Averages of bond enthalpies

[Allison Li 2F]

If the molecule is not diatomic, the bond enthalpies are averages from many different molecules, which makes this method the least accurate method to calculate enthalpy. I'm just wondering why they are averages. Thanks!

[Henry_2A]

Hi!

I believe it's because the strength of the bond depends on the environment of where the bond is, meaning the nearby elements.

[Anna Wilborn-1L]

For his exmaple of a carbon carbon double bond, it is an average because there are many organic molecules were carbon carbon double bonds exist. For each molecule, the each carbon may have additional atoms bond to them which differ by organic molecule. The other atoms and their bonds to the carbons effect the bond enthalpy of the carbon carbon double bond slightly. When you look on a table for the bond enthalpy of a carbon carbon double bond, it is an average of the slightly different bond enthalpies.

[Mandy Mg 2J]

I believe Professor Lavelle used a carbon carbon double bond as an example to explain why bond enthalpies of non-diatomic molecules were averages in his lecture. Essentially, carbon carbon double bonds exist in many organic molecules and have various atoms bonded to them, and these additional bonds slightly affect the bond enthalpy of the double bond. So to find the bond enthalpy of the carbon carbon double bond, averages of its bond enthalpies in many organic molecules must be accounted for. Because bond enthalpies are basically an overall average of other averages, other methods such as enthalpy of formation are much more accurate when finding reaction enthalpies.