Most reactions that occur in a chemistry class are very fast reactions, often occurring in only a few seconds or even less time. However, in the real world, many reactions occur very slowly. Apples brown over the course of five or ten minutes, silver tarnishes over months, cars rust over the course of a decade. It is now time to consider why some reactions are faster than others, and how the rate of reactions can be changed. The study of the rates of reactions is called kinetics.
So the questions that must be asked are:
What is a mechanism?