Vapor-Liquid Equilibrium for Non-Ideal Solutions: Interactive Simulation

These simulations were prepared using Mathematica. Download the free Wolfram player, and then download the simulation CDF file (link given below or click on figure to download). Try to predict the behavior when a parameter changes before using a slider to change that parameter. A screencast below explains how to use this simulation.

Most solutions exhibit deviations from Raoult’s law. For positive deviations, the bubble pressure is greater than that predicted by Raoult’s law; this indicates that the attractive interactions between A and B molecules are weaker than the attractive interactions between A-A and B-B molecules. A negative deviation means the bubble pressure is smaller than that predicted by Raoult’s law, implying stronger mutual interactions between unlike molecules. The blue and green curves represent the partial pressures of A and B, respectively, and the black curve shows the total vapor pressure. The dashed lines represent the behavior expected is the solution were ideal (Raoult’s law).¬†
This simulation was prepared by S. M. Blinder and is located here. Open content licensed under CC BY-NC-SA.

Try to answer these questions before determining the answer with the simulation. We suggest that you write down the reasons for your answers.

  1. As the solution deviates more from Raoult’s law, does the partial pressure of component B deviate more at low or high values of \(x_b\)?
  2. Under what type of deviations from Raoult’s law would an azeotrope form?
  3. Is Henry’s law behavior for component A observed at low or high mole fractions of component B?

This demonstration presents P-x-y and T-x-y diagrams for vapor-liquid equilibrium (VLE) of a benzene/ethanol mixture. Drag the black dot to change the benzene mole fraction and the temperature or pressure. This liquid mixture is non-ideal and the system has an azeotrope at the conditions used. The bar chart displays the relative amounts of liquid (blue) and vapor (green) in equilibrium and the mole fraction of benzene in each phase. The blue line represents the liquid-phase boundary (bubble point), and the green line represents the vapor-phase boundary (dew point).

Try to answer these questions before determining the answer with the simulation. We suggest that you write down the reasons for your answers.

  1. This system has a maximum-pressure azeotrope. Does it have a minimum- or maximum-temperature azeotrope?
  2. If the temperature increases, how does the azeotrope change? Does its pressure increase or decrease?