Collision theory states that reacting particles must collide with sufficient energy and in the correct orientation for a reaction to occur. Collision theory can be used to explain and predict how changes to concentration, temperature, pressure, and the surface area of reactants will affect the rate of reaction. It also explains how the presence of catalysts will increase the rate of reaction.

Concentration

Changes to concentration will affect the frequency of collisions between reactants. For example, an increase in the concentration of reactants will increase the frequency of collisions, therefore increasing the rate of reaction.

As concentration increases, molecules get closer and closer together, and therefore are more likely to collide with enough energy to react.

Temperature

Changes to temperature will affect the frequency of collisions and the proportion of collisions that are successful. For example, an increase in the temperature of a system will lead to an increase in the frequency of collisions and an increase in the proportion of collisions that are successful, therefore increasing the rate of reaction. The frequency of collisions will increase because a higher temperature means that the reactants will have a greater average kinetic energy (meaning that they are now moving around with greater speed). The proportion of collisions that are successful will increase because the proportion of reactants with enough kinetic energy to overcome the activation energy barrier has increased.

Pressure

Changes to pressure will affect the frequency of collisions between gaseous reactants. For example, if one or more of the reactants is a gas, then an increase in the pressure of the system will increase the frequency of collisions between reactants, therefore increasing the rate of reaction.

If the pressure of a gas is increased, it is being squeezed into a smaller space. This increase in pressure (and thus decrease in volume) means that the gas molecules are closer together and thus more likely to react.

Surface Area

Changes to the surface area of a solid-state reactant will affect the frequency of collisions between reactants. Because of this, a fine powder will react faster than large chunks of the same substance. The fine powder and its relatively large surface area will react faster than the large chunks because there are far more available sites for reactant interaction – the frequency of collisions has increased.  

Catalysts

A catalyst is a substance that increases the rate of reaction, but is not consumed in the reaction itself. Catalysts achieve this by providing an alternate reaction pathway with a lower activation energy, meaning that a greater proportion of reactants have the kinetic energy needed to successfully collide.