Problem 129 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following can form minimum boiling point azeotropic mixture? (a) Ethyl alcohol + water (b) Carbon tetrachloride \(+\) chloroform (c) Ethyl alcohol + water (d) Acetone \(+\) chloroform

Step-by-Step Solution

Answer

Ethyl alcohol + water forms a minimum boiling point azeotropic mixture.

Step 1: Identify Azeotropes

Azeotropic mixtures are mixtures that vaporize without a change in composition. They have either a maximum or a minimum boiling point.

Step 2: Understand Minimum Boiling Azeotropes

A minimum boiling azeotrope occurs when the boiling point of the mixture is lower than that of any of its constituents. This is often due to a positive deviation from Raoult's Law, where the intermolecular forces between unlike molecules are weaker than those between like molecules.

Step 3: List Known Minimum Boiling Azeotropes

Ethyl alcohol (ethanol) and water are well-known to form a minimum boiling point azeotrope, with a boiling point lower than either ethanol or water individually.

Step 4: Analyze Other Options

Neither carbon tetrachloride with chloroform nor acetone with chloroform form known minimum boiling azeotropes. In contrast, these combinations may exhibit different boiling behaviors or form maximum boiling azeotropes.

Step 5: Conclude Based on Analysis

The known minimum boiling azeotrope from the given options is ethyl alcohol with water.

Key Concepts

Minimum Boiling Azeotropes

Minimum boiling azeotropes are special mixtures that possess a unique property: they boil at a lower temperature than any of their individual components. This curious behavior arises because the interactions between the different molecules in the mixture result in a weaker overall structure, leading to a lower boiling point.

Some important aspects of minimum boiling azeotropes include:

• They demonstrate a boiling point reduction compared to their pure components.
• The formation is often due to positive deviations from Raoult's Law.
• They lead to unique distillation challenges, as they vaporize with a constant composition.
• Common examples include ethanol and water.

Understanding minimum boiling azeotropes is crucial when dealing with solutions or mixtures, particularly in fields such as chemical engineering and physical chemistry.

Raoult's Law

Raoult's Law is a principle in chemistry that describes how the vapor pressures of individual components in an ideal solution contribute to the total vapor pressure. For ideal solutions, the law can be expressed as:
The partial vapor pressure of a component in the solution (\( P_i \)) is equal to the mole fraction of the component in the liquid phase (\( x_i \)) times the vapor pressure of the pure component (\( P_i^0 \)).
Formula: \[P_i = x_i \times P_i^0\]
In the context of azeotropes, Raoult's Law helps explain why certain mixtures behave as they do. However, azeotropes often show deviation from Raoult's Law:

• Positive deviation occurs when the intermolecular forces between different components are weaker than those between like components, leading to a minimum boiling azeotrope.
• Negative deviation happens when these forces are stronger, but this leads to maximum boiling azeotropes.

By understanding these deviations, scientists can better predict and manipulate the behaviors of non-ideal mixtures.

Intermolecular Forces

Intermolecular forces are crucial determinants of a substance's physical properties, including boiling and melting points, vapor pressure, and viscosity. These forces arise from interactions between molecules in a substance, and they vary in strength based on the types of molecules involved.

Common types of intermolecular forces include:

• Dipole-Dipole Interactions: Occur between polar molecules where positive and negative charges attract each other.
• London Dispersion Forces: Present in all molecules, these are weak forces due to instantaneous polarization.
• Hydrogen Bonds: A special type of dipole interaction, significantly stronger, occurring in molecules with H-F, H-O, or H-N bonds.

In the context of azeotropes, the balance of these forces can lead to different boiling behaviors. For instance, ethanol and water form a minimum boiling azeotrope due to weaker intermolecular forces between the different molecules compared to the forces in pure components. Understanding these forces helps in predicting how mixtures will behave and is essential for designing separation processes in chemical industries.