Physics:Entropy of fusion
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.[1] It is denoted as [math]\displaystyle{ \Delta S_{\text{fus}} }[/math] 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.
- [math]\displaystyle{ \Delta G_{\text{fus}} = \Delta H_{\text{fus}} - T \times \Delta S_{\text{fus}} \lt 0, }[/math]
where [math]\displaystyle{ \Delta H_\text{fus} }[/math] is the enthalpy of fusion. Since this is a thermodynamic equation, the symbol [math]\displaystyle{ T }[/math] refers to the absolute thermodynamic temperature, measured in kelvins (K).
Equilibrium occurs when the temperature is equal to the melting point [math]\displaystyle{ T = T_f }[/math] so that
- [math]\displaystyle{ \Delta G_{\text{fus}} = \Delta H_{\text{fus}} - T_f \times \Delta S_{\text{fus}} = 0, }[/math]
and the entropy of fusion is the heat of fusion divided by the melting point:
- [math]\displaystyle{ \Delta S_{\text{fus}} = \frac {\Delta H_{\text{fus}}} {T_f} }[/math]
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.[2]
See also
Notes
- ↑ Atkins & Jones 2008, p. 236.
- ↑ Ott & Boerio-Goates 2000, pp. 92–93.
References
- Atkins, Peter; Jones, Loretta (2008), Chemical Principles: The Quest for Insight (4th ed.), W. H. Freeman and Company, p. 236, ISBN 978-0-7167-7355-9
- Ott, J. Bevan; Boerio-Goates, Juliana (2000), Chemical Thermodynamics: Advanced Applications, Academic Press, ISBN 0-12-530985-6
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