What is bond enthalpy? Explain bond enthalpy in polyatomic molecules like $H_2O$.

(N/A) Bond enthalpy is defined as the amount of energy required to break one mole of bonds of a particular type between two atoms in the gaseous state.
The unit of bond enthalpy is $kJ \ mol^{-1}$.
For diatomic molecules,it is the bond dissociation enthalpy. For example,for $H_2$:
$H_{2(g)} \rightarrow H_{(g)} + H_{(g)} ; \Delta_{a} H^{\ominus} = 435.8 \ kJ \ mol^{-1}$
In polyatomic molecules like $H_2O$,the energy required to break the two $O-H$ bonds is not the same due to the change in the chemical environment after the first bond breaks:
$H_2O_{(g)} \rightarrow H_{(g)} + OH_{(g)} ; \Delta_{a} H_{1}^{\ominus} = 502 \ kJ \ mol^{-1}$
$OH_{(g)} \rightarrow H_{(g)} + O_{(g)} ; \Delta_{a} H_{2}^{\ominus} = 427 \ kJ \ mol^{-1}$
The average bond enthalpy for $O-H$ in water is the mean of these values: $(502 + 427) / 2 = 464.5 \ kJ \ mol^{-1}$.
This difference in energy values occurs because the electronic environment of the $O-H$ bond changes once one hydrogen atom is removed.