Raoult's law – non-ideal solutions and azeotropes
ChemistryKinetics & EquilibriumAges 16–17Virtual lab
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Kinetics & Equilibrium
Mix two volatile liquids: set the mole fraction and the temperature, and see the partial pressures and the total vapour pressure compared with Raoult's law for benzene–toluene (ideal), ethanol–propanone and ethanol–water (positive deviation), propanone–trichloromethane and nitric acid–water (negative deviation). Boil mixtures to build the boiling point–composition diagram, find the azeotrope and see why fractional distillation cannot separate it. The Molecules tab compares the attractions between molecules.
Lesson: Solutions: Raoult's law, positive and negative deviations, azeotropes and fractional distillation
What it shows
Raoult's law says that each volatile liquid in an ideal solution contributes a partial vapour pressure p_A = x_A·p°_A. Benzene and toluene behave almost ideally. When A–B attractions are weaker than A–A and B–B attractions, the vapour pressure is higher than Raoult's law predicts (positive deviation); when they are stronger, it is lower (negative deviation). Large deviations give azeotropes: mixtures that boil at a constant temperature with a vapour of the same composition, so fractional distillation cannot separate them.
How to use
Choose the Pair. In Vapour pressure, set the Mole fraction x_A and the Temperature and press Record reading. In Boiling point, press Boil for several compositions; after five, press Show curves, find the azeotrope and press Fractional distillation. Molecules compares the attractions and the signs of ΔmixH and ΔmixV.
Parameters you can change
- Tab Vapour pressure, Boiling point, Molecules
- Pair Benzene (A) – toluene (B), Ethanol (A) – propanone (B), Ethanol (A) – water (B), Propanone (A) – trichloromethane (B), Nitric acid (A) – water (B)
- Mole fraction x_A 0–1
- Temperature 10–80 °C
Questions to explore
- At the same temperature, how do the total vapour pressures of benzene–toluene and ethanol–propanone compare with Raoult's law?
- Why does the vapour above benzene–toluene contain more benzene than the liquid does?
- Why can fractional distillation of dilute ethanol never give pure ethanol?
