This is to give an average value to work from since the precise enthalpy value for a bond may be different in different molecules. The bond enthalpies quoted in the data book are the energies ...

Energy in = 436 + 243 = 679 kJ/mol (this is the energy absorbed when the bonds of the reactants break). Energy out = 2 × 432 = 864 kJ/mol (this is the energy released when the bonds of the ...