Ideal and Non-Ideal Solutions

Full Notes

Solutions can be classified based on how closely they follow Raoult’s Law, giving two main categories: ideal and non-ideal solutions.

Ideal Solutions

An ideal solution is one in which the enthalpy of mixing and volume change on mixing are both zero:

• ΔH_mix = 0 → No heat is absorbed or evolved.
• ΔV_mix = 0 → No expansion or contraction occurs.

Key Characteristics:

• The interactions between molecules of different components (A–B) are similar to those between like molecules (A–A or B–B).
• Obeys Raoult’s Law over the entire range of concentration.

Raoult’s Law:

For component A and B in a binary liquid solution:

p_A = x_A · p_A⁰
p_B = x_B · p_B⁰

Total vapor pressure:

p_total = p_A + p_B

Non-Ideal Solutions

Non-ideal solutions are those which deviate from Raoult’s Law.

These deviations arise due to differences in the strength of intermolecular interactions between A–A, B–B, and A–B.

Positive Deviations from Raoult’s Law

This is where total pressure (p_total) is greater than expected.

• Interactions between unlike molecules (A–B) are weaker than those between like molecules.
• Molecules escape more readily meaning vapor pressure increases.
• ΔH_mix > 0, heat is absorbed.
• ΔV_mix > 0, volume increases.
• Examples:
  • Ethanol + acetone
  • Carbon disulphide + acetone

These show increased vapor pressure and may form minimum boiling azeotropes (see below).

Negative Deviations from Raoult’s Law

This is where total pressure (p_total) is lower than expected.

• Interactions between unlike molecules are stronger than like interactions.
• Fewer molecules escape meaning vapor pressure decreases.
• ΔH_mix < 0, heat is released.
• ΔV_mix < 0, volume contracts.
• Examples:
  • Acetone + chloroform (hydrogen bonding)
  • Chloroform + aniline

These may form maximum boiling azeotropes (see below).

Azeotropes

Azerotropes are binary mixtures (contain two compounds) that boil at a constant temperature and have same composition in liquid and vapour phase. They cannot be separated by fractional distillation.

Types of Azeotropes:

• Minimum Boiling Azeotrope:
  • Formed by solutions with positive deviation from Raoult’s Law.
  • Example: Ethanol + Water
  • On distillation, mixture stabilises at ~95% ethanol.
  • Beyond this point, composition remains unchanged in both phases.
• Maximum Boiling Azeotrope:
  • Formed by solutions with negative deviation from Raoult’s Law.
  • Example: Nitric Acid + Water
  • Approximate composition: 68% HNO₃ and 32% water by mass.
  • Boiling point: 393.5 K

Summary

• Ideal solutions obey Raoult’s law with ΔH_mix = 0 and ΔV_mix = 0.
• Non-ideal solutions show positive or negative deviations based on A–B interaction strength.
• Positive deviations raise vapor pressure and can give minimum boiling azeotropes.
• Negative deviations lower vapor pressure and can give maximum boiling azeotropes.
• Azeotropes boil at constant composition and cannot be separated by fractional distillation.