Bond Enthalpy

Bond enthalpy is defined as the minimum amount of energy required to break one mole of a particular bond in molecules that are in their GASEOUS state. Its unit is kJ mol⁻¹. The larger the bond enthalpy, the STRONGER the bond. Bond enthalpy depends on the size of the bonded atoms and on the number of bonds (multiplicity) between them; the larger the atom involved in the bond, the LOWER the bond enthalpy tends to be (see the trend across H-F, H-Cl, H-Br, H-I in Table 10.4).

Average bond enthalpy. For polyatomic molecules that contain two or more bonds of the SAME type, the individual bonds may not all require exactly the same energy to break -- so an AVERAGE bond enthalpy (the arithmetic mean of the individual bond-energy values) is used. Water is the textbook example: it has two O-H bonds, but they do not break with equal ease:

The average O-H bond enthalpy in water is therefore $\dfrac{502 + 427}{2} = 464.5$ kJ mol⁻¹. …