Editing 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 [[Entropy (order and disorder)|degree of disorder]] increases in the [[phase transition|transition]] from an organized [[crystal]]line solid to the disorganized structure of a [[liquid]]; the only known exception is [[helium]].{{sfn|Atkins|Jones|2008|p=236}} It is denoted as <math>\Delta S_{\text{fus}}</math> and normally expressed in [[joules]] per [[Mole (unit)|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>\Delta G_{\text{fus}} = \Delta H_{\text{fus}} - T \times \Delta S_{\text{fus}} < 0,</math>
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 [[kelvin]]s (K).
 
Equilibrium occurs when the temperature is equal to the [[melting point]] <math>T = T_f</math> so that 
:<math>\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>\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.{{sfn|Ott|Boerio-Goates|2000|pp=92–93}}

==See also==
*[[Entropy of vaporization]]

==Notes==
{{Reflist}}

==References==
*{{Citation |last1=Atkins |first1=Peter |last2=Jones |first2=Loretta |year=2008 |title=Chemical Principles: The Quest for Insight |edition=4th |publisher=W. H. Freeman and Company |isbn=978-0-7167-7355-9 |page=236}}
*{{Citation |last1=Ott |first1=J. Bevan |last2=Boerio-Goates |first2=Juliana |year=2000 |title=Chemical Thermodynamics: Advanced Applications |publisher=Academic Press |isbn=0-12-530985-6}}

{{DEFAULTSORT:Entropy Of Fusion}}
[[Category:Thermodynamic entropy]]
[[Category:Thermodynamic properties]]