Ideal vs Non-Ideal Solutions and Azeotropes

CHERaoult's Law & Vapour Pressure

Solutions are classified as ideal or non-ideal by whether they obey Raoult's law at all compositions, and non-ideal ones show positive or negative deviations linked to changes in $\Delta H$ and $\Delta V$ on mixing. Azeotropes are constant-boiling mixtures that cannot be separated by simple distillation.

Ideal solution (Raoult's law obeyed)

p_A = p_A^\circ x_A, \quad \Delta H_{mix} = 0, \quad \Delta V_{mix} = 0

A-B interactions equal A-A and B-B interactions

Positive deviation

p > p_A^\circ x_A + p_B^\circ x_B, \quad \Delta H_{mix} > 0, \quad \Delta V_{mix} > 0

A-B forces weaker than pure-component forces

Negative deviation

p < p_A^\circ x_A + p_B^\circ x_B, \quad \Delta H_{mix} < 0, \quad \Delta V_{mix} < 0

A-B forces stronger than pure-component forces

Ideal solutions obey Raoult's law throughout, with $\Delta H_{mix} = 0$ and $\Delta V_{mix} = 0$ ; A-B interactions equal A-A and B-B (e.g. benzene + toluene, n-hexane + n-heptane).
Positive deviation: A-B forces weaker than A-A/B-B, so vapour pressure is higher than Raoult's prediction, with $\Delta H_{mix} > 0$ (endothermic) and $\Delta V_{mix} > 0$ ; e.g. ethanol + water (H-bonds break).
Negative deviation: A-B forces stronger than pure-component forces, vapour pressure lower than predicted, with $\Delta H_{mix} < 0$ (exothermic) and $\Delta V_{mix} < 0$ ; e.g. chloroform + acetone (new H-bond forms).
Azeotrope: a binary mixture that boils at constant temperature with the vapour having the same composition as the liquid — it distils unchanged.
Minimum-boiling azeotrope arises from large positive deviation (e.g. $\approx 95\%$ ethanol + water); maximum-boiling azeotrope from large negative deviation (e.g. nitric acid + water).
Ethanol cannot be made 100% pure by distilling an ethanol-water mixture because it forms a minimum-boiling azeotrope at about $95\%$ ethanol.
Ideal solutions also have additive volumes and zero enthalpy of mixing, and each component obeys Raoult's law over the entire composition range.
In chloroform + acetone, the acidic $C-H$ of chloroform forms a new H-bond with the carbonyl oxygen of acetone, strengthening A-B interaction and causing the negative deviation.
At the azeotropic composition the boiling-point curve shows a maximum (negative-deviation systems) or minimum (positive-deviation systems), and liquid and vapour have identical composition there.
Because liquid and vapour compositions are equal at an azeotrope, fractional distillation cannot enrich either component beyond that point — only methods other than simple distillation can break it.
Minimum-boiling azeotropes boil below both pure components (high vapour pressure, positive deviation); maximum-boiling azeotropes boil above both (low vapour pressure, negative deviation).
Trap: positive deviation pairs with minimum-boiling azeotropes, negative deviation with maximum-boiling — keep the pairing straight.

Where the marks go

Swapping the pairing: positive deviation gives a minimum-boiling azeotrope, negative deviation gives a maximum-boiling one.
Getting the sign of $\Delta H_{mix}$ wrong — positive deviation is endothermic ( $\Delta H > 0$ ) and negative deviation is exothermic ( $\Delta H < 0$ ).
Claiming an azeotrope can be separated by fractional distillation; its identical liquid and vapour composition makes that impossible.
Mislabelling examples — ethanol + water is positive deviation, while chloroform + acetone and $HNO_3$ + water are negative deviation.
Forgetting that ideal solutions require $\Delta H_{mix} = 0$ AND $\Delta V_{mix} = 0$ together, not just obedience to Raoult's law at one composition.

How the board asks it

Define / stateazeotrope definition and the ethanol-water azeotrope
Define an azeotrope. Why can a solution of ethanol and water containing about $95\%$ ethanol not be made $100\%$ pure by fractional distillation?
Give reasonsintermolecular forces and sign of $\Delta H_{mix}$
Account for the fact that a mixture of ethanol and water shows a positive deviation from Raoult's law.
Predict the productdeviation type and the type of azeotrope formed
State, with reason, the type of deviation from Raoult's law shown by a mixture of chloroform and acetone, and the kind of azeotrope it forms.
Distinguishideal solution vs negative deviation
Distinguish between an ideal solution and a non-ideal solution showing negative deviation from Raoult's law, giving one example of each.
Structure / namingvapour-pressure vs composition curve for negative deviation
Draw a labelled graph of vapour pressure against composition for a binary liquid mixture showing negative deviation from Raoult's law, indicating the curve expected for an ideal solution.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.