When a catalyst is used in a reaction, the heat absorbed or evolved during the reaction

remains unchanged

The correct answer is Option (3) → remains unchanged

A catalyst is a substance that changes the rate of a chemical reaction without being consumed in the process.

• Mechanism: A catalyst works by providing an alternative reaction pathway that has a lower activation energy ($E_a$).
• Energy States: The catalyst affects the energy of the transition state.
• Initial and Final States: However, a catalyst does not change the energy levels of the initial reactants or the final products.

The enthalpy change ($\Delta H$) of the reaction is defined as the difference between the energy of the products ($H_{\text{products}}$) and the energy of the reactants ($H_{\text{reactants}}$):

$\Delta H = H_{\text{products}} - H_{\text{reactants}}$

Since the catalyst only lowers the activation energy ($E_a$) and does not change the energy of the reactants or products, the difference in enthalpy ($\Delta H$) remains the same.

Therefore, the heat absorbed (for an endothermic reaction, $\Delta H > 0$) or the heat evolved (for an exothermic reaction, $\Delta H < 0$) during the reaction remains unchanged when a catalyst is used.