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Bond Order and Enthalpy

Question 51Wh...

Question

Question 51

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

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Solution

All the similar bonds in a molecule do not have the same bond enthalpies. e.g., in $H_{2} O (H - O - H)$ molecule after the breaking of the first O-H bond, the second O-H bond undergoes some change because of a changed chemical environment.

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., $H_{2} O (g) \to H (g) + O H (g) △_{a} H_{1}^{0} = 502 k J m o l e^{- 1} O H (g) \to H + O (g); △_{a} H_{2}^{0} = 427 k J m o l^{- 1} Average O-H bond enthalpy = \frac{502 + 427}{2} = 464.5 k J m o l^{- 1}$

The bond enthalpies of O - H bond in $C_{2} H_{5} O H a n d H_{2} O$ are different because of the different chemical (electronic) environment around oxygen atom.

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