Collision theory explains how conditions change the rate of a reaction. A reaction can only happen when reacting particles collide with each other and with enough energy. The minimum energy that particles must have to react is the activation energy. Many collisions do not lead to a reaction because the particles do not have this much energy.
Increasing the concentration of a solution, the pressure of a gas or the surface area of a solid puts more reacting particles in the space where they can meet. Collisions then happen more frequently, so the rate of reaction increases. If the concentration is doubled there are twice as many collisions every second, so the rate roughly doubles. The rate is proportional to the frequency of collisions.
The size of the pieces of a solid is described by the surface area to volume ratio. A cube with sides of 1 cm has a surface area of 6 cm2 and a volume of 1 cm3, a ratio of 6 to 1. A cube with sides of 2 cm has a surface area of 24 cm2 and a volume of 8 cm3, a ratio of 3 to 1. Smaller pieces have a larger surface area to volume ratio, so more collisions happen each second.
Increasing the temperature makes the particles move faster. They collide more often and the collisions are more energetic, so a greater proportion of collisions reaches the activation energy. Both effects increase the rate of reaction.
