Classification of Non-Ideal solution

It is of two types:

(A) Non-ideal solutions showing positive deviation:

In this solution, solute-solvent intermolecular attraction forces are lower than the solute-solute or solvent-solvent intermolecular attraction forces. For example, mixture of ethanol and cyclohexane. In pure alcohol intermolecular hydrogen bonds are present but on adding cyclohexane, its molecules get in-between the molecules of ethanol and thus reduce ethanol-ethanol attractions. Here, in order to break some hydrogen bonds certain heat is absorbed (ΔH = positive) and a slight increase in volume is observed (ΔV = positive). It should be noted that weaker attraction forces increase the tendency of solute and solvent molecules to leave the liquid layer and hence vapour pressure increases, which causes the elevation of
boiling point. It means, these solutions have lower boiling point than calculated.

Characteristics of an ideal solution:

(i) P_A> P^°_AX_A and P_B > P°_BP_B
(ii) ΔH _mixing > 0 i.e., solution becomes cold
(iii) ΔV _mixing > 0 i.e., expansion of solution takes place

Examples:

(i) Acetone and CS₂
(ii) Acetone and C₂H₅OH
(iii) Acetone and Benzene
(iv) Methyl alcohol and water
(v) Ethyl alcohol and water
(vi) CCI₄ and CHCI₃
(vii) CCI₄ and Benzene or toluene

(B) Non-ideal solution showing negative deviation:

In this solution, solute-solvent intermolecular attraction forces are higher than the solute-solute or solvent-solvent intermolecular attraction forces. For example, when HCI is mixied in water, it forms intermolecular hydrogen bonds with water molecules with evolution of some heat (ΔH = -ve) and some decrease in volume (ΔV = -ve). Due to stronger intermolecular attraction force formation, the tendency of molecules to leave the liquid surface reduces, hence, vapour pressure decreases and the boiling point increases.

Characteristics of a non-ideal solution:
(i) P_A < P°_AX_A and P_B < P°_BX_B
(ii) ΔH _mixing < 0 i.e., solution becomes hot
(iii) ΔV _mixing < 0 i.e., contraction of solution takes place

Examples:

(i) HCI and water
(ii) Chloroform and benzene or diethyl ether
(iii) HNO₃ and water
(iv) Acetic acid and pyridine
(v) Water and HI
(vi) Water and HCIO₄

Composition of the vapour:

The composition of the liquid and vapour that are in mutual equilibrium are not necessarily the same. Common Sense suggests that the vapour pressure should be richer in the more volatile component.

Let the mole fraction of A and B in vapour phase be Y_A and Y_B, then from Dalton's law,
Y_A = 

and Y_B = 

Note: Y_A + Y_B = 1