The rate law basically uses the rate constant as well as the concentrations of reactants to basically express mathematically what the rate of that chemical reaction is as a whole. So let's suppose we have the following elementary reaction. So what do we mean by an elementary reaction? Well, recall from general chemistry, an elementary reaction is a reaction that takes place in a single step. And the important thing about elementary reactions that you have to remember is in any elementary reaction we can use the coefficient of that reactant to basically determine what the order of that reactant is within that rate law. So we can use the coefficient in front of the reactant to basically tell us what the exponent value is of that particular reactant in that rate law.