Since reactions occur as a result of collisions between particles, the particles are in a reaction vessel, the the concentration of particles, the more collisions are possible. The collisions that occur, the faster the reaction rate.

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Explanation:

For a reaction to occur, the reactant particles must collide with each other. This forms the basis of the collision theory. However, not all collisions leads to a chemical reaction or forms a product during a chemical reaction. The collisions that causes the formation of a product is known as effective collision. Effective collision occurs when the reactant particles hit each other well enough to surmount the activation energy of the reaction and hence for the formation of product. Activation energy is the minimum amount of energy required for a reaction to occur.

From the above, it can be deduced that the higher the concentration of the reacting particles, the higher the chances of collision and subsequently effective collision. And if effective collision increases, the rate of the reaction also increases.

One way to increase the number of effective collisions is the addition of catalyst which reduces the activation energy. Once activation energy is reduced, more collisions will lead to product since there will be more effective collisions.