What is the law that states the rate of a reaction is proportional to the concentration of the reactants?

The law that states the rate of a reaction is proportional to the concentration of the reactants is known as the rate law. The rate law provides a mathematical relationship between the rate of a chemical reaction and the concentrations of the reactants involved. It can be expressed in the general form:

Rate = k [A]^m [B]^n

In this equation, k represents the rate constant, [A] and [B] are the concentrations of the reactants, and m and n are the orders of the reaction with respect to each reactant, which can be determined experimentally. This relationship is fundamental in studying chemical kinetics, as it allows chemists to predict how changes in concentration will affect the speed of the reaction.

Other concepts such as Le Chatelier's Principle focus on the equilibrium of reactions rather than their rates, while the Arrhenius equation provides insight into the temperature dependence of reaction rates. Hess's law pertains to the enthalpy changes in chemical reactions rather than their rates. Thus, the rate law specifically captures the essence of how reactant concentrations influence reaction rates, making it the correct answer.