CBSE Class 12 Chemistry: Solutions - Marking Scheme Solution
(a) Example and Reason for Negative Deviation: [1 Mark]
- Example: A mixture of Phenol and Aniline or Acetone and Chloroform ($CH_3COCH_3 + CHCl_3$).
- Reason: In these solutions, the intermolecular attractive forces between the solute and solvent molecules ($A-B$ interactions) are stronger than those between the pure components ($A-A$ and $B-B$ interactions). This leads to a decrease in vapor pressure.
(b) (i) Raoult's Law for Volatile Components: [1 Mark]
Step 1 (Governing Law): Raoult's law states that for a solution of volatile liquids, the partial vapor pressure of each component in the solution is directly proportional to its mole fraction present in the solution.
Step 2 (Mathematical Expression): For a component $i$, the partial pressure $p_i$ is given by:
$$p_i = p_i^0 \cdot x_i$$Where $p_i^0$ is the vapor pressure of the pure component and $x_i$ is the mole fraction.
Step 3 (Conclusion): The total pressure $P_{total} = p_1^0 x_1 + p_2^0 x_2$.
OR
(b) (ii) Raoult's Law as a Special Case of Henry's Law: [1 Mark]
Step 1 (Comparison): According to Raoult's law, $p_i = p_i^0 x_i$. According to Henry's law, $p_i = K_H x_i$.
Step 2 (Deduction): If we compare the two equations, we observe that they are identical in form. The only difference is the proportionality constant ($p_i^0$ vs $K_H$).
Step 3 (Conclusion): Raoult's law becomes a special case of Henry's law when the proportionality constant $K_H$ becomes equal to the pure component vapor pressure $p_i^0$.
(c) Characteristics of an Ideal Solution: [2 Marks]
Step 1 (Thermodynamic Conditions): An ideal solution must satisfy the following criteria:
- Enthalpy of mixing: $\Delta_{mix}H = 0$ (No heat is absorbed or evolved during mixing).
- Volume of mixing: $\Delta_{mix}V = 0$ (The total volume of the solution is equal to the sum of the volumes of the components).
Step 2 (Molecular Interaction): The solute-solute ($A-A$) and solvent-solvent ($B-B$) interactions are identical to the solute-solvent ($A-B$) interactions.
💡 EL10 Concept Insight:
Think of an ideal solution like mixing two identical brands of water; the molecules don't "care" which bottle they came from, so nothing changes in volume or heat. In real life, molecules are like different groups of friends—sometimes they like each other more (negative deviation) or less (positive deviation) than their own group, which changes how easily they escape into vapor!