activation energy

In chemistry, the minimum energy required in order to start a chemical reaction. Some elements and compounds will react together merely by bringing them into contact (spontaneous reaction). For others it is necessary to supply energy (heat, radiation, or electrical charge) in order to start the reaction, even if there is ultimately a net output of energy. This initial energy is the activation energy.

The point at which the reaction begins is known as the energy barrier. When the energy barrier is reached, the chemical bonds in the reactants are broken, enabling them to proceed from reactants to products.

In some reactions, such as the combustion of fuels, the activation energy required for the chemical reaction to take place is very small, resulting in a rapid reaction. Other chemical reactions, such as the rusting of iron (a type of oxidation) have a very large energy barrier and take place slowly.

A chemical equation only describes the energy of reaction; the activation energy is not shown. The total chemical energy involved can be represented in an energy level diagram; this also shows whether a reaction is exothermic (giving off energy) or endothermic (absorbing energy).

For example, the energy level diagram for the composition of water from its chemical constituents, hydrogen and oxygen, shows that this is an exothermic reaction:

The reverse chemical reaction, the breakdown of water into its chemical constituents, is an endothermic reaction. The amount of heat energy required is equivalent to the heat energy released during the formation of the compound.