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
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
H2(g) + ŻO2(g)
DH = +57.8 kcal mole-1
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.