Trouton's rule

Trouton's rule is the empirical observation that many liquids have similar molar entropies of vaporization at their normal boiling points, typically about 85–88 J·mol⁻¹·K⁻¹.

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Entropy of vaporization: The molar entropy increase when a liquid becomes vapor under specified conditions. Trouton's rule compares this quantity across substances at their normal boiling points.
Entropy: A thermodynamic state function measuring energy dispersal and the number of accessible microscopic arrangements. The rule concerns a particular entropy change during liquid-to-vapor conversion.
Benzene: An aromatic hydrocarbon with molecular formula C₆H₆ and a boiling point near 80 °C. Its entropy of vaporization near the rule's typical range illustrates ordinary behavior.
Trouton's rule exceptions: Substances whose molar entropies of vaporization differ markedly from Trouton's typical range. Strong association and unusual molecular behavior produce the clearest departures.
Enthalpy of vaporization: The heat required to convert one mole of liquid into vapor at constant pressure. At a liquid's boiling point, this enthalpy divided by temperature gives its entropy of vaporization.
Enthalpy: A thermodynamic state function equal to internal energy plus pressure times volume. Vaporization enthalpy helps determine the entropy change at boiling.
Water: A molecular compound with formula H₂O that is liquid near room temperature at ordinary pressure. Hydrogen bonding makes its vaporization entropy depart from the simplest Trouton estimate.
Kistiakowsky's rule: An empirical refinement of Trouton's rule that accounts for molecular complexity in vaporization entropy. It adjusts the simple near-constant estimate for structural differences among liquids.
Normal boiling point: The temperature at which a liquid's vapor pressure equals one standard atmosphere. The rule compares liquids at this shared pressure condition.
Phase transition: A transformation between distinct physical states of matter. Vaporization is the phase transition whose entropy change the rule compares.