As already has been mentioned, the third-law entropies, Sí298,
(tabulated alongside heats of formation in appendix
2) were calculated entirely from calorimetric, or heat, measurements
and owe nothing to any theories about entropy and randomness. Nevertheless,
it is a tribute to Boltzmann's genius that we can explain these
relative entropy values so well using the concept of disorder and
what we know about the structures and physical properties of substances.
If Boltzmann did not exist, it would be necessary to invent him.
Let us look at matter through Boltzmann's eyes, and interpret measured
third-law entropies in terms of order and disorder. We can see several
clear-cut trends, and they all become obvious if we replace the
word "entropy" with "disorder."
A. The entropy (or disorder) increases whenever a liquid
or solid is converted to a gas (left). Sodium metal, liquid bromine,
and water provide examples.
The units of entropy are calories per-degree per mole, or entropy
units per mole, abbreviated e.u. mole-l. The molar entropies
(S0) of sodium, bromine, and water all increase as the
substances are vaporized because the gases are more disordered than
the condensed phases.