Entropy of fusion

In <a href="/facts/Thermodynamics/q4hIx3yP">thermodynamics</a>, the entropy of fusion is the increase in <a href="/facts/Entropy/s52IXeFz">entropy</a> when <a href="/facts/Melting/zhhDBo5V">melting</a> a <a href="/facts/Solid/hj5UsXno">solid</a> substance. This is almost always positive since the <a href="/facts/Entropy_(order_and_disorder)/2aXkcJHM">degree of disorder</a> increases in the <a href="/facts/Phase_transition/eLP2BY8R">transition</a> from an organized <a href="/facts/Crystal/e2R2mk3E">crystalline</a> solid to the disorganized structure of a <a href="/facts/Liquid/t9GkLWzt">liquid</a>; the only known exception is <a href="/facts/Helium/yRul93TG">helium</a>. It is denoted as 
 
 
 
 Δ
 
 S
 
 fus
 
 
 
 
 {\displaystyle \Delta S_{\text{fus}}}
 
 and normally expressed in <a href="/facts/Joules/9ndq9jD2">joules</a> per <a href="/facts/Mole_(unit)/kCMFwCc0">mole</a>-<a href="/facts/Kelvin/E3trzC4C">kelvin</a>, J/(mol·K).
A natural process such as a phase transition will occur when the associated change in the <a href="/facts/Gibbs_free_energy/bvenDuzh">Gibbs free energy</a> 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 <a href="/facts/Enthalpy_of_fusion/eLJIURcp">enthalpy of fusion</a>.
Since this is a thermodynamic equation, the symbol ⁠
 
 
 
 T
 
 
 {\displaystyle T}
 
⁠ refers to the absolute <a href="/facts/Thermodynamic_temperature/JXeJJPa6">thermodynamic temperature</a>, measured in <a href="/facts/Kelvin/E3trzC4C">kelvins</a> (K).
Equilibrium occurs when the temperature is equal to the <a href="/facts/Melting_point/MY0w0h0k">melting point</a> 
 
 
 
 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}}}}

Entropy of fusion open-in-new

Entropy of fusion