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

1. Raoult’s Law can be generally defined as, ‘For any solution, the partial vapour pressure of each volatile component in the solution is directly proportional to its mole fraction.’ However, this law has its limitations, especially in the case of a large number of solutions with high concentrations and complexity of components whose intermolecular forces vary considerably from this law.

2. Hence, liquid-liquid solutions are classified into ideal and non-ideal solutions on the basis of Raoult’s Law.

3. Ideal solutions are mixtures which obey the law over the entire range of their concentrations. Two important properties of these solutions are (i) Enthalpy of mixing the pure components to form the solution is zero where no heat is absorbed or evolved when the two components are mixed [ΔmixH = 0] and (ii) Volume of mixing of the solution is zero, where the volume of the solution would be equal to the sum of volumes of the two components without an increase or decrease in volume [ΔmixV = 0].

4. Ideal mixtures do not exist practically but some binary solutions behave ideally to an extent. Examples include n-hexane and n-heptane, benzene and toluene, propan-1-ol and propan-2-ol, bromoethane and chloroethane and so on.

5. A simpler definition of Raoult’s law states ‘The partial vapor pressure of a component in a mixture is equal to the product of the vapour pressure of the pure component at that temperature and its mole fraction in the mixture’. A non-ideal solution does not obey the law and does not follow the properties of ideal solutions. The vapour pressure of these type of solutions are experimentally higher or lower than that predicted by the equation derived from the law.

6. The ideal and non-ideal behaviour of solutions can be explained by understanding the intermolecular interactions of these solutions. The molecules of the two components of a binary solution can be named A and B. In pure components, the intermolecular interactions will be between the molecules themselves i.e. A-A and B-B. In binary solutions, the interactions will be between the two sets of the molecules i.e. A-B in addition to the two before. In a pure solvent, some of the molecules will gain enough energy to overcome the intermolecular attractions and escape from the liquid surface to vapour form. This process of escape is dependent on intermolecular forces. If the forces are of smaller magnitude, more molecules will escape the surface at a constant temperature. In the case of an ideal mixture, this tendency of molecules A as well as B will be the same. If ideal solutions existed, the intermolecular forces between A-A, B-B and A-B will be identical, which is impossible in most cases. It happens to an extent in the examples mentioned previously because the sizes of the constituent molecules are similar and so are the Van der Waals forces. Ideal solution behaviour is also assumed for very dilute solutions.

7. Considering non-ideal solutions, the enthalpy of the mixture as well as the volume of mixing is not equal to zero [ΔmixH ≠ 0; ΔmixV ≠ 0]. Heat is either evolved [ΔmixH < 0] or absorbed [ΔmixH > 0] on mixing the components to form a solution, and the volume of the solution is usually not equal to the sum of volumes of the components.

8. Non-ideal solutions are formed in two situations, one where there is positive deviation from Raoult’s law, and the other where there is negative deviation from Raoult’s law.

9. In case of positive deviation from Raoult’s law, the intermolecular forces between solute-solvent molecules are weaker than solute-solute and solvent-solvent particles. That is, A-B interactions are weaker than A-A and B-B interactions. In these solutions, it will be easier for A or B molecules to escape than be in a pure state. This increases vapour pressure and results in a positive deviation. Here, p1 > p10 x1 and p2 > p02 x2, as the total vapour pressure (p10 x1 + p02 x2) is higher than expected from Raoult’s law. The resulting solution has larger enthalpy than its constituents, causing the reaction to be endothermic and heat is absorbed. The volume of mixing is positive as the volume increases on mixing the constituents. ΔmixH > 0 and ΔmixV > 0.

10. In case of negative deviation of Raoult’s law, intermolecular forces between A-A and B-B are weaker than A-B, that is, solute-solvent interactions are stronger than solute-solute and solvent-solvent interactions, which leads to molecules staying in the pure state than escaping, leading to a reduction in vapour pressure and thus negative deviation. Here, p1 < p10 x1 and p2 < p02 x2, as the total vapour pressure (p10 x1 + p02 x2) is lower than expected from Raoult’s law. The resulting solution has negative enthalpy than its constituents, causing the reaction to be exothermic and heat is released. The volume of mixing is negative as the volume decreases on mixing the constituents. ΔmixH < 0 and ΔmixV < 0.

Figure 1: Solution showing positive deviation from Raoult's Law