Why is it possible to have an accurate value of the energy of a hydrogen-hydrogen bond, but not of a carbon-oxygen bond?
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?
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
   |
   O
   |
   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.
To understand why it is possible to have an accurate value of the energy of a hydrogen-hydrogen bond, but not of a carbon-oxygen bond, we need to consider some key factors.
1. Molecular Complexity: The energy of a chemical bond depends on the specific molecular arrangement and environment. Hydrogen-hydrogen bonds are relatively simple, involving two hydrogen atoms sharing a pair of electrons. As a result, the energy associated with this bond can be accurately calculated and measured.
On the other hand, carbon-oxygen bonds can be more complex. Carbon and oxygen can form different types of bonds, such as single bonds (C-O), double bonds (C=O), or even triple bonds (C≡O). These multiple bonding possibilities introduce variations in bond length and strength, making it challenging to obtain a single, accurate value for a carbon-oxygen bond energy.
2. Bond Variation: The existence of multiple bonding possibilities affects the overall bond energy and stability of carbon-oxygen bonds. In the case of carbon dioxide (CO2), it has a linear molecular structure with two carbon-oxygen double bonds. Each double bond has a different bond energy compared to a single bond or triple bond.
The variation in bond energy arises from differences in bond length and bond strength. Double bonds are shorter and stronger than single bonds, containing more shared electrons between carbon and oxygen. This additional electronic sharing results in increased bond energy.
The complexity of accurately determining the energy of a carbon-oxygen bond arises from the fact that it can exist in various molecular contexts, each with its own specific arrangement and electronic distribution.
In summary, the accurate determination of bond energy depends on the simplicity and uniformity of the bonding situation. The single, uniform nature of hydrogen-hydrogen bonds allows for accurate predictions and measurements. On the other hand, the multiple bonding possibilities and molecular arrangements associated with carbon-oxygen bonds lead to variations in bond energy, making it more challenging to obtain a single, definitive value.