump − energy of the products. The products stay where they are and the top comes down by the same amount, so it decreases by that same amount.
This is why a catalyst speeds up the forward and the backward reactions to the same extent; the equilibrium is reached sooner but is not changed.
Why the other options are incorrect
Option (1) It increases by the same amount by which the activation energy of the forward reaction decreases. — Incorrect. A lower hump cannot make the climb from the products taller.
Option (2) It does not change, because the catalyst acts on the forward reaction and not on the reverse reaction. — Incorrect. Both directions pass over the same barrier, so lowering it helps both reactions.
Option (4) It decreases, but by a smaller amount than the activation energy of the forward reaction. — Incorrect. Since the energy of the products does not change, the fall is equal, not smaller.
NCERT (2026-27 edition):
3.4.1 Effect of Catalyst – “It is believed that the catalyst provides an alternate pathway or reaction mechanism by reducing the activation energy between reactants and products and hence lowering the potential energy barrier as shown in Fig. 3.11.”
“…it helps in attaining the equilibrium faster, that is, it catalyses the forward as well as the backward reactions to the same extent so that the equilibrium state remains same but is reached earlier.”
Fig. 3.11: Effect of catalyst on activation energy