Ideal & Non-Ideal Solutions - Chemistry Class 12 - NEET PDF Download

Raoult's Law states that the partial vapour pressure of each component in an ideal liquid solution is directly proportional to its mole fraction in the solution.
On the basis of Raoult's Law, liquid-liquid solutions are classified into two types of solutions, they are:
•  Ideal Solutions
•  Non-ideal Solutions

What is an Ideal Solution?

An ideal solution is a mixture in which molecules of different species are distinguishable but the intermolecular forces between unlike molecules are the same as those between like molecules. When these forces are the same for all molecular pairs, the solution behaves ideally.

Characteristics of Ideal Solution

• The solutions that obey Raoult's Law at every range of concentration and at all temperatures are called ideal solutions.
• Ideal solutions are obtained by mixing two components (solute and solvent) having similar molecular size and structure so that their intermolecular interactions are nearly identical.
  Example: Consider two liquids A and B. In the mixture the following intermolecular forces exist:
  • A - A intermolecular attraction
  • B - B intermolecular attraction
  • A - B intermolecular attraction

• The solution is ideal only when the A - A, B - B and A - B intermolecular forces are nearly equal.

• They follow Raoult's Law, which means the partial pressures of components A and B in a solution are P_A = P_A⁰ x_A and P_B = P_B⁰ x_B, where P_A⁰ and P_B⁰ are the vapour pressures of the pure components and x_A, x_B are their mole fractions.
• The enthalpy of mixing is zero: Δ_mix H = 0. No heat is evolved or absorbed on mixing.
• The volume of mixing is zero: Δ_mix V = 0. The total volume of the solution equals the sum of the volumes of the pure components (no contraction or expansion).
• Solute-solute and solvent-solvent interactions are nearly equal to solute-solvent interactions.

Note: Perfectly ideal solutions are rare; only some pairs of liquids show nearly ideal behaviour.

Examples of Ideal Solutions

• n-Hexane and n-heptane
• Bromoethane and chloroethane
• Benzene and toluene
• CCl₄ and SiCl₄
• Chlorobenzene and bromobenzene
• Ethyl bromide and ethyl iodide
• n-Butyl chloride and n-butyl bromide

What is a Non-Ideal Solution?

Solutions that do not obey Raoult's Law over the entire concentration range and at all temperatures are called non-ideal solutions. They show deviations from ideal behaviour because the interactions between unlike molecules differ from those between like molecules.

Types of Non-Ideal Solutions

Characteristics of Non-Ideal Solution

• The solute-solute and solvent-solvent interactions differ from the solute-solvent interaction.
• The enthalpy of mixing is not zero: Δ_mix H ≠ 0. If Δ_mix H < 0 heat is released on mixing; if Δ_mix H > 0 heat is absorbed.
• The volume of mixing is not zero: Δ_mix V ≠ 0. There may be contraction or expansion on mixing.

Types of Non-Ideal Solution

• Non-ideal solutions showing positive deviation from Raoult's Law
• Non-ideal solutions showing negative deviation from Raoult's Law

Positive Deviation from Raoult's Law

Example: Consider two components A and B with vapour pressures P_A, P_B, pure vapour pressures P_A⁰, P_B⁰ and mole fractions x_A, x_B.

These liquids will show positive deviation from Raoult's Law when

• P_A > P_A⁰ x_A and P_B > P_B⁰ x_B, so that the total vapour pressure (P_A⁰ x_A + P_B⁰ x_B) is less than the observed pressure.
• The solute-solvent interaction is weaker than the solute-solute or solvent-solvent interactions, i.e. A - B < A - A or B - B.
• The enthalpy of mixing is positive: Δ_mix H > 0, because breaking the original interactions requires more energy than is released in forming new ones.
• The volume of mixing is positive: Δ_mix V > 0, so the solution expands on mixing.

Examples: Following are examples of solutions showing positive deviation from Raoult's Law

• Acetone and carbon disulphide
• Acetone and benzene
• Carbon tetrachloride and toluene (or chloroform)
• Methyl alcohol and water
• Acetone and ethanol
• Ethanol and water

Negative Deviation from Raoult's Law

• P_A < P_A⁰ x_A and P_B < P_B⁰ x_B, so that the observed total vapour pressure is lower than the Raoult's Law value.
• The solute-solvent interaction is stronger than solute-solute and solvent-solvent interactions, i.e. A - B > A - A or B - B.
• The enthalpy of mixing is negative: Δ_mix H < 0, because formation of new interactions releases more heat than is required to break the original ones.
• The volume of mixing is negative: Δ_mix V < 0, so the solution contracts on mixing.

Examples: Following are examples of solutions showing negative deviation from Raoult's Law

• Chloroform and benzene
• Chloroform and diethyl ether
• Acetone and aniline
• Nitric acid (HNO₃) and water
• Acetic acid and pyridine
• Hydrochloric acid (HCl) and water
  
  
   Image 5: Graph between vapour pressure and mole fraction

What are Azeotropes?

Azeotropes are mixtures of two (or more) liquids that boil at a constant temperature and have the same composition in the liquid and vapour phases at their boiling point.

• Azeotropes cannot be separated by simple fractional distillation because the vapour formed during boiling has the same composition as the liquid.
• Because of this uniform composition at the boiling point, azeotropes are also called constant-boiling mixtures.

Each azeotrope has a characteristic boiling point.
The boiling point of an azeotrope is either lower than the boiling points of any of its constituents (a minimum-boiling or positive azeotrope) or higher than the boiling points of its constituents (a maximum-boiling or negative azeotrope).

Types of Azeotropes

• Maximum-boiling azeotrope
• Minimum-boiling azeotrope

A maximum-boiling azeotrope is formed when a mixture shows a large negative deviation from Raoult's Law at a certain composition, giving a higher boiling point than either pure component.

Examples:

• Nitric acid (HNO₃) (68%) and water (32%) form a maximum-boiling azeotrope at a boiling temperature of 393.5 K
• Hydrochloric acid (HCl) (20.24%) and water form a maximum-boiling azeotrope at a boiling temperature of 373 K

A minimum-boiling azeotrope is formed when a mixture shows a large positive deviation from Raoult's Law at a certain composition, producing a boiling point lower than that of either pure component.
Example: Ethanol (95.5%) and water (4.5%) form a minimum-boiling azeotrope at a boiling temperature of 351.5 K