What is meant by the term average bond enthalpy? Why is there difference in bond enthalpy of O-H bond in ethanol `(C_(2)H_(5)OH)` and water `(H_(2)O)` ?

Explanation

The average of the bond dissociation enthalpies of different bonds in polyatomic molecules (which contain more than one bond of the same type) is called average bond enthalpy.
The `O-H` bonds in ethanol, `H-overset(H)overset(|)underset(H)underset(|)C-overset(H)overset(|)underset(H)underset(|)C-OH` and water `H-underset(H)underset(|)O` donot have similar environment around oxygen atom. Hence, their `O-H` bond enthalpies are different.