These are the rates of C and D. So the rate of C is the change in C with respect to T and the rate of D is change in D with respect to T. Now, these equations aren't very useful. What we actually want to be able to express is the entire rate of that entire equation as a whole. And this is where the rate law and the rate constant comes into play. 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?