Why do reactions speed up when the reactants are more concentrated?

Answer 1

Imagine a large clear plastic jar filled about 1/20 full with pea-sized round rubber pellets. Now imagine shaking that jar very rapidly (as if it were on a paint mixer.) Now imagine that same jar shaking at the same speed, but this time the jar is 3/4 full.

The more often a collision takes place between the reactants, the more often the reactants combine to make their product.
Collisions will happen at a much higher rate in the jar that is 3/4 full (representing a higher concentrated solution) than in the jar that's only 1/20 full (representing the less concentrated solution).

Hope that helps.

Answer 2

there is more of each reactant, so there is a large chance that each reactant will come in contact and the reaction can take place.

ie:
reds hate blues.
they fight whenever they meet.
you have a room that 1 mile by 1 mile.
if you put 1 red and 1 blue. how likely is it taht they meet and fight?
not very.
now put a larger concentration of each: 50 reds and 50 blues.
is it more likely that a red meets a blue and they fight?
i think so.