Ideal and non-ideal solutions; azeotropes
Ideal solutions obey Raoult's law, with ΔmixH = 0 and ΔmixV = 0. Non-ideal ones show positive or negative deviation; some form constant-boiling azeotropes.
What makes a solution ideal
In an ideal solution, the attraction between unlike molecules (A-B) is nearly the same as between like molecules (A-A and B-B). So mixing takes in or gives out no heat (ΔmixH = 0), and the volumes simply add up (ΔmixV = 0). Each component obeys Raoult's law at every concentration. Perfectly ideal solutions are rare. Nearly ideal ones are n-hexane + n-heptane, bromoethane + chloroethane and benzene + toluene.
Positive vs negative deviation
| Feature | Positive deviation | Negative deviation |
|---|---|---|
| A-B attraction | Weaker than A-A and B-B | Stronger than A-A and B-B |
| Escaping tendency of molecules | Higher | Lower |
| Vapour pressure | Higher than Raoult's law predicts | Lower than Raoult's law predicts |
| ΔmixH | Positive: heat is absorbed, the mixture cools | Negative: heat is given out, the mixture warms |
| ΔmixV | Positive: the volume increases | Negative: the volume decreases |
| Examples | Ethanol + acetone; carbon disulphide + acetone; ethanol + water | Chloroform + acetone; phenol + aniline; nitric acid + water |
| Azeotrope formed (if the deviation is large) | Minimum boiling: ethanol + water, about 95% ethanol by volume | Maximum boiling: 68% nitric acid + 32% water by mass, boiling at 393.5 K |
Why these examples deviate
- Ethanol + acetone (positive): pure ethanol is held together by hydrogen bonds. Acetone molecules get in between and break some of these hydrogen bonds, so the attractions become weaker.
- Carbon disulphide + acetone (positive): the dipolar attractions between CS₂ and acetone molecules are weaker than those within each pure liquid.
- Chloroform + acetone (negative): the H of CHCl₃ forms a hydrogen bond with the O of acetone, written (CH₃)₂C=O···H-CCl₃. The A-B attraction is stronger.
- Phenol + aniline (negative): the H of the phenol OH group forms a strong hydrogen bond with the lone pair on the N of aniline.
How to draw the deviation graphs
- Use the same axes as for Raoult's law: composition on the x-axis (x₁ from 1 to 0, x₂ from 0 to 1) and vapour pressure on the y-axis.
- Draw the ideal lines for p₁, p₂ and p_total as dotted straight lines.
- Positive deviation: draw p₁, p₂ and p_total as solid curves that bulge above the dotted lines. The p_total curve may show a maximum.
- Negative deviation: draw the solid curves sagging below the dotted lines. The p_total curve may show a minimum.
- Label p₁°, p₂°, p₁, p₂, p_total and the dotted 'Raoult's law' lines.
Azeotropes
An azeotrope is a binary mixture that has the same composition in the liquid and in the vapour, and boils at a constant temperature. So it cannot be separated by fractional distillation. A large positive deviation gives a minimum boiling azeotrope: when fermented sugar solution is distilled, the ethanol cannot be taken beyond about 95% by volume. A large negative deviation gives a maximum boiling azeotrope, such as 68% nitric acid and 32% water by mass, which boils at 393.5 K.
Ethanol blended into petrol in India must be almost water-free, but distilling fermented sugarcane molasses stops near 95% ethanol, the azeotrope. Extra drying steps are needed to go beyond it.
2 marks: “What is an azeotrope? Give one example each of a minimum boiling and a maximum boiling azeotrope.” A binary mixture with the same composition in liquid and vapour, which boils at a constant temperature. Minimum boiling: ethanol and water (about 95% ethanol by volume). Maximum boiling: nitric acid and water (68% HNO₃ by mass).
3 marks: “A mixture of chloroform and acetone shows negative deviation from Raoult's law. Explain why, give the signs of ΔmixH and ΔmixV, and draw the vapour pressure graph.” A hydrogen bond forms between the H of CHCl₃ and the O of acetone, so A-B attraction is stronger and fewer molecules escape. ΔmixH < 0 and ΔmixV < 0. Graph: solid curves below the dotted Raoult lines, with a minimum in p_total.
1 marks: “Mixing ethanol and acetone makes the liquid slightly cooler. Name the type of deviation shown.” Positive deviation (heat is absorbed, so ΔmixH > 0).
Positive deviation: everything plus (ΔH, ΔV, vapour pressure), so the boiling point is a minimum. Negative deviation: everything minus, so the boiling point is a maximum.