Question

Question:
Why is there a discrepancy between an enthalpy of reaction obtained from calorimetry and one obtained from bond energies?

Answer 1

1. Bond energies use to neglect whether the compound is gaseous, liquid or solid, but in a reaction enthalpy it's quite visible. As well, molecules store heat as rotations, vibrations... that bond energies don't estimate properly.
2. The bond energy changes due to its environment, so, they have to average all of the different ones and so therefore they are not exact, but rather approximate values.
3. Bond energy of a bond is different in different molecules which causes variations in calculating enthalpy of reaction.For example,$C-H$ bond energy in $CH_4$ and $C_2H_6$ are not same.But we use the same values of this bond in every compound containing $C-H$ bond.