Ideal & Non-Ideal Solutions
Azeotropes & Deviations from Raoult's Law | Class 12 Solutions
1. Ideal Solutions
Solutions that obey Raoult's Law over the entire range of concentration are called Ideal Solutions.
Characteristics:
- Intermolecular interactions: A-B $\approx$ A-A or B-B.
- Enthalpy of mixing: $\Delta_{mix}H = 0$ (No heat absorbed or evolved).
- Volume of mixing: $\Delta_{mix}V = 0$ (Total volume = Sum of individual volumes).
- Vapor Pressure: $P_{total} = P_A^\circ \chi_A + P_B^\circ \chi_B$.
Examples: Benzene + Toluene, n-Hexane + n-Heptane, Bromoethane + Iodoethane.
2. Non-Ideal Solutions
Solutions that do not obey Raoult's Law are called Non-Ideal Solutions. They show either positive or negative deviation.
A. Positive Deviation
The vapor pressure is higher than predicted by Raoult's Law.
- Interactions: A-B interactions are weaker than A-A or B-B. Molecules escape easily.
- Thermodynamics: $\Delta_{mix}H > 0$ (Endothermic), $\Delta_{mix}V > 0$ (Expansion).
- Examples: Ethanol + Acetone, Carbon Disulfide + Acetone, Ethanol + Water.
B. Negative Deviation
The vapor pressure is lower than predicted by Raoult's Law.
- Interactions: A-B interactions are stronger than A-A or B-B (e.g., H-bonding). Molecules held tightly.
- Thermodynamics: $\Delta_{mix}H < 0$ (Exothermic), $\Delta_{mix}V < 0$ (Contraction).
- Examples: Phenol + Aniline, Chloroform + Acetone, $HNO_3$ + Water.
3. Azeotropes (Constant Boiling Mixtures)
Binary mixtures having the same composition in liquid and vapor phase and boil at a constant temperature. They act like pure liquids.
| Type | Caused By | Boiling Point | Example |
|---|---|---|---|
| Minimum Boiling Azeotrope | Large Positive Deviation | Lower than either component | Ethanol (95%) + Water (5%) (BP 351.15 K) |
| Maximum Boiling Azeotrope | Large Negative Deviation | Higher than either component | Nitric Acid (68%) + Water (32%) (BP 393.5 K) |
Note: Azeotropes cannot be separated by fractional distillation.
Practice Quiz
Test your knowledge on Solution behavior.
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