Which statement best describes how a catalyst affects a reaction’s energy profile?

• A. A catalyst increases the reaction enthalpy.
• B. A catalyst changes the reaction stoichiometry.
• C. A catalyst provides an alternative pathway with lower activation energy, increasing rate without changing ΔH.
• D. A catalyst converts an endothermic reaction into an exothermic one.

Answer: (C)

A catalyst works by altering the reaction’s energy landscape to provide a different mechanism with a lower activation energy, so more reacting molecules can reach the transition state and the reaction proceeds faster. Importantly, this speeds things up without changing the overall enthalpy change (ΔH) of the reaction, because the thermodynamics—the difference in energy between reactants and products—are unchanged by a catalyst. Catalysts also don’t alter the stoichiometry or convert an endothermic process into an exothermic one. So the best description is that a catalyst offers an alternative, lower-energy pathway that increases the rate while leaving ΔH the same.