13. How To Measure Disorder   Previous PageNext Page
       THIRD-LAW ENTROPIES AND COMMON SENSE

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.

 

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