Entropy question

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From: Geoff Ruth (gruth@leadershiphigh.org)
Date: Thu Feb 17 2005 - 08:53:18 PST 

Message-Id: <p06110403be3a7b3d4220@[192.168.1.158]>
Date: Thu, 17 Feb 2005 08:53:18 -0800
From: Geoff Ruth <gruth@leadershiphigh.org>
Subject: Entropy question

My AP Chem kids have posed a question which I can't answer: why some
standard entropies are either 0 (in the case of H+(aq)) or negative
(such as F-(aq) or OH-(aq)).

One way of stating the 3rd law of thermodynamics is that the entropy
of a pure crystalline substance at 0K is 0. But if we can't reach
zero K, then it seems like all entropies should be positive numbers.
How then does it make sense to define S(H+) as 0, or to have some
substances with negative entropies?

I fear that the answer to this question is going to involve a lot of
abstract physical chemistry and calculus. Is there a conceptually
logical answer to how some standard entropies can be 0 or negative?

Thanks,
Geoff
Leadership HS

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