Which describes a catalyst's role in a reaction mechanism, with an example?

• A. Provides an alternative lower-energy pathway, increasing the rate; example: Pt catalyst in hydrogenation.
• B. Is consumed during the reaction and becomes part of the products.
• C. Increases the activation energy barrier, slowing the reaction.
• D. Changes the equilibrium constant of the reaction.

Answer: (A)

A catalyst works by providing an alternative, lower-energy pathway for the reaction to proceed, which lowers the activation energy and lets more reacting molecules cross the energy barrier at a given temperature. Because of this lower barrier, the rate of the reaction increases. Importantly, the catalyst itself is not consumed in the process.

Platinum in hydrogenation is a classic example: the metal surface brings reactant molecules together, facilitates the dissociation of hydrogen gas, and enables rapid addition of hydrogen to the substrate through a mechanism that requires less energy than the uncatalyzed route. This speeds up the reaction without the Pt catalyst being used up.

Catalysts don’t change the overall amount of products at equilibrium, so they don’t alter the equilibrium constant. They also don’t become part of the final products, and they don’t raise the activation energy (they do the opposite by lowering it).