Accurate Bond Enthalpies

[Michelle Zhang 1J]

Dr. Lavelle mentioned in lecture that the bond enthalpies of diatomic molecules are accurate. Does that mean the bond enthalpy, for example, between Cl-Cl only in Cl2 is accurate? Or does that mean the bond enthalpy for Cl-Cl in any molecule that contains two Cl's connected together is accurate?

[Stephen_Kim_1D]

He means that only diatomic molecule bond enthalpies are exactly accurate (Cl-Cl bond enthalpy is exactly accurate to Cl2). A C-H bond enthalpy, however, would not be exactly accurate to the energy required to break a bond in CH4, for example (the C-H bond enthalpy is an average between many types of hydrocarbons). So your first statement is right and your second is wrong.

[Talia Tam 3L]

I think that he means only bonds between two atoms of the same type in a molecule with only those two atoms are accurate. The example of Cl2 that you gave is correct.

[Allison Li 2F]

I think he means only diatomic molecule bonds are accurate. The Cl-Cl bond in Cl2 is only for diatomic Cl2, whereas Cl-Cl bonds in other molecules are influenced by the bond enthalpies, making them "averages".

[Barbara Soliman 1G]

It is only diatomic molecules whose bond enthalpies are accurate. For a Cl-Cl bond in any molecule, you are still getting "pulls" from the other atoms in the molecules, thus an average would be needed across all molecules that include a Cl-Cl bond. This would make it not as accurate. Hope this helps!

[Molly McAndrew 1 1H]

Does this only apply to the 7 specific diatomic molecules, or are there any other examples that are considered 100% accurate?

[Kaitlyn Bateman 1L]

Because molecules with multiple bonds are difficult to measure the bond energy of each individual bond, those are averages and would be less accurate. If it a simple diatomic molecule, then it can be measured and it would be accurate.