We began this chapter with a mention of making and breaking of bonds, but quickly began talking as if heats of reactions were nothing more than experimentally observable numbers, to be manipulated in whatever way was useful. Where do these heats come from? We can come a long way toward understanding chemical reactions by thinking of heats of reactions solely in terms of individual bond energies in molecules.

The water molecule, H-O-H, has two O-H bonds. How much energy is required to tear these bonds apart and form isolated H and O atoms? This process is represented diagrammatically in the picture story below. We can make an experimental measurement of the standard heat of the reaction

H₂O(g) � H₂(g) + �O₂(g)   DH = +57.8 kcal mole^-1 H₂O

This is not precisely what we were after, however. After the water molecule is pulled into H and O atoms, the situation is complicated by the formation of H-H and O=O bonds.