What is a catalyst?

• A. A chemical agent that selectively speeds up a chemical reaction without being consumed
• B. A substance that increases the overall rate by heating the mixture
• C. A solvent that dissolves reactants
• D. A product formed at the end of the reaction

Answer: (A)

A catalyst speeds up a chemical reaction without being consumed by providing an alternative pathway with lower activation energy. By offering this easier route, more reactant molecules can reach the transition state and convert into products at a faster rate, all while the catalyst is regenerated in the process. Because it is not used up, the same amount of catalyst can repeatedly catalyze further reactions.

This is why the description fits best. Heating can increase reaction rate by giving molecules more kinetic energy, but it’s not a substance that facilitates the reaction through a new mechanism. A solvent’s role is simply to dissolve reactants and help them mix; it doesn’t introduce a faster pathway. A product formed at the end is simply the outcome, not a facilitator of the process. Catalysts can be either in the same phase as the reactants (homogeneous) or in a different phase (heterogeneous), and they alter the rate without changing the final equilibrium composition.