How many of the following change when a catalyst is added to a reaction mixture at a fixed temperature?
Activation energy, Gibbs energy change (ΔG), Equilibrium constant, Rate constant, Reaction pathway, Time taken to attain equilibrium
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 4
A catalyst gives the reaction an alternate pathway with a lower activation energy, so the reaction becomes faster. The reactants and products stay the same, so the quantities that depend only on them do not change.
Change:
Activation energy (it is lowered)
Rate constant (it increases): by \( k = A\,e^{-E_a/RT} \), a lower Ea gives a larger k
Reaction pathway (an alternate pathway or mechanism)
Time taken to attain equilibrium (it becomes shorter; the same equilibrium is reached earlier)
Do not change:
Gibbs energy change (ΔG) — depends only on the reactants and products.
Equilibrium constant — the forward and backward reactions are catalysed to the same extent.
Quantities that change: activation energy, rate constant, reaction pathway and time to attain equilibrium = 4.
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…”
“It is clear from Arrhenius equation (3.18) that lower the value of activation energy faster will be the rate of a reaction.”
\( \displaystyle k = A\,e^{-E_a/RT} \qquad (3.18) \)
“…A catalyst does not alter Gibbs energy, ΔG of a reaction. It catalyses the spontaneous reactions but does not catalyse non-spontaneous reactions. It is also found that a catalyst does not change the equilibrium constant of a reaction rather, 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.”