Answer:
You can only use bond enthalpies directly if everything you are working with is in the gas state.
Explanation:
If you have one or more liquids present, you need an extra energy term to work out the enthalpy change when you convert from liquid to gas, or vice versa
Of course, bond energies exist for liquids and solids. But we measure it for gas only, because there there is no influence from the solvent. We work at low pressure to be able to use the main hypothesis of perfect gas : one molecule exists as if it were alone, free of the interaction of other molecules. Thus the energy we measure by breaking the molecule is as close to the real bond energy as possible.Hope it helps, and hope it's right !
