Question

According to the generally accepted definition of the ideal solution there are equal interaction forces acting between molecules belonging to the same or different species. (This is equivalent to the statement that the activity of the components equals the concentration.) Strictly speaking, this concept is valid in ecological systems (isotopic mixtures of an element, hydrocarbons mixtures, etc.). It is still usual to talk about ideal solutions as limiting cases in reality since very dilute solutions behave ideally with respect to the solvent. This law is further supported by the fact that Raoult’s law empirically found for describing the behaviour of the solvent in dilute solutions can be deduced thermodynamically via the assumption of ideal behaviour of the solvent.
Answer the following questions:
(a) Give one example of miscible liquid pair which shows negative deviation from Raoult’s law. What is the reason for such deviation?
(b) (i) State Raoult’s law for a solution containing volatile components. 
OR 
(ii) Raoult’s law is a special case of Henry’s law. Comment. 
(c) Write two characteristics of an ideal solution.

Hint:

Negative deviation occurs when the intermolecular attraction between dissimilar molecules is stronger than between similar molecules.

Answer 1

(a) Example of Negative Deviation from Raoult’s Law:

Example: Acetone and Chloroform mixture.

Reason: Acetone and chloroform exhibit strong hydrogen bonding between their molecules, which is stronger than the interactions in the pure components. This stronger interaction leads to a decrease in vapour pressure than expected from Raoult's law, hence showing negative deviation.

(b) (i) Raoult’s Law for Volatile Components:

Statement: Raoult’s law states that the partial vapour pressure of each component in a solution is directly proportional to its mole fraction in the solution and the vapour pressure of the pure component.

Mathematical Expression: For a binary solution of components A and B,

• \( P_A = X_A \cdot P_A^0 \)
• \( P_B = X_B \cdot P_B^0 \)
• Total vapour pressure, \( P_{\text{total}} = P_A + P_B \)

OR

(b) (ii) Raoult’s Law as a Special Case of Henry’s Law:

Raoult’s law can be considered a special case of Henry’s law when the solute is a gas and the solvent is a liquid.

Henry’s law: The partial pressure of a gas in a liquid solution is proportional to its mole fraction in the solution.

Raoult’s law resembles this when the gas behaves ideally and dissolves uniformly in the solvent. So, for an ideal solution, Raoult’s law (for volatile solvents) and Henry’s law (for gas solutes) give a similar linear relationship between vapour pressure and mole fraction.

(c) Characteristics of an Ideal Solution:

1. There is no change in enthalpy of mixing (ΔH_mix = 0) when the components are mixed.
2. There is no change in volume upon mixing (ΔV_mix = 0).
3. The solution obeys Raoult’s law at all compositions and temperatures.