To understand why certain variables affect the rates of reactions, we need to understand what happens in a chemical reaction on a molecular level.

Collision theory is a set of principles that describes the effects of various factors on the rate of a reaction. In its application, collision theory relies on statistics and other mathematical models. But the concepts behind it are simple--and important to the understanding of reactions.

There are two major ideas in collision theory:

molecules or atoms must collide in order to react
not all collisions result in reaction

A basic understanding of collision theory enables us to predict the kinds of things we might be able to do that will change the rate of a chemical reaction.