Page 44 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:36 9in x 6in b985-ch04 Elementary Physical Chemistry

Chapter 4

The Third Law of Thermodynamics

Up to now we could describe only differences in H, E, i.e. ∆H,∆E, but
not absolute values of H or E. In the case of entropy, on the other hand,
we can describe the concept in absolute values. What makes this possible
is the Third Law of Thermodynamics. There are several statements of the
ThirdLaw.The oneweareusinghereis
The entropy of a perfect crystalline solid at absolute zero is zero.

Comments: Note the restriction perfect. Perfect means that the system
is in perfect thermodynamic equilibrium. But how can you tell
whether this is so experimentally at these very low temperatures?
You can’t.

What can be done is to measure the heat capacity over a temperature
range around 0 K and compare the results with the statistical entropy
calculated by statistical mechanics, which assumes perfect thermody-
namic equilibrium. If the entropies of the calorimetric measurements
and the statistical values agree, the crystalline solid is “perfect”.
Otherwise, the solid is imperfect.

There are many systems which are not perfect: CO, N 2 O, H 2 Oare
examples. All cases, observed so far, can be explained or rationalized. For
example, CO has a very small dipole moment. At absolute zero the dipoles
should all point in the same direction. But that does not happen. As the
solid cools, there are some “frozen-in” structures, with dipole moments
pointing in opposite directions. The low temperature crystalline solid is
then not in “perfect” thermodynamic equilibrium. Similar arguments have
been presented for the other exceptions.

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