In chemical reactions, chemical bonds are broken and reformed, and preexisting chemical bonds that contained a greater amount of energy than the newly created bonds will release their excess bond energy. When energy is released from a chemical reaction through the reformation of a chemical bond requiring less energy, the energy that was stored within the previous bond as chemical potential energy becomes free energy, such as heat, when it exits the reaction. The breaking and reformation of chemical bonds involves the transfer or sharing of electrons between two or more substances and may also require an input of energy into the reaction.
The release of energy that can result from a chemical reaction is usually converted to heat. This type of reaction is called an exothermic reaction and is the result of a net release of energy when chemical bonds are broken and reformed. The degree of heat released can be significantly high in an exothermic chemical reaction, such as when fossil fuels are oxidized. The release of energy can also be much less noticeable on the macroscopic level, such as when biochemical reactions take place at the cellular level and the reformation of chemical bonds serves to provide living organisms with the metabolic energy required to sustain life.
An endothermic reaction is one that requires more energy to be invested into it than the amount it will release. When the new chemical bonds require more energy than the preexisting ones, there will be a net gain of energy involved in the bond reformation process. If an endothermic reaction were to take place within a flask or beaker in a laboratory setting, the container holding the reactants would begin to feel cold to the touch as the reforming chemical bonds draw in heat from the surrounding environment.