Collision theory, the theory used to predict chemical reaction rates, particularly for gases. The theory of collision is based on the assumption that the reacting species (atoms or molecules) need to come together or collide with one another for a reaction.There are three important parts to the collision theory, the interacting substances have to collide, they have to collide with enough energy and they have to collide with the correct orientation.

Collision theory is a theory proposed independently by Max Trautz in 1916 and William Lewis in 1918, that qualitatively explains how chemical reactions occur and why reaction rates differ for different reactions. ... The reaction rate therefore increases. Collision theory is closely related to chemical kinetics.