Question

The system that forms maximum boiling azeotrope is:

• A. Benzene-toluene
• B. Acetone-chloroform
• C. Carbon-di-sulphide-acetone
• D. Ethyl alcohol-water

Hint:

Negative deviation from Raoult’s law \(\Rightarrow\) lower vapour pressure \(\Rightarrow\) higher boiling point \(\Rightarrow\) maximum boiling azeotrope.

Answer 1

The Correct Option is B

Concept: Azeotropes are mixtures of two or more liquids that boil at a constant temperature and possess the same composition in both liquid and vapour phases. They are broadly classified into:
• Minimum boiling azeotropes \(\rightarrow\) formed by positive deviation from Raoult’s law.
• Maximum boiling azeotropes \(\rightarrow\) formed by negative deviation from Raoult’s law.

Step 1: Understanding maximum boiling azeotropes.
A maximum boiling azeotrope is formed when the intermolecular attractions between unlike molecules are stronger than those between like molecules. Due to these stronger attractions:
• escaping tendency of molecules decreases,
• vapour pressure decreases,
• boiling point increases. Thus, the solution boils at a temperature higher than either of the pure liquids.

Step 2: Analyzing acetone-chloroform system.
Acetone contains a carbonyl oxygen atom having lone pair of electrons, while chloroform contains a highly polarized hydrogen atom. Strong hydrogen bonding occurs: \[ (CH_3)_2CO \cdots H-CCl_3 \] This strong interaction causes a large negative deviation from Raoult’s law.

Step 3: Comparing with other options.

• Benzene-toluene behaves nearly ideally.
• Ethanol-water forms a minimum boiling azeotrope due to positive deviation.
• CS\(_2\)-acetone also shows positive deviation. Therefore, acetone-chloroform forms a maximum boiling azeotrope.