An ideal solution obeys Raoult’s law at every composition. In real solutions the attraction between unlike molecules differs from that between like ones, so the vapour pressure is higher or lower than Raoult’s law predicts.
An ideal solution obeys Raoult’s law at every composition. Its total vapour pressure is a straight line between the vapour pressures of the two pure liquids.
An ideal solution has two other properties. No heat is released or absorbed when the liquids are mixed, and the volume of the solution equals the sum of the volumes of the liquids mixed:
ΔmixH = 0 ΔmixV = 0
The molecular condition for an ideal solution is that the attraction between unlike molecules, A–B, is the same as the attractions between like molecules, A–A and B–B. Then replacing an A neighbour with a B neighbour changes no energy, so mixing absorbs or releases no heat. Benzene with toluene, and n-hexane with n-heptane, are close to ideal. Their molecules are similar in size and in the kind of attraction between them.
In most real solutions, the A–B attraction is different from the A–A and B–B attractions. These solutions are non-ideal: their vapour pressures do not follow Raoult’s law. The deviation has one of two signs.
In a positive deviation, the vapour pressure is higher than Raoult’s law predicts. The total vapour pressure curve lies above the straight line.
The cause is that the A–B attraction is weaker than the A–A and B–B attractions. Molecules in the solution are held less strongly than in the pure liquids, so they escape into the vapour more readily. Mixing such liquids absorbs heat (ΔmixH > 0) and the volume increases slightly (ΔmixV > 0).
Ethanol and acetone show positive deviation. Pure ethanol molecules are held together by hydrogen bonds. Acetone molecules come between them and break some of these bonds, and the ethanol–acetone attractions that replace them are weaker.
In a negative deviation, the vapour pressure is lower than Raoult’s law predicts. The curve lies below the straight line.
The cause is that the A–B attraction is stronger than the A–A and B–B attractions. Molecules are held more strongly in the solution, so fewer escape. Mixing releases heat (ΔmixH < 0) and the volume decreases slightly (ΔmixV < 0).
Chloroform and acetone show negative deviation. The hydrogen atom of chloroform forms a hydrogen bond with the oxygen atom of acetone. Neither pure liquid has this bond.
A mixture of ethanol and acetone has a vapour pressure higher than Raoult’s law predicts. What does this show about the attractions between the molecules?
The ethanol–acetone attraction is weaker than the attractions in the pure liquids. This is a positive deviation.
In pure ethanol, molecules are held by hydrogen bonds. Acetone molecules break some of them, and the attractions that replace them are weaker. Molecules at the surface are held less strongly, so more escape per second, and the vapour pressure is higher than for an ideal solution.
The same weaker attraction means energy must be supplied to mix the liquids, so the mixture cools slightly when the liquids are combined.
With a large deviation, the total vapour pressure curve has a maximum (positive deviation) or a minimum (negative deviation) at some composition.
Distillation separates two liquids because the vapour that boils off has a different composition from the liquid: it contains more of the more volatile component. Condensing the vapour and boiling it again, many times, separates the liquids.
At the composition of the maximum or minimum, the vapour has the same composition as the liquid. Boiling it produces vapour of the same composition, so no separation occurs. A mixture with this property is an azeotrope. It boils at a constant temperature without changing composition.
A positive deviation gives a maximum in vapour pressure. Higher vapour pressure means the mixture boils at a lower temperature, so this is a minimum-boiling azeotrope. Ethanol and water form one at about 95% ethanol by volume. It boils at about 78.2°C, slightly below pure ethanol (78.4°C) and well below water (100°C).
A negative deviation gives a minimum in vapour pressure, and a maximum-boiling azeotrope. Nitric acid and water form one at about 68% nitric acid by mass, boiling at about 120°C, above either pure liquid.
Distilling a water–ethanol mixture cannot raise the ethanol content above about 95%. Why?
At about 95% ethanol, the mixture is an azeotrope.
Below that composition, the vapour contains more ethanol than the liquid, so each distillation step increases the ethanol content. At the azeotrope, the vapour and liquid have the same composition. Distilling it again gives the same mixture, so the content stops increasing.
To obtain nearly pure ethanol, a different method is needed, such as adding a substance that absorbs the remaining water.
1An ideal solution is one that…
2For an ideal solution, the enthalpy of mixing and the volume of mixing are…
3A positive deviation from Raoult’s law means the vapour pressure is…
4A negative deviation occurs when the attraction between unlike molecules is…
5Classify each mixture by the sign of its deviation.
6An azeotrope cannot be separated by distillation because…
7Why can simple distillation not produce pure ethanol from a water–ethanol mixture?
Next: colligative properties, which depend only on the number of solute particles.
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