standard molar entropy

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standard molar entropy

Postby AsthaPatel4B » Fri Feb 17, 2017 4:53 pm

I wanted to clarify on a concept I'm not 100% sure about. If temperature is 25 degrees Celsius, the molar entropy of a molecule increases with molar complexity and if the temperature is 0 degrees Celsius, the molar entropy of a molecule increases with molar mass. Is this true or is it the other way around? Please explain. Thank you!

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Re: standard molar entropy

Postby Amy_Bugwadia_3I » Fri Feb 17, 2017 7:40 pm

If you're given the piece of information that the temperature is 0 K, then you are looking for the residual entropy. Residual entropy has to do with degeneracy, W, which essentially means the number of microstates/atomic positions that a compound can exist in. Because the residual entropy is given by S = kb * ln(W), S increases as W increases. If it isn't specified that we are at 0 K, then we use standard molar entropy. The general trend with standard molar entropy is that the greater the molar mass of the compound, the higher the standard molar entropy.

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Re: standard molar entropy

Postby Amy_Shao_2D » Fri Feb 17, 2017 8:22 pm

Also the reason why we only look at residual entropy at 0K is because at 0K there is no vibrational entropy. I think the increasing molar mass has to do with generally, more molar mass means more complexity which means more interactions between the atoms which leads to vibrations but at 0K it just depends on the number of positional states.

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