Question

Which one of the following forms minimum boiling azeotrope at one atmosphere?

• A. acetone-chloroform
• B. benzene-toluene
• C. hydrochloric acid-water
• D. ethanol-water

Hint:

Quick rule of thumb for Azeotropes: - Positive Deviation from Raoult's law \( \rightarrow \) Higher total vapor pressure \( \rightarrow \) Minimum Boiling Azeotrope (Example: Ethanol + Water). - Negative Deviation from Raoult's law \( \rightarrow \) Lower total vapor pressure \( \rightarrow \) Maximum Boiling Azeotrope (Example: Acetone + Chloroform, \( \text{HNO}_3 \) + Water, HCl + Water).

Answer 1

The Correct Option is D

Concept: An azeotrope is a unique liquid mixture of two or more components that boils at a constant temperature and retains the exact same composition in both its liquid and vapor phases during distillation. Azeotropic behaviors arise from non-ideal solution characteristics that deviate from Raoult's Law:

• Positive Deviation from Raoult's Law: The intermolecular forces between unlike molecules (A-B forces) are weaker than those between like molecules (A-A and B-B forces). This increases the total vapor pressure of the system above ideal predictions, producing a maximum in the vapor pressure curve and a corresponding

minimum boiling azeotrope.

• Negative Deviation from Raoult's Law: The intermolecular forces between unlike molecules (A-B forces) are stronger than those between like molecules (A-A and B-B forces). This decreases the total vapor pressure of the system below ideal predictions, producing a minimum in the vapor pressure curve and a corresponding

maximum boiling azeotrope.

Step 1: Evaluating the listed mixtures individually.
Let us analyze each solution mixture option given in the problem statement:

• Acetone--Chloroform: This binary mixture forms strong hydrogen bonds between the hydrogen atom of chloroform and the carbonyl oxygen atom of acetone. This strong attraction causes a significant

negative deviation from Raoult's law, resulting in a maximum boiling point azeotrope (boiling point approximately \( 64.7^\circ\text{C} \), which is higher than both pure acetone and pure chloroform).

• Benzene--Toluene: These are structurally similar hydrocarbons that form a nearly

ideal solution system. This mixture obeys Raoult's law closely across all concentrations and does not form an azeotrope.

• Hydrochloric acid (HCl)--Water: This system exhibits a strong exothermic interaction in solution, showing a significant

negative deviation from Raoult's law. It forms a maximum boiling azeotrope at approximately 20.2 wt% HCl with a boiling point of about \( 108.6^\circ\text{C} \).

• Ethanol--Water: Ethanol and water exhibit a notable

positive deviation from Raoult's law due to structural breaking disruptions in the pure liquid hydrogen-bonding networks when mixed. This positive deviation creates a maximum vapor pressure peak, which produces a

minimum boiling azeotrope at 1 atm with a composition of approximately 95.6% ethanol by weight and a boiling point of \( 78.17^\circ\text{C} \) (which is lower than the boiling points of both pure ethanol, \( 78.3^\circ\text{C} \), and pure water, \( 100^\circ\text{C} \)).

Step 2: Conclusion.
Based on this analysis, the ethanol-water system is a classic example of a minimum boiling azeotrope at atmospheric pressure.