What is the effect of increasing concentration on reaction rate, all else equal?

• A. It generally decreases the rate.
• B. It has no effect on the rate.
• C. It is only relevant when temperature changes.
• D. It generally increases the rate.

Answer: (D)

Raising the concentration increases the rate because it raises the frequency of encounters between reacting particles. More particles in the same volume means more collisions per second, and a higher chance that collisions lead to product formation. The exact effect follows the rate law: the rate depends on each reactant’s concentration raised to a power equal to the reaction’s order with respect to that reactant. So doubling the concentration of a reactant that's first-order with respect to that reactant doubles the rate; for a second-order dependence on a single reactant, doubling the concentration quadruples the rate. In general, if a reactant has a positive dependence on concentration, increasing it speeds up the reaction. There are rare zero-order cases where changing concentration doesn’t affect the rate, but with all else held constant, the typical outcome is a faster rate.