Gibbs Free Energy and Phase Separation: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • For two phases to mix, the Gibbs free energy of the mixture must be lower than the sum of the Gibbs free energies of the starting phases.
  • If a mixture can obtain a lower Gibbs free energy by splitting into two phases, then phase separation occurs.
  • On a plot of Gibbs free energy versus mole fraction for a binary mixture, if a tangent line can be drawn between two phase compositions to yield a lower Gibbs free energy than obtained for complete mixing, then those two phases will be in equilibrium.
  • The lever rule can be used on a G vs. mole fraction plot for a binary mixture to determine the amounts of each phases when two phases are in equilibrium.
  • When positive deviations from Raoult’s law are large enough, a maximum-pressure azeotrope (minimum temperature azeotrope) exists. Large enough deviations result in phase separations.
From studying this module, you should now be able to:
  • Explain how phase stability is related to minimizing Gibbs free energy at constant temperature and pressure.
  • Predict phase separation from Gibbs free energy dependence on composition.
  • Interpret the meaning of large activity coefficients and what they indicate about phase separations.
  • Calculate equilibrium phase compositions, given activity coefficients and some phase compositions.


Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder