How does a catalyst affect a reaction's activation energy and overall enthalpy change?

• A. Catalyst raises the activation energy.
• B. Catalyst changes the enthalpy change of the reaction.
• C. Catalyst changes the equilibrium constant.
• D. Catalyst provides a lower-energy pathway; lowers the effective activation energy without changing the overall enthalpy change.

Answer: (D)

A catalyst speeds up a reaction by altering the pathway the reaction follows, not by changing the overall energetics. It provides an alternative mechanism with a lower-energy transition state, so the energy barrier—the activation energy—needed to reach the activated complex is reduced. Because this is a kinetic shortcut and not a thermodynamic change, the enthalpy change of the reaction (the difference in energy between reactants and products) remains the same.

Since a catalyst accelerates both the forward and reverse reactions by the same mechanism, it does not shift the equilibrium composition, and the equilibrium constant stays unchanged at a given temperature. It also does not raise the activation energy. So the correct view is that the catalyst offers a lower-energy pathway, lowering the effective activation energy without altering the overall enthalpy change.