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Entropy of fusion

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(Redirected from Standard entropy change of fusion) Increase in entropy when a solid melts
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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 Δ S fus {\displaystyle \Delta S_{\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.

Δ G fus = Δ H fus − T × Δ S fus < 0 , {\displaystyle \Delta G_{\text{fus}}=\Delta H_{\text{fus}}-T\times \Delta S_{\text{fus}}<0,}

where ⁠ Δ H fus {\displaystyle \Delta H_{\text{fus}}} ⁠ is the enthalpy of fusion. Since this is a thermodynamic equation, the symbol ⁠ T {\displaystyle 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 {\displaystyle T=T_{f}} so that

Δ G fus = Δ H fus − T f × Δ S fus = 0 , {\displaystyle \Delta G_{\text{fus}}=\Delta H_{\text{fus}}-T_{f}\times \Delta S_{\text{fus}}=0,}

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

Δ S fus = Δ H fus T f {\displaystyle \Delta S_{\text{fus}}={\frac {\Delta H_{\text{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

Notes

  1. Atkins & Jones 2008, p. 236.
  2. 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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