Why concentration affects rate

The image below shows a solution with a low concentration of the two reactants "A" and "BC."

As the reactants move around, they may hit each other (allowing reactions) but most of the time, they will simply bump into water molecules.



As the concentration increases, the particles become more crowded.

In the second diagram, you will find the same number of water molecules, but a much larger number of "A"  and "BC" particles.

As a result, the "A's" and "BC's" will collide more often. Since reactions require collisions, the more collisions there are, the more opportunities there are for reactions.


Thus, increasing the concentration will increase the rate of reaction.