How does the concentration of an acid or base affect the speed of an acid-base-catalyzed reaction?

In acid-base catalysis, there is generally a direct relationship between the velocity of the reaction and the concentration of the catalyzing substance. For many reactions, the rate of product formation is proportional to the concentration of the reactant and the catalyst, often expressed by the rate expression $k_1[A][C]$, where $[A]$ is the reactant concentration and $[C]$ is the catalyst concentration.1

The effect of concentration can vary significantly depending on the conditions:

• Dilute Solutions: In dilute solutions of strong acids, the acidity—and consequently the catalytic power—increases in direct proportion to the concentration of the acid.2
• Concentrated Solutions: At concentrations exceeding approximately one molar, the catalytic ability of strong acids often increases much more rapidly than the concentration itself. This is largely attributed to the depletion of water, which increases the proton-donating power of the solution. In these cases, the catalytic power is more accurately measured by the acidity function, $H_0$, rather than simple concentration.2

Ultimately, the specific impact of concentration on reaction speed depends on the reaction mechanism and whether the rate-determining step involves the formation or decomposition of an intermediate species.12