Concept 02 / 07Solutions
Non-Ideal Solutions: Positive and Negative Deviations and Azeotropes
Deviations from Raoult's law, molecular origin of positive and negative deviations, signs of enthalpy and volume of mixing, examples, and minimum-boiling and maximum-boiling azeotropes that cannot be separated by fractional distillation.
Solutions that do not obey Raoult's law over the whole composition range are non-ideal. Their vapour pressure lies above or below the Raoult prediction (see figure).
Positive deviation. A-B interactions are weaker than the A-A and B-B interactions, so molecules escape more easily and the vapour pressure is higher than predicted.
(endothermic; energy is needed to break the stronger like-like interactions); (volume expands).
Examples: ethanol + acetone (acetone breaks up ethanol's hydrogen bonds), ethanol + water, carbon disulfide + acetone, ethanol + cyclohexane, benzene + acetone, carbon tetrachloride + toluene.
Negative deviation. A-B interactions are stronger than A-A and B-B, so the vapour pressure is lower than predicted.
(exothermic); (contraction).
Examples: chloroform + acetone (the C-H of chloroform hydrogen-bonds to the carbonyl oxygen of acetone), phenol + aniline, nitric acid + water, HCl + water, acetic acid + pyridine.
Azeotropes. Binary mixtures that boil at constant temperature with the same composition in liquid and vapour, so they cannot be separated by fractional distillation.
Minimum-boiling azeotropes form from large positive deviations: the vapour pressure passes through a maximum, so the boiling point passes through a minimum. Ethanol and water form one at about ethanol by volume (b.p. ), which is why fermentation liquor cannot be distilled beyond this purity (absolute alcohol needs other methods).
Maximum-boiling azeotropes form from large negative deviations: nitric acid and water at by mass (b.p. ); HCl and water at about HCl.
Key Takeaways:
- Positive deviation: weaker A-B, higher vapour pressure, , , minimum-boiling azeotrope.
- Negative deviation: stronger A-B, lower vapour pressure, , , maximum-boiling azeotrope.
- Azeotropes distil unchanged: ethanol-water (), -water ().
- Pitfall: a higher vapour pressure means a lower boiling point; positive deviation gives a minimum-boiling azeotrope.
Brain-Teaser 1: When of ethanol and of water are mixed, the volume is less than , though the system shows positive deviation. How is this possible?
Solution: The sign rules ( for positive deviation) are general tendencies from a simple model. For ethanol-water, strong new hydrogen bonds and efficient packing of the small water molecules around ethanol cause contraction (about ), while the vapour pressure still shows positive deviation because much of water's hydrogen-bond network is disrupted (the mixing is actually slightly exothermic at some compositions). Real systems can be more complex than the simple sign rules.
Brain-Teaser 2: Classify each as showing positive or negative deviation: (a) acetone + chloroform, (b) acetone + carbon disulfide, (c) HCl + water.
Solution: (a) Negative: new C-H...O hydrogen bonds form. (b) Positive: weak dipole-induced dipole interactions replace stronger dipole-dipole forces in acetone. (c) Negative: HCl ionises and ions are strongly hydrated.
Brain-Teaser 3: Why can pure ethanol not be obtained by fractional distillation of a dilute aqueous solution?
Solution: Ethanol and water form a minimum-boiling azeotrope (about ethanol) with a lower boiling point than either pure component. Fractional distillation drives the distillate towards this azeotrope, whose vapour has the same composition as the liquid, so no further enrichment is possible. Absolute alcohol is obtained by adding benzene (forming a ternary azeotrope) or using a drying agent.