Whenever chemical bonds are broken, energy is released. When new bonds are formed, energy is stored. This energy may be given off as heat, or it may be used to do work. Sometimes, the products of the reaction store more energy in their chemical bonds than the reactants had stored. This stored energy can be held for later use.
Exergonic reactions are energy-releasing reactions.
Endergonic reactions are energy-storing reactions. There are many types of chemical reactions. They can be loosely classified by the type of chemical change taking place. There are several basic reaction types that will be encountered repeatedly in cellular processes. It is a good idea to become familiar with these reactions and the terms that describe them.