Advanced #64

Water has a high entropy of vaporization

The entropy of vaporization of water deviates significantly from Trouton's rule.

Scientific Explanation

Trouton’s rule states that the molar entropy of vaporization of most liquids at their boiling point is approximately 85 to 88 joules per mole per kelvin. Water deviates sharply upward at 109 joules per mole per kelvin — a sign that liquid water is unusually ordered.

The entropy of vaporization measures the increase in disorder when transitioning from liquid to gas. A high value means the liquid is relatively ordered and loses a great deal of order upon vaporizing. In water’s case, the extensive hydrogen-bond network creates an exceptionally strong short-range order for a liquid. When a molecule enters the gas phase, it loses not only positional binding but also the orientational order imposed by the directional bonds.

Ethanol also shows an elevated entropy of vaporization (110 J/mol K) because it likewise forms hydrogen bonds. Benzene, with only dispersion forces, falls right at the Trouton value of 87 J/mol K. Acetic acid has an unusually low value of 61 J/mol K because it vaporizes as a dimer, losing less order than expected.

Entropy of Vaporization - Trouton's Rule Deviation Bar chart comparing the entropy of vaporization of water (109 J per mol per K) with the Trouton rule prediction of approximately 88 J per mol per K, plus ethanol (110) and acetic acid (61). Water significantly exceeds the Trouton rule because of strong hydrogen bonding. ΔSᵛ₁₀ (J mol⁻¹ K⁻¹) 0 88 120 Trouton rule (~88) 109 Water 110 Ethanol 87 Benzene 61 Acetic Acid Entropy of Vaporization — Trouton's Rule
Entropy of vaporization of various liquids. The dashed line marks the Trouton value of 88 J/mol K.

Everyday Relevance

Water’s high entropy of vaporization has consequences for weather and the water cycle. When water evaporates, the vapor carries away not only energy but also a large amount of “order information.” Upon condensation in clouds, this entropy is released, contributing to the warming of surrounding air and driving convection currents — one of the driving forces behind thunderstorm formation and tropical cyclones.