9.23 (molar enthalpy between molecules) [ENDORSED]

[Angela 1K]

The question asks: Which would you expect to have a higher molar entropy at T = 0, single crystals of BF₃ or of COF₂? Why?

The answer key says COF₂, but I don't really understand its explanation for why. Could anyone rephrase the explanation or perhaps explain it in a different way in hopes that I'd be able to better understand it? Thanks.

[Chem_Mod]

Here we more appropriately make use of the statistical definition of entropy:



where W = (# configurations)^{# components in the system}. Since the molecule COF₂ can be arranged in more unique orientations (configurations) than BF₃, it has more entropy in a single crystal (no defects) at T = 0. This is called "residual entropy."

[Natalie LeRaybaud 1G]

In addition, molecules that are considered more complex tend to have greater entropies. Since CoF2 can be arrange in more different orientations when compared to BF3, it is considered more complex and this has a greater entropy.