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?

Explanation

All the similar bonds in a molecule do not have the same bond enthalpies. E.g. In undergoes some change becuase of charged chemical envrionment.
Therefore, in polyatomic molecules the term mean or average bond enthalpy is used. It is obtained by dividing total bond dissociation enthalpy by the number of bonds broken.
e.g. `H_(2)O to H(g) + OH(g)`
`Delta_(a)H_(1)^(@)=502 kJ mol^(-1) OH(g) to H + O(g)`
`Delta_(a)H_(2)^(@)=427 kJ mol^(-1)`
Average O-H bond enthalpy =`(502+427)/(2) = 464.5 kJ mol^(-1)`
The bond enthalpies of O-H bond in `C_(2)H_(5)OH` and `H_(2)O` are different because of the different chemical (electronic) environment around oxygen atom.
`underset((C_(2)H_(5)OH))(H-overset(H)overset(|)underset(H)underset(|)(C)-overset(H)overset(|)underset(H)underset(|)(C)-O)-H, " "underset((H_(2)O))(H-O-H)`