Fusion and vaporization increase entropy, equal to enthalpy change divided by temperature.
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At a phase transition such as melting or boiling, temperature remains constant while the system absorbs latent heat. The entropy change at the transition temperature is delta S equals delta H divided by T, since the transition is reversible at equilibrium. Vaporization produces a much larger entropy increase than fusion, because a gas occupies far more volume and has far more microstates than a liquid. Trouton's rule notes that many liquids have a molar entropy of vaporization near 88 J per mole per kelvin, a useful empirical guide. Phase change entropies are essential for calculating absolute entropies from calorimetric data.
Key ideas: At the transition temperature, delta S equals delta H divided by T, with T in kelvin and delta H in joules. Boiling gives larger values than melting since the volume change is much greater. Trouton's rule of about 88 J per mole kelvin works for non-associated liquids.
Common mistakes and connections: Water's value is higher than 88, around 109, because hydrogen bonding orders the liquid. Do not apply the formula away from the transition temperature, where the process is not reversible. These values are used to build absolute entropy at high temperature by summing contributions.
Entropy of phase change = latent heat / T. Vaporization has much larger entropy jumps than fusion, near 85 to 110 J/mol/K (Trouton's rule) for many liquids. The interactive lesson shows this as a live animation or chart.
Water vaporizes at 373 K with delta H of about 40.7 kJ per mole, so delta S is about 40.7 kJ divided by 373 K, which is near 109 J per mole per kelvin.
Fusion and vaporization increase entropy, equal to enthalpy change divided by temperature.
Entropy of Phase Change is a intermediate topic in Thermodynamics & Energetics. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Thermodynamics & Energetics first: Third Law of Thermodynamics, Standard Molar Entropy, Entropy Changes in Reactions.
The next topics in Thermodynamics & Energetics are Gibbs Free Energy and Gibbs Energy and Spontaneity.
Yes. The interactive lesson includes a self-check question on Entropy of Phase Change, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Entropy of Phase Change is topic 36 of 91 in Thermodynamics & Energetics.