## positional residual entropy

Simrina Desar Dis 1H
Posts: 49
Joined: Fri Sep 29, 2017 7:04 am

### positional residual entropy

How is positional residual entropy different from regular entropy? Would this affect any calculations?

Nishma Chakraborty 1J
Posts: 51
Joined: Fri Sep 29, 2017 7:04 am

### Re: positional residual entropy

Residual entropy relates to the entropy of a substance as T≈0 kelvin. There are fewer degenerative states at that temperature. I think residual entropy is calculated using the absolute entropy equation, S=kb*ln(W). Regular (or change in) entropy is calculated using the change in pressure, change in temperature, or change in volume, equation using a constant value. For example: deltaS=Cv*ln(Tf/Ti).

Hope this helps! :)
Last edited by Nishma Chakraborty 1J on Sun Feb 04, 2018 9:42 pm, edited 1 time in total.

CameronJohari1J
Posts: 66
Joined: Fri Sep 29, 2017 7:06 am
Been upvoted: 1 time

### Re: positional residual entropy

Positional residual entropy is basically the absolute entropy and the equation was derived statically. The formula is S=Kb*lnW. Regular entropy to my understanding is usually delta S of the system.

Xihui Yin 1I
Posts: 40
Joined: Fri Sep 29, 2017 7:06 am

### Re: positional residual entropy

Recall the third law - entropy in a perfect crystal at 0K is 0. This means that at 0K there is no thermal entropy, hence only entropy remaining is positional residual entropy, which is based off the "randomness" of the positions of the molecules/atoms arranged. For example a non-polar molecule has more positional residual entropy than a polar one, because its lack of electric dipole means that no intermolecular arrangement is favoured over the other, hence it can take on random positions.

This thus means that you can use the equation listed by the replies above to calculate positional entropy, because the equation takes into account both thermal and positional entropies and positional can be calculated once thermal is excluded.

Xihui Yin 1I
Posts: 40
Joined: Fri Sep 29, 2017 7:06 am

### Re: positional residual entropy

Xihui Yin 1I wrote:Recall the third law - entropy in a perfect crystal at 0K is 0. This means that at 0K there is no thermal entropy, hence only entropy remaining is positional residual entropy, which is based off the "randomness" of the positions of the molecules/atoms arranged. For example a non-polar molecule has more positional residual entropy than a polar one, because its lack of electric dipole means that no intermolecular arrangement is favoured over the other, hence it can take on random positions.

This thus means that you can use the equation listed by the replies above to calculate positional entropy, because the equation takes into account both thermal and positional entropies and positional can be calculated once thermal is excluded.

My bad, apparently the equation already represents the positional entropy at 0K