What is the effect of increasing concentration on the rate of a solution-phase reaction?

• A. Faster rate.
• B. Slower rate.
• C. No change.
• D. Rate depends only on temperature.

Answer: (A)

Increasing concentration raises the collision frequency between reacting molecules in solution. With more particles per unit volume, encounters happen more often, so there are more opportunities for successful reactions each second. The rate then increases according to the rate law, meaning if the reaction depends positively on that concentration, the overall rate goes up (often in a proportional or higher-order manner). In general, higher concentration leads to a faster rate, so the best choice is faster rate.