Dependence of Rate on Concentration

Rate as a function of concentration

At a fixed temperature, the rate of a chemical reaction can depend on the concentration of one or more of the reactants — and sometimes even on the concentration of a product. Writing the rate of a reaction in terms of the concentrations of the reacting species is called the rate law (also called the rate equation or the rate expression).

The pattern seen experimentally

As a reaction proceeds, the reactants get used up, so their concentration keeps falling — and the rate of the reaction falls right along with it, decreasing steadily as time passes. The reverse also holds: if a reaction is started with a higher initial concentration of reactants, it generally proceeds faster. This concentration–rate link is exactly why a rate law is needed — it is the tool used to state that dependence precisely, and the following sections build it up piece by piece.

Formally, for a reaction $aA + bB \rightarrow \text{products}$, the rate law expresses this dependence as …