Entropy and Equilibrium

• According to the Second Law of Thermodynamics, a change takes place in the direction which corresponds to an increase in S_total.
• This is the case for the freezing of water below temperatures of 0^oC; however, above this temperature the S_total for the freezing of ice is negative and so the reverse occurs; the ice melts.
• When the S_total is zero, there will be no tendency for the reaction to go in either direction; it will be at equilibrium.
• This can be shown once again for the freezing and melting of water:
  
• At 273K, the S_total for the equilibrium reaction is:
  
  
• The reaction is at equilibrium; liquid water and ice exist together at 0^oC.
  At equilibrium, the S_total is equal to zero.
  
• Even though theoretically the equilibrium occurs at 0^oC, the temperature does not have to be exactly equal to this for it to occur.
• If the conditions happen to be standard, then the standard entropy change will be zero; however, the change rarely is zero at equilibrium.

Useful books for revision