Bond Enthalpies

[AlyssaBei_1F]

In lecture on Friday, Dr. Lavelle said bond enthalpies of diatomic molecules are accurate, but all others are averages of many different molecules. Can someone give me an example of what he means?

[Samantha Kwock 1D]

Bond enthalpies of diatomic molecules are averages because the bond is identical in all cases, with only two atoms of the same element. For example, with H2 or Cl2. However, for other bond enthalpies, the other chemical species bonded to the atoms affect its energy. For example, for a C-H bond, the bond enthalpy would be slightly different for a C-H bonded to another C-H group vs a C-H bonded to an Oxygen. This is why their bond enthalpies are averages of many different molecules.

[Vicky Lu 1L]

To add, this is why bond enthalpy is the least accurate of the three methods since its value is made up from the average of data of many different molecules of different reactions.