We must correct for the energy absorbed in taking these diatomic
2H(g, atoms) DH
= +52.1 kcal per mole of H atoms
2O(g, atoms) DH
= +59.2 kcal per mole of O atoms
The desired process and its DH
can be determined from the previous reactions:
This is what we were after: the energy required to tear a
water molecule apart, not into elements in their standard
states, but into isolated atoms. Half of this total energy
can then be ascribed to each bond, giving an O-H bond energy
of 110.6 kcal per mole of bonds.
From calculations such as this, we can obtain the table of
bond energies shown above. These bond energies are not perfect
for any one molecule, but are the best average values for
a great number of molecules with the same kind of bond.