Just what is a rate, and why are some rates faster than others? Can we do anything about the rates that are slower or faster than we would like? These questions---and their answers--- are crucial to the resolution of the fourth question which we posed at the beginning of the year: If a reaction occurs, how fast will it go? In this unit, we will try to address that question.

We can begin with the concept of RATE. We are familiar with rates in everyday life: the dollars per hour that someone is paid, the miles per hour at which an automobile moves. These rates, and others, all measure the change in some quantity for a set period or change in time. We might use the notation of mathematics to express a rate as:

O.K., so a rate is the change in some quantity per time. But what quantity? That depends on the reaction you wish to study.