Positional/Residual Entropy

[Alison Trinh 1E]

What makes positional/residual entropy different from entropy in general?

[Eugene Chung 3F]

residual entropy is the left-over entropy at 0K. (perfect crystal). The entropy in this case would only depend on the ways in which molecule can position/orient themselves.

[Venus_Hagan 2L]

If a molecule consists of all the same atoms or the atoms can be in multiple places within the molecule (resonance) then it has no positional entropy and at 0 Kelvins, the molecule would have no entropy. Ex BF3. If the molecule has different molecules like COF2, then the oxygen can be in multiple places and thus has positional entropy. At 0 Kelvin, it would still have some entropy.

[rohun2H]

A molecule at 0 K will have 0 residual entropy, but only if it is perfectly ordered will the molecule have 0 entropy, if it is not perfectly ordered then some entropy will still be present.