Why is it possible to have an accurate value of the energy of a hydrogen-hydrogen bond, but not of a carbon-oxygen bond?

Question

I know carbon and oxygen can form both double and single bonds, and hydrogen can only form single bonds. But apparently C=O has two different bond energies. It's different when it is in CO₂... Any hints on explaining this?

Answer 1

The bond energy of atoms usually is spoken of in terms of average bond energy becuase the bond energy of C=O in a hydrocarbon type molecule is influenced by what is around it. For example, in the R-COOH carboxylic acid, we draw it as
O=C-R
&nbsp&nbsp |
&nbsp&nbsp O
&nbsp&nbsp |
&nbsp&nbsp H
but remember that the true picture is one of resonance and the O=C bond isn't exactly a double bond and the other one isn't an exact single bond. Then if we stick Cl or other groups on the molecule that changes the picture again. So bond energies really are averages of several C=O bonds in various molecules.
O=C=O, on the other hand, isn't influenced by "other" atoms attached to it.