Chemical reactions can be divided into two types based on whether they require energy to proceed or release energy as they proceed.
Endergonic reactions absorb energy. In this case, the products have more energy than the reactants. These chemical reactions are nonspontaneous – they require an input of energy in order to occur. The products of these reactions have more stored energy than the starting molecules. These products can be thought of as energy-storing molecules. An endergonic reaction will not take place on its own without the addition of energy.
Exergonic reactions release energy. Think: exergonic means energy is exiting the system. These reactions are also referred to as spontaneous reactions – they will occur by themselves without an input of energy. Their products have less stored energy than the starting molecules. An important distinction must be drawn between the term spontaneous and the idea of a chemical reaction occurring immediately. Contrary to the everyday use of the term, a spontaneous reaction is not one that suddenly or quickly occurs. The rusting of iron is an example of a spontaneous reaction that occurs slowly, little by little, over time.
One more important concept must be considered regarding exergonic reactions. Exergonic reactions require a small amount of energy input to get going, before they can proceed with their energy-releasing steps. This small amount of energy input necessary for all chemical reactions to occur is called the activation energy. Think of a firecracker: it won’t explode until you add a small amount of energy by lighting the fuse. The energy added by lighting the fuse can be compared to the activation energy of a chemical reaction.
Text adapted from: OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016 http://email@example.com