How do rates of reaction change with concentration?
An increased concentration will have the affect of increasing the reaction rate, as there are more particles to react.
In order for any reaction to happen, the particles must first collide. This is true whether both particles are in solution, or whether one is in solution and the other a solid. If the concentration is higher, the chances of collision are greater.
For example: in the lab, zinc granules react fairly slowly with dilute hydrochloric acid, but much faster if the acid is concentrated. Concentrated acid has greater number of acid molecules. The greater the number of molecules, the greater the number of collisions between Zinc particles and acid molecules, and the greater the chances of effective collisions (causing the reaction to go fast).