Entropy of fusion explained

In thermodynamics, the entropy of fusion is the increase in entropy when melting a solid substance. This is almost always positive since the degree of disorder increases in the transition from an organized crystalline solid to the disorganized structure of a liquid; the only known exception is helium. It is denoted as

\Delta S _{fus} and normally expressed in joules per mole-kelvin, J/(mol·K).

A natural process such as a phase transition will occur when the associated change in the Gibbs free energy is negative.

\Delta G _{fus} = \Delta H _{fus} - T x \Delta S _{fus} < 0 , where is the enthalpy of fusion.Since this is a thermodynamic equation, the symbol refers to the absolute thermodynamic temperature, measured in kelvins (K). Equilibrium occurs when the temperature is equal to the melting point T = T _{f} so that \Delta G _{fus} = \Delta H _{fus} - T _{f} x \Delta S _{fus} = 0 , and the entropy of fusion is the heat of fusion divided by the melting point:\Delta S _{fus} = (\Delta H _{fus})/(T _{f})

Helium

Helium-3 has a negative entropy of fusion at temperatures below 0.3 K. Helium-4 also has a very slightly negative entropy of fusion below 0.8 K. This means that, at appropriate constant pressures, these substances freeze with the addition of heat.

See also

• Entropy of vaporization