Chemical Forums
Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: InquiringStudentMF on April 08, 2012, 09:27:05 PM
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Everything I have read on-line says that that entropy can never be negative. However, my chemistry book lists some aqueous solutions as having negative entropy such as OH-, F-, S2-). Does anyone know why this is? Any help is appreciated.
Thanks!!
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The entropy of the universe must always increase in a spontaneous process. So, apparently, if you have a fluoride solution the entropy of forming the solution is negative but the entropy of the rest of the universe must have been positive enough to outweigh the entropy decrease of the solution resulting in a net gain of entropy for the universe.
Entropy can't increase for every subsystem of the universe or else we could never, say, freeze water!
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The distinction that permits negative entropy values for aqueous solutions is that their entropy is not measured in an absolute sense. Rather, they are measured relative to the entropy of a solution of H+ ions, which is arbitrarily assigned an entropy value of 0.
When an aqueous ion is listed as having a negative entropy value, it means that the ion is imposing order on its surroundings, thereby decreasing the entropy. For solvated ions, this can be thought of as an ion's solvation shell. The water molecules become more ordered around the solvated ion because of electrostatic interactions.
The reason we don't see negative entropies in the solid state is because molecules in the solid state don't necessarily impose order on their surroundings like ions in the aqueous state do.
Source: http://www.madsci.org/posts/archives/may99/926112274.Ch.r.html (http://www.madsci.org/posts/archives/may99/926112274.Ch.r.html)