entropy of fusion

{{Short description|Increase in entropy when a solid melts}}

{{Refimprove|date=December 2009}}

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.{{sfn|Atkins|Jones|2008|p=236}} It is denoted as $\backslash Delta\; S\_\{\backslash text\{fus\}\}$ 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.

:

where {{tmath|\Delta H_\text{fus} }} is the enthalpy of fusion.

Since this is a thermodynamic equation, the symbol {{tmath|T}} 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

:$\backslash Delta\; G\_\{\backslash text\{fus\}\}\; =\; \backslash Delta\; H\_\{\backslash text\{fus\}\}\; -\; T\_f\; \backslash times\; \backslash Delta\; S\_\{\backslash text\{fus\}\}\; =\; 0,$

and the entropy of fusion is the heat of fusion divided by the melting point:

: $\backslash Delta\; S\_\{\backslash text\{fus\}\}\; =\; \backslash frac\; \{\backslash Delta\; H\_\{\backslash text\{fus\}\}\}\; \{T\_f\}$