Trouton's Rule
Trouton's rule states that the molar entropy of vaporization (ΔSvap) for many pure liquids is approximately constant and its value is around 85 to 88 JK-1mol-1 at their boiling points.
∆Svap ≅ 88 JK-1mol-1
This approximate value of entropy of vaporization (ΔSvap) around 85 JK-1mol-1 indicates that the change in entropy during evaporation is relatively the same in different liquids, despite their different molecular structures.
By using Trouton's rule, enthalpy of vaporization (∆Hvap) of a liquid can be calculate when its boiling point (Tb) is known.
The entropy of vaporization of toluene, benzene and chloroform is approximately 87.30, 89.45 and 87.92 JK-1mol-1 respectively which obey the Trouton's rule.
Limitations of Trouton's Rule
Trouton's rule is mostly applicable for non-polar liquids or those having minimum intermolecular forces, whereas structured liquids or those with strong hydrogen bonding may deviate significantly from the expected values.
Entropy of vaporization of water is lower than expected value due to strong hydrogen bonding, which reduces the free movement in the liquid phase. Similarly, ethanol also shows positive deviation from the expected value because of its hydrogen bonding interactions. Formic acid show negative deviation from Trouton's rule, indicating an orderly structure (form dimers) in the gas phase, which reduces the expected entropy.
✍︎ Remember
Positive Deviation: Liquids with strong hydrogen bonding (e.g., water, ethanol), where the entropy of vaporization is higher than expected shows positive deviation from Trouton's rule.
Negative Deviations: Substances (e.g. formic acid) where molecular interactions in the gas phase lead to a more ordered structure and reduce the expected entropy gain during vaporization shows negative deviation from Trouton's rule.
