What is the function of a catalyst in a chemical reaction?

The correct option is "does not affect the equilibrium constant of reaction".

To understand why this is correct, let us analyze the role of a catalyst in a chemical reaction step-by-step:

1. Definition and Mechanism of a Catalyst:
A catalyst is a substance that increases the rate of a chemical reaction by providing an alternative reaction pathway with a lower activation energy ($E_a$). Because it lowers the energy barrier for both the forward and the reverse reactions equally, it increases both the forward rate constant ($k_f$) and the reverse rate constant ($k_r$) by the same factor.

2. Effect on Chemical Equilibrium:
The equilibrium constant ($K_{\mathrm{eq}}$) of a reversible reaction is defined as the ratio of the rate constant of the forward reaction to that of the reverse reaction:

$K_{\mathrm{eq}}=\frac{k_f}{k_r}$

Since the catalyst increases both $k_f$ and $k_r$ by the exact same proportion, the ratio remains unchanged. Therefore, the catalyst does not affect the position of equilibrium or the value of the equilibrium constant ($K_{\mathrm{eq}}$). It only helps the reaction reach that equilibrium state faster.

3. Evaluating the Other Options:
- "decrease rate constant of reaction": Incorrect. A catalyst increases the rate constant.
- "increases activation energy of reaction": Incorrect. A catalyst decreases the activation energy.
- "reduces enthalpy of reaction": Incorrect. The enthalpy of reaction ($\mathrm{\Delta H}$) is a state function representing the difference in energy between reactants and products. Since a catalyst does not change the energy levels of the reactants or products, it has no effect on the enthalpy of the reaction.