A chemical reaction occurs by the rearrangement of atoms and
molecules in the reactant (starting) molecules and the end product
molecules. Some bonds are broken while others are reformed. The
process of breaking and forming bonds results in a net energy
needed or given off for a reaction.
In the example above and to the left, the combustion reaction
of methane and oxygen to form carbon dioxide and water is shown
broken into steps to show the entire energy "using"
and "forming" process. First it takes energy to break
bonds, all four of the C-H bonds in methane must be broken. The
energy units are kilojoules, a positive sign means that the process
is endothermic or energy is required to break the bonds.
In a similar fashion, two diatomic oxygen molecules are broken
apart which requires more energy. Now all of the individual atoms
in the reactant molecules have been broken apart.
On the right side of the diagram in a second step, the various
atoms form new bonds in new molecules of carbon dioxide and water.
The formation of new bonds is an exothermic process where
heat is given off. Again the energy given off is totaled to form
new bonds in carbon dioxide and water molecules.
Finally, the overall reaction yields an excess of energy given
off -802 kj. (the minus sign means that this is an exothermic
process). In more familiar units this is equivalent to 191 kilocalories
per 16 grams of methane. This is a little more than the 150 calories
in a can of Coke.
The excess of energy given off is mainly in the form of heat.
Chemical energy stored in the bonds of molecules is transformed
into heat and light energy. Most chemical reactions are of this
type and thus are exothermic. Less energy is required to break
old bonds than is given off in the process of forming new bonds.