Entopy change in a physical (phase) transformation can be determined particularly for evaporation of liquids at the boiling points, using Trouton’s rule.
According to this rule, the heat of vaporization (ΔHvap)in calories per mole divided by the boiling point of the liquid in Kelvin is a constant equal to 21 cal deg-1 mole-1 and is known as the entropy of vaporization.
ΔSVap = ΔHvap/Tb = 21 cal deg-1 mole-1
ΔHvap = Enthalpy change of vapourisation = Latent heat of vapourisation.
This equation is useful for estimating the molar heat of vaporization of a liquid of known boiling point. Substances that deviate from this rule are as follows:
i) Low boiling liquids such as hydrogen and Helium which boil only a little above 0K.
ii) Polar substances like water, alcohol which form hydrogen bonded liquids and exhibit very high boiling points as well as high ΔHvap.
iii) Liquids such as acetic acid whose molecules are partially associated in the vapor phase and possess very low entropy vaporization which is very much less than 21 cals/ mol/deg.
Those liquids that obey Troutons rule are said to behave ideally.