Chemical kinetics is the study of the rates and mechanisms of chemical reactions. The reaction rate is a kinetic property and depends on the mechanism. In contrast, thermodynamic properties (discussed in Section 2.7) are independent of mechanism. Thermodynamics indicates whether or not a reaction is spontaneous, but kinetics indicates how fast the reaction occurs, and whether it happens fast enough to be of any interest or value.
The easiest way to measure the rate of reactions is by measuring the disappearance of a reactant, that is, the change in concentration of a reactant with time. Thus,
where AC = the change in the concentration of the reactant and At = the elapsed time. The subscripts 1 and 2 refer to concentrations and corresponding times, respectively. The reactant concentration decreases with time, making (C2 - Cj) a negative quantity. The minus sign is used to
make it a positive quantity.
The reaction rate can be calculated just as well from the change in concentration with time of a product, in which case AC/At is a positive quantity. It is important to remember that the reaction rate is constantly changing, so AC/At represents an average rate. The instantaneous rate is given by the first derivative dC/dt.