For a chemical reaction to occur two or more molecules must come together and react. That is, they must "hit" each other. The form of the equation representing the speed of a chemical reaction, A + B --> products, is something like
If everytime two things (A & B) "hit" they were to react nature would not look as it does. For one thing, we would not be here! If,somehow, we could look from above we would see a mish-mash of muddy stew. Most molecules in chemical reactions do not react everytime they make an encounter-the speed is just too fast to work well for life. In fact, it is necessary for molecules to have a certain energy ("speed") in order to overcome a barrier to reaction. This energy barrier, which slows the reaction compared to the "reacts on every encounter" speed, is called the activation energy. We understand the speed as the speed due to the molecular temperature-hot molecules move faster than cold ones. This energy of activation needs to be over come for reaction and the speed of reaction depends on its magnetude.
Without going through a handwaving explanation the expression for the speed of a reaction is proportional to concentrations and the rate constant whose temperature dependence is given by the Arrhenius equation:
If we wanted to know the change in the rate constant between two temperatures, knowing a value for the activation energy, we would assume that the factor A is constant and form the ratio
Why does a refrigerator keep food from spoiling? Spoiling is a chemica reaction. If the activation energy for food spoiling is 77 kJ/mol and we compare the rates of reaction between room temperature (77F = 25C = 298K) and refrigerator freezer temperature (4F = -16C = 257K) pluging into the expression above shows that the ratio of rate constants, or the ratio of reaction speeds, is reduced by 142 from room temperature. In otherwords, the speed of reaction in a freezer is very much slower than at room temperature.