Reactions in solution

Abstract

This chapter explains that most chemical reactions take place in solution, and so it is important to understand what controls their rates. The concept of the rate-determining step plays an important role for reactions in solution where it leads to the distinction between ‘diffusion control’ and ‘activation control’. In the diffusion-controlled limit, the condition for the encounter rate to be rate-determining is not that it is the slowest step, but that the reaction rate of the encounter pair is much greater than the rate at which the pair breaks up without reacting. In the activation-controlled limit, the condition for the rate of energy accumulation to be rate-determining is a competition between the rate of reaction of the pair and the rate at which the pair breaks up, and all three rate constants control the overall rate. The chapter then looks at Fick's first law of diffusion and Fick's second law of diffusion.