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Potential chemical energy is energy stored in a material that may be released through a chemical reaction. This energy can come from combining atoms or molecules or from breaking molecules apart. It is released in the form of heat, light, or both. Usually some type of trigger is necessary to release potential energy, but this trigger may be as simple as just mixing two materials together, such as adding the highly reactive metal, potassium, with water, resulting in a very strong reaction that releases a great deal of heat.
The formation and breaking of bonds between atoms is the source of all potential chemical energy. Depending on the strength of such bonds, the amount of stored energy will vary. Very strong bonds store a small amount of energy, and weak bonds store larger amounts. Strong bonds are very stable and require added energy to break, meaning less energy is released when broken. The opposite is true of weak bonds, so they require little added energy as a trigger to break and release a great deal of energy.
This principle of potential chemical energy is the basis for our use of many of the world's energy sources. Hydrocarbon fuels, also known as fossil fuels, such as coal, petroleum, and its refined derivatives, contain very large amounts of potential chemical energy. When burned, the molecules of these substances are combined with oxygen. This results in the breaking of some of the molecular bonds between certain atoms in the molecules and the formation of others as oxygen atoms are incorporated into the molecular structure, a process known as oxidation. The result is the release of potential chemical energy in the form of heat and light, but particularly heat, which is harnessed and used to power machines and converted to electricity.
Stored energy is expressed in standard international units (SI) of megajoules per kilogram (Mj/kg), and the amount of energy stored in a substance in relation to a given mass is called energy density. This allows for comparison of the amount of potential chemical energy stored in one substance to another by mass. This type of energy may be released in different ways. For fossil fuels, it is usually released by burning. For substances like foods, it is released during the body's metabolic processes, which are chemically identical to burning but are carried out a much slower and controlled rate.
Explosives such as dynamite and nitroglycerin release their potential chemical energy very quickly, which gives them their explosive properties. Most explosives have a relatively low amount of potential chemical energy by mass, even in comparison to things like sugar, but their chemical properties allow for this energy to be released almost instantaneously. For example, nitroglycerine contains 6.5 Mj/kg and raw sugarcane contains 19 Mj/kg.
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