Advanced Placement (AP) Chemistry Practice Exam

A catalyst plays a crucial role in a chemical reaction by speeding up the reaction rate without being consumed in the process. It accomplishes this by providing an alternative pathway for the reaction that has a lower activation energy compared to the uncatalyzed reaction. This reduction in activation energy allows more reactant molecules to possess the necessary energy to undergo the reaction at a given temperature, effectively increasing the rate at which products are formed.

For example, in many enzymatic reactions, enzymes act as biological catalysts that lower the activation energy needed for biochemical reactions, allowing them to occur at a much faster rate than they would without the enzyme.

This mechanism does not alter the overall yield of the products or the position of equilibrium in a reversible reaction; rather, it only allows the system to reach equilibrium more quickly. Thus, while catalysts can enhance the efficiency of a reaction, they do not increase the total amount of product produced or change the thermodynamics of the reaction, which is why the other options do not accurately describe the role of a catalyst.