From a chemical standpoint, combustion is a process in which chemical bonds are broken and new chemical bonds formed. The net result of these changes is a release of energy, the heat of combustion. For example, suppose that a gram of coal is burned in pure oxygen with the formation of carbon dioxide as the only product. In this reaction, the first step is the destruction of bonds between carbon atoms and between oxygen atoms. In order for this step to occur, energy must be added to the coal/oxygen mixture. For example, a lighted match must be touched to the coal.
Once the carbon-carbon and oxygen-oxygen bonds have been broken, new bonds between carbon atoms and oxygen atoms can be formed. These bonds contain less energy than did the original carbon-carbon and oxygen-oxygen bonds. That energy is released in the form of heat, the heat of combustion. The heat of combustion of one mole of carbon, for example, is about 94 kcal.
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