NEET 2020 Chemistry Question 115 Solution

In solutions exhibiting positive deviation from Raoult's law, the intermolecular attractive forces between solute and solvent molecules ($$A-B$$) are weaker than those between like molecules ($$A-A$$ and $$B-B$$). As a result, molecules escape more easily into the vapor phase, leading to higher vapor pressures ($$P_A > P_A^\circ x_A$$, $$P_B > P_B^\circ x_B$$), endothermic mixing ($$\Delta_{\text{mix}} H > 0$$), and volume expansion ($$\Delta_{\text{mix}} V > 0$$).

In pure ethanol, molecules are strongly associated via intermolecular hydrogen bonding. When acetone is mixed with ethanol, its molecules slip in between host ethanol molecules and disrupt their hydrogen-bonded network. Because the new ethanol-acetone dipole-dipole attractions are weaker than the original ethanol-ethanol hydrogen bonds, the escaping tendency of both components increases, displaying positive deviation.

- Benzene + Toluene: Both are non-polar hydrocarbons of similar size and structure, forming an nearly IDEAL solution ($$\Delta H = 0, \Delta V = 0$$). - Chloroethane + Bromoethane: Ideal solution of homologous haloalkanes. - Acetone + Chloroform: Chloroform forms a new, strong intermolecular hydrogen bond with the carbonyl oxygen of acetone, displaying NEGATIVE deviation.

Ethanol + Acetone uniquely exhibits positive deviation from Raoult's law.