Notice that the rate law we have used to this point expresses rate as a function of concentration. For example, for the decomposition of NO₂ we have defined:

which tells us (once we have determined the value of n) exactly how the rate depends on the concentration of the reactant, NO₂. A rate law that expresses how the rate depends on concentration is technically called the differential rate law, but it is often simply called the rate law. Thus when we use the term the rate law in this text, we mean the expression that gives the rate as a function of concentration.

A second kind of rate law, the integrated rate law, also will be important in our study of kinetics. The integrated rate law expresses how the concentrations depend on time. A given differential rate law is always related to a certain type of integrated rate law, and vice versa. That is, if we determine the differential rate law for a given reaction, we automatically know the form of the integrated rate law for the reaction.

This means that once we determine experimentally either type of rate law for a reaction, we also know the other one.

Which rate law we choose to determine by experiment often depends on what types of data are easiest to collect. If we can conveniently measure how the rate changes as the concentrations are changed, we can readily determine the differential (rate/concentration) rate law. On the other hand, if it is more convenient to measure the concentration as a function of time, we can determine the form of the integrated (concentration/ time) rate law. We will discuss how rate laws are actually determined in the next several sections.