How does the rate of reaction change over time?

• A. It remains constant throughout.
• B. It fluctuates randomly.
• C. It is fast at the start. Then, it becomes slower and gradually stops.
• D. It increases steadily.

Answer: (C)

Reaction rate depends on how much reactant is present, and in most reactions the amounts of reactants decrease as the reaction proceeds. At the start, concentrations are high, so there are many effective collisions per second and the rate is fast. As the reactants are consumed, their concentrations drop, fewer collisions occur, and the rate slows down. Eventually, when reactants are nearly exhausted, the rate becomes very small and the reaction effectively stops. This pattern—a fast start followed by a gradual slowdown and eventual cessation—is the typical way rates change over time in standard reactions.