A rate law, also known as the rate equation, is a mathematical expression that describes the relationship between the rate of a chemical reaction and the concentrations of its reactants/12%3A_Kinetics/12.3%3A_Rate_Laws). It is an empirical differential mathematical expression for the reaction rate of a given reaction in terms of concentrations of chemical species and constant parameters only. The rate law generally takes the form of a power law, where the initial reaction rate has some functional dependence on the concentrations of the reactants/12%3A_Kinetics/12.3%3A_Rate_Laws). The sum of the partial orders of the reactants in the rate law expression gives the overall order of the reaction. The rate law cannot be deduced from the chemical equation and must be determined by experiment. The order of a reaction provides insight into the change in the rate of the reaction that can be expected by increasing the concentration of the reactants. The rate law is determined experimentally using the method of initial rates, where the instantaneous reaction rate is measured immediately on mixing the reactants.