When a catalyst is used in a reaction, the heat absorbed or evolved during the reaction
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → remains unchanged
The heat absorbed or evolved in a reaction is its enthalpy change, \( \Delta H = H_{\text{products}} - H_{\text{reactants}} \). Enthalpy is a state function: \( \Delta H \) depends only on the initial and final states, not on the path between them.
A catalyst provides an alternative path of lower activation energy, but the reactants and products are the same. It lowers the activation energy of the forward and the backward reactions by the same amount, say x, and \( \Delta H \) is the difference between the two:
\( \displaystyle \Delta H = E_{a,\text{f}} - E_{a,\text{b}} \)
\( \displaystyle \Delta H_{\text{cat}} = (E_{a,\text{f}} - x) - (E_{a,\text{b}} - x) = E_{a,\text{f}} - E_{a,\text{b}} = \Delta H \)
So the heat absorbed or evolved remains unchanged. For the same reason a catalyst does not change \( \Delta G \) or the equilibrium constant; it only lets the reaction reach equilibrium faster.
NCERT (2026-27 edition):
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. … A catalyst does not alter Gibbs energy, ΔG of a reaction. … 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.”
