Standard molar entropy vs. residual entropy

Boltzmann Equation for Entropy:

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Christina Cen 2J
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Joined: Sat Jul 22, 2017 3:01 am

Standard molar entropy vs. residual entropy

Postby Christina Cen 2J » Wed Feb 14, 2018 1:35 pm

I'm a bit confused about the distinction between these two.

In the winter 2013 midterm 5B, it tells us that
CH3F<CH2F2<CHF3<CF4 in terms of increasing standard molar entropy. I think this is because F is more massive, so that it has more elementary particles.

But then, doesn't CH3F have more possible arrangements than CF4, which only has one possible arrangement, meaning CH3F has a higher residual entropy?

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Joined: Sat Jul 22, 2017 3:00 am

Re: Standard molar entropy vs. residual entropy

Postby TarynLane2J » Wed Feb 14, 2018 1:57 pm

I think that residual entropy is the entropy left over once you cool a substance to absolute zero (or as close as you can get to absolute zero). So this would mean that all substances, except for perfect crystals, have some amount of residual entropy because the molecules can have multiple orientations or something. But standard molar entropy is different because it's the entropy that one mole of that substance has under standard conditions (298K, 1 atm, etc.) I don't know if this completely answers your question but hope it helps

Lourick Bustamante 1B
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Joined: Wed Nov 16, 2016 3:02 am

Re: Standard molar entropy vs. residual entropy

Postby Lourick Bustamante 1B » Wed Feb 14, 2018 2:10 pm

Residual entropy is the measure of how many different arrangements a molecule can have. Molar entropy is a measure of a molecule's chaos. Both of your statements above are true, and they don't oppose one another. You can compare two molecules, and have one molecule have higher residual entropy AND a lower molar entropy. Just because a molecule has more residual entropy compared to another doesn't necessarily mean that it has to have a higher overall molar entropy as well.

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