Question

Explain the terms ideal and non-ideal solutions in the light of forces of interactions operating between molecules in liquid solutions.

Answer 1

Ideal solution: The binary solution of two volatile liquids following Rault's law at all concentration and at all temperature is termed as 'Ideal solution'

i.e. `P_A = P_(0A xx A), P_B = P_(0B xx B)`

If the intermolecular attractive forces between the solute(A) - solvent (B) (i.e. A-B interaction) are nearly equal to those between the solvent-solvent (A-A) and solute-solute (B-B) it forms ideal solutions. In such an ideal solution, Enthalpy of mixing, Δ mixing H = 0. Volume change on mixing, Δ mixing V = 0.

Examples: n-hexane nd n-heptane.

Non Ideal solution: These are binary solutions of two vollatile liquids which do not obey raout's law at all concentration and at all temperature.

i.e. `P_A ≠ P_(0A xx A), P_B ≠ P_(0B xx B)`

Further, if the intermolecular attractive forces bwtween the solute-solvent (A-B interaction) are not equal (either stronger or weaker) to those between the solvent-solvent (A-A) and solute-solute (B-B) particles, it forms non-ideal solutions. Enthalpy of mixing, `Δ_("mixing")` H is not equal to 0 and Volume change on mixing, `Δ_("mixing")` V is not equal to 0.

Example: CS₂ and acetone.

where, `P_(oA), P_(oB)` represent the vaour pressures of pure solvent and PA, PB are the partial vapour pressures of components A and B repectively in solution, X represents mole fractions of the two components represented as subscripts A and B respectively.