Residual Entropy

[Bronson Mathos 1H]

Hello, I was a bit lost doing the first Sapling problem of week 5 and 6 involving determining which molecules have residual structure and I was wondering what exactly I should look for and what exactly residual entropy is?

[chinmayeec 2H]

Residual entropy is basically entropy that exists at T=0 and arises from the different positions that atoms can take in a molecule. For example, N2O has residual entropy since the atoms can be arranged as O-N-N or N-N-O. However, a diatomic molecule like Cl2 does not have residual entropy. Molecules that are symmetrical will not have residual entropy, basically.

[Xinyu Li 1C]

From my understanding, residual entropy ignores thermal motion and can be calculated by using the Boltzmann equation (S = Kb * ln W).

[t_rasul2I]

Residual entropy is referring to the entropy caused by the permutations of molecular arrangements. If a molecule is like o-o where there can only be one permutation, there is no residual entropy. Moreover, this is because the number of arrangements to the power of the molecules would equal 1 and ln of 1 is 0 which would give an overall S value of 0.

[Zoe FC 1C]

To add on to this, I think residual entropy is the entropy that remains after molecules stop moving (which happens when T=0 Kelvin) and so the residual entropy comes from disorder in the structure of the molecules.

[Brennan McGurrr 3C]

Residual entropy is more used for comparison for questions in this class. Basically, the more complex a molecule is, the more residual entropy it has. Questions will ask you which one has more or less.