Question

An azeotropic mixture of two liquids has boiling point higher than either of them when it:

• A. is saturated
• B. shows negative deviation from Raoult's law
• C. shows positive deviation from Raoult's law
• D. shows no deviation from Raoult's law

Hint:

Summary of Azeotrope Boiling Points: - Negative Deviation from Raoult's Law \( \rightarrow \) Lower vapor pressure \( \rightarrow \) Maximum Boiling Azeotrope (Boiling point is higher than either pure component). - Positive Deviation from Raoult's Law \( \rightarrow \) Higher vapor pressure \( \rightarrow \) Minimum Boiling Azeotrope (Boiling point is lower than either pure component).

Answer 1

The Correct Option is B

Concept: A binary solution exhibiting a non-ideal vapor-liquid equilibrium profile can form an azeotropic state where the liquid composition matches the vapor composition exactly at a specific temperature. The direction of deviation from Raoult's Law dictates the temperature characteristics of the azeotropic point:

• Positive Deviation: Cohesive forces between unlike components are weaker than those within pure components. This increases the total vapor pressure of the mixture, producing a maximum vapor pressure and a corresponding

minimum boiling point.

• Negative Deviation: Cohesive forces between unlike components are stronger than those within pure components. This decreases the total vapor pressure of the mixture, producing a minimum vapor pressure and a corresponding

maximum boiling point.

Step 1: Analyzing negative deviations from Raoult's Law.
When a binary liquid mixture exhibits negative deviation from Raoult's law, the intermolecular attractive forces between the different components (A-B interactions) are stronger than the forces between like molecules (A-A and B-B interactions). Examples include mixtures of acetone and chloroform, or strong mineral acids with water. Because the molecules bind more tightly to each other in solution, they have a lower tendency to escape into the gas phase. Consequently, the total vapor pressure of the mixture at any given temperature is lower than what would be predicted for an ideal solution by Raoult's law: \[ P_{total} \lt x_A \cdot P_A^{sat} + x_B \cdot P_B^{sat} \]

Step 2: Connecting the vapor pressure profile to boiling points.
A boiling point is reached when the total vapor pressure of a liquid equals the external atmospheric pressure. Because a mixture with negative deviation has a lower vapor pressure, it requires more thermal energy to raise its vapor pressure to match the atmospheric pressure. This creates a minimum in the vapor pressure-composition curve, which corresponds to a maximum on the boiling point-composition curve. At this maximum point, the azeotropic mixture has a boiling point higher than either of the pure components. Therefore, an azeotropic mixture has a boiling point higher than its individual constituents when it shows a

negative deviation from Raoult's law.