Vapor-Liquid Equilibrium for Non-Ideal Solutions: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • When a vapor mixture is cooled or its pressure increases, both components condense when the dew point is reached.
  • Bubble pressure is the pressure where the first bubble of vapor forms as the pressure above a liquid decreases at constant temperature.
  • Bubble temperature is the temperature where the first bubble of vapor forms as the temperature of a liquid increases at constant pressure.
  • Dew pressure is the pressure where the first drop of liquid forms as the pressure of a vapor increases at constant temperature.
  • Dew temperature is the temperature where the first drop of liquid forms as the temperature of a vapor decreases at constant pressure.
  • A positive deviation from Raoult’s law means the pressure above the liquid mixture is greater than that calculated from Raoult’s law because the interactions between A and B molecules are weaker than the average A-A and B-B interactions.
  • A negative deviation from Raoult’s law means the pressure above the liquid mixture is less than that calculated from Raoult’s law because the interactions between A and B molecules are stronger than the average of A-A and B-B interactions.
  • An azeotrope corresponds to a maximum or minimum in the pressure versus mole fraction plot. At an azeotrope, the liquid and vapor compositions are identical, which makes separations by distilliation difficult.
  • A system with maximum-pressure azeotrope will have a minimum-temperature azeotrope.
  • A system with minimum-pressure azeotrope will have a maximum-temperature azeotrope.
From studying this module, you should now be able to:
  • Explain the meaning of positive and negative deviations from Raoult’s law and explain why they occur in terms of molecular interactions.
  • Calculate vapor-liquid equilibrium using the modified Raoult’s law.
  • Calculate activity coefficients from vapor-liquid equilibrium data.
  • Determine what phases are present at specified conditions, given a phase diagram.
  • Calculate pressure bubble points and dew points using the modified Raoult’s law and a model for the liquid phase activity coefficients.
  • Calculate temperature bubble points and dew points using the modified Raoult’s law and a model for the liquid phase activity coefficients.
  • Calculate activity coefficients for liquid phases from excess Gibbs free energy expressions.
  • Calculate Gibbs free energy of mixing and excess Gibbs energy from VLE equilibrium compositions.
  • Construct a P-x-y diagram at a given temperature for two miscible liquids that form a non-ideal solution, given saturation pressures at that temperature and equations that model the non-ideal behavior of the liquid solution.
  • Construct a T-x-y diagram at a given pressure for two miscible liquids that form a non-ideal solution, given Antoine equations (saturation pressure versus temperature) for each component and equations that model the non-ideal behavior of the liquid solution.
  • Explain the properties of azeotropes, and explain why they make separations by distillation difficult.

 

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