Which of the following describes how concentration affects the rate of a reaction?

• A. Higher concentration changes activation energy.
• B. Higher concentration increases the number of collisions per second, increasing rate.
• C. Higher concentration has no effect.
• D. Higher concentration decreases rate.

Answer: (B)

The key idea is collision frequency. When you increase the concentration of reactants, more particles are present in the same space, so they collide with each other more often—more collisions per second. Since reactions proceed when collisions have enough energy to overcome the activation energy, having more collisions increases the number of successful collisions, so the reaction rate goes up. Activation energy itself isn’t changed by concentration; it’s the energy barrier that must be overcome, and concentration just changes how often particles meet. So describing the effect as higher concentration increasing the number of collisions per second, thus increasing the rate, is the best explanation. The other statements don’t fit because they either imply the energy barrier is altered or that rate is unchanged or reduced, which isn’t the case.