Chemical Energy

The energy held in the covalent bonds between atoms in a molecule is called chemical energy. Every bond has a certain amount of energy. To break the bond requires energy -- in chemical language it is called endothermic. These broken bonds then join together to create new molecules, and in the process release heat -- chemists call this exothermic. If the total heat given out is more than the heat taken in then the whole reaction is called exothermic, and the chemicals get hot. The burning of methane in oxygen is an example of this. If the heat taken in is more than the heat given out then the whole reaction is endothermic and the chemicals get cold. Combining carbon and hydrogen to make methane is an example. We rarely meet such reactions in every day life. They happen in living cells, the energy being supplied by sunlight or some other source.

ATP is the molecule used by life to carry chemical energy. The bond between two of its phosphate groups carries a lot of energy because both phosphates have negative electric charge.