New trends in physics teaching, v.4; The ... - unesdoc - Unesco

Entropy and Information
15. If the free enthalpy of one of the two phases would be lower than in the other phase the absolute minimum
of the free enthalpy would be completely on the side of this phase and the two phases would not coexist.
16. The experimental values for the increase in entropy - with the exception of helium - are larger than the
theoretical values calculated from the information loss during evaporation. Since the liquid and its steam
have the same temperature at the boiling point, the momentum distribution should agree according to the
equipartition theorem. Certain degrees of freedom, e.g. rotation of the molecules can be ‘frozen’ in the liquid,
however. This leads to a possible increase of the thermal motion, of the angular momentum and similar
quantities in the gaseous phase as compared to the liquid and thus to a further increase in entropy. This
increase is especially large for polar, non-linear molecules. This is the reason for the large heat of evaporation
of water, which is 80 per cent higher than the simple theoretical value calculated here.
The reason for the exceptional properties of helium is the fact that the density of this liquid is much lower
than the size of the molecules would indicate. Due to quantum fluctuations, the average distance between
particles is 0.45 nm rather than 0.1 nm. The increase in volume during evaporation and the corresponding
increase in entropy is therefore much lower than for standard liquids.
17. Close to the melting point, a high concentration of vacancies occurs. In this case, the creation of additional
vacancies requires an energy lower than E.
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