Final answer:
A catalyst lowers the activation energy of a reaction by changing the mechanism of the reaction, allowing more reactant molecules to have effective collisions.
Step-by-step explanation:
A catalyst works by lowering the activation energy of a reaction by changing the mechanism of the reaction. The activation energy is the energy required for the reactants to undergo a chemical transformation and form products. By providing an alternative pathway with a lower activation energy, the catalyst speeds up the reaction by allowing more reactant molecules to have effective collisions.