Collision Theory
The collision theory states that for a chemical reaction to occur, the reacting particles must collide with one another.
The rate of the reaction depends on the frequency of collisions.
The theory also tells us that reacting particles often collide without reacting. Certain requirements must be met if the collisions are effective enough to cause a reaction. For a collision to be effective, the reacting species must:
(1)	possess at least a certain minimum energy necessary to rearrange outer electrons in breaking bonds and forming new ones and 
(2)	have the proper orientations toward one another at the time of collision

Animations:  http://www.saskschools.ca/curr_content/chem30_05/2_kinetics/kinetics2_1.htm
When we examine factors that increase the rate of a reaction, we will therefore look at factors that can influence at least one of the following:
•	how often collisions occur. 
•	more effective collisions in terms of collisions occurring with sufficient energy
•	more effective collisions in terms of collisions occurring with the proper orientation.
As a result, the rate of a reaction can be said to be proportional to the number of collisions times the number of effective collisions. 

			r α number of collisions x number of effective collisions

The five factors (temperature, concentration, surface area, catalyst, and chemical nature) affecting a rate of a reaction influence either the number of collisions(concentration, surface area, temperature)  or the fraction of effective collisions (chemical nature, catalyst, temperature).