We just determine the rate law for a specific reaction. It is possible to write a general rate law for any balanced chemical equation. Consider the following general equation for a chemical reaction;
The rate law for the reaction is;
Where k is the rate constant for the reaction and we refer to 'm' as the order of the reaction with respect to A. Similarly 'n' is the order of the reaction with respect to B. The sum of 'm + n' is the overall order of the reaction. For our purposes the sum of the exponents in the rate law will only have values of 0, 1, 2 or 3. Although half orders are possible most of the problems we will concern ourselves with will be whole number exponents. The implication of the exponents and their values will be covered when we discuss mechanism of chemical reactions. While it is possible to write the general rate law given the balanced chemical equation the exact values of 'm', 'n' and 'k' can only be determined from experimental data. It should be noted that the units on the rate constant depend on the order of the reaction.
As shown earlier the order of each reactant must be determined by experiment. When more then one reactant is involved the order is somewhat more difficult to determine experimentally. One straight forward method is to determine the rate by changing one of the reactants concentration while holding the other species concentration constant. From the rates measured we can find the order of the reaction with respect to the reactant whose initial concentration is changed. Let's look at a sample problem where this method is used.