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Q.An azeotropic mixture of two liquids has a boiling point higher than either of the two liquids when it: (A) shows large negative deviation from Raoult's law (B) shows no deviation from Raoult's law (C) shows large positive deviation from Raoult's law (D) obeys Raoult's law

CBSECBSE Class XII Board 2023MCQ· 1mImportance★★★★★
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A positive azeotrope (maximum-boiling) forms when strong intermolecular attractions between unlike molecules cause large negative deviation from Raoult's law, lowering vapor pressure and raising the boiling point above either pure component.

Types of Azeotropes and Deviations from Raoult's Law

An azeotrope is a constant-boiling mixture where the liquid and vapor have identical compositions, so it cannot be separated by simple distillation. The key to understanding which type of azeotrope has a higher or lower boiling point lies in how the mixture deviates from ideal behavior.

Raoult's law predicts that the partial vapor pressure of each component is proportional to its mole fraction: PA=xAPA0P_A = x_A P_A^0. Real solutions deviate from this when intermolecular forces between unlike molecules (AA–BB interactions) differ from those between like molecules (AA–AA and BB–BB interactions).

Negative deviation occurs when AA–BB attractions are stronger than the average of AA–AA and BB–BB attractions. The molecules "prefer" to stay in the liquid phase, so the total vapor pressure is lower than Raoult's law predicts. Lower vapor pressure means a higher boiling point.

Positive deviation occurs when AA–BB attractions are weaker than the average of pure-component interactions. Molecules escape more readily into the vapor phase, giving higher vapor pressure and thus a lower boiling point.


Step-by-Step Reasoning

  1. Identify the azeotrope type

    The question asks for an azeotropic mixture with a boiling point higher than either pure liquid. This is called a maximum-boiling azeotrope or positive azeotrope (confusingly named, but "positive" refers to the boiling-point elevation, not the deviation type).

  2. Connect boiling point to vapor pressure

    A liquid boils when its vapor pressure equals atmospheric pressure. If a mixture has a lower vapor pressure than expected, it must be heated to a higher temperature to reach the boiling point.

  3. Determine the required deviation

    For the vapor pressure to be lower than predicted by Raoult's law, the solution must show negative deviation. This happens when intermolecular forces between the two components are unusually strong—think hydrogen bonding between unlike molecules (e.g., chloroform and acetone, or nitric acid and water). …

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