Miscible liquids and Raoult’s law
A two-component system contains two substances, for example two liquids that mix completely. In an ideal mixture of liquids A and B the molecules attract each other just as much as they attract their own kind. Raoult’s law says each component’s vapour pressure is its mole fraction times the vapour pressure of the pure liquid.
Choose the mole fraction of A and the vapour pressures of the pure liquids. Find the total vapour pressure above the mixture.
Deviations and fractional distillation
| Mixture | Vapour pressure | Reason | Example |
|---|---|---|---|
| Ideal | Follows Raoult’s law | A–B forces same as A–A and B–B | Hexane and heptane |
| Positive deviation | Higher than expected | A–B forces weaker; molecules escape more easily | Ethanol and water, ethanol and hexane |
| Negative deviation | Lower than expected | A–B forces stronger | Water and hydrochloric acid, trichloromethane and propanone |
The vapour above a boiling mixture is richer in the more volatile component (the one with the lower boiling point). Fractional distillation repeats evaporation and condensation up a column, so that the more volatile component can be collected as nearly pure liquid. Mixtures with large deviations form an azeotrope (constant-boiling mixture) that cannot be separated completely: 95.6% ethanol and water boil together at 78.2 °C.
Immiscible liquids and partition
When two liquids do not mix (for example water and oil), each exerts its full vapour pressure independently. The mixture boils when the sum of the two vapour pressures reaches atmospheric pressure, which is below the boiling point of either liquid. This is steam distillation, used to extract oils from plants (lemon grass, eucalyptus) at less than 100 °C without destroying them.
A solute shared between two immiscible solvents distributes itself in a fixed ratio called the partition coefficient K = concentration in solvent 1 ÷ concentration in solvent 2 at a given temperature. Solvent extraction uses this to remove a substance from water into an organic solvent.
Key terms
- Mole fraction
- Moles of a component divided by the total moles in the mixture.
- Azeotrope
- A mixture that boils at constant composition and cannot be separated by distillation.
- Partition coefficient
- The ratio in which a solute distributes itself between two immiscible solvents.
- Volatile
- Evaporates easily.
Summary
- Raoult's law: p_A = x_A p°_A; an ideal mixture follows it.
- Positive deviations give a minimum boiling-point azeotrope; negative deviations a maximum.
- Fractional distillation separates miscible liquids by boiling point; steam distillation separates immiscible ones.
- The partition coefficient is the ratio of concentrations in two solvents.