Problem 22 - A Complete Resource Book in Chemistry for JEE Main

Question

The azeotropic mixture of water and ethonal boils at \(78.15^{\circ} \mathrm{C}\). When this mixture is distilled, it is possible to obtain (a) pure \(\mathrm{H}_{2} \mathrm{O}\) (b) pure \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) (c) pure \(\mathrm{H}_{2} \mathrm{O}\) as well as pure \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) (d) neither \(\mathrm{H}_{2} \mathrm{O}\) nor \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) in their pure state

Step-by-Step Solution

Answer

(d) Neither water nor ethanol in their pure state.

Step 1: Understanding Azeotropic Mixtures

An azeotropic mixture is a mixture of two or more liquids whose composition is unchanged by simple distillation. This happens because they boil at a constant temperature and their vapor has the same proportions of components as the liquid mixture.

Step 2: Identifying Water and Ethanol Azeotrope

The azeotropic mixture described here is of water (\(\text{H}_2\text{O}\)) and ethanol (\(\text{C}_2\text{H}_5\text{OH}\)), which boils at \(78.15^{\circ}\text{C}\). At this temperature, the mixture's vapor and liquid phases are identical in composition.

Step 3: Distillation Limitations with Azeotropes

Due to the azeotropic nature, when distillation is performed, the vapor collected during boiling will condense into a liquid with the same composition as the original azeotropic mixture. Thus, it is not possible to obtain \(\text{H}_2\text{O}\) or \(\text{C}_2\text{H}_5\text{OH}\) in pure states through simple distillation.

Step 4: Conclusion on Pure Component Possibility

Given the characteristics of azeotropic mixtures, neither water nor ethanol can be obtained in their pure form from the azeotrope by regular distillation methods.

Key Concepts

Distillation

Distillation is a common technique used for separating mixtures based on the differences in their boiling points. This process typically involves heating a liquid mixture to form vapor, which is then condensed back into liquid to achieve separation.
For example, if a mixture contains substances with different boiling points, distillation can be used to isolate each component based on its unique boiling temperature.

• The mixture is heated, causing the component with the lower boiling point to vaporize first.
• The vapor is then cooled in a condenser, turning it back into liquid form.
• This condensed liquid, known as distillate, is collected separately.

However, when dealing with special types of mixtures like azeotropes, simple distillation alone cannot separate the components because their vapor composition is identical to the liquid composition.

Water and Ethanol Azeotrope

An azeotropic mixture, like that of water and ethanol, presents a unique challenge in separation processes. This particular mixture boils at a temperature of 78.15°C, at which point both the liquid and vapor phases have the same proportion of water and ethanol.

• In the case of the water-ethanol azeotrope, the mixture contains about 95% ethanol and 5% water.
• This composition remains constant during the boiling process, inhibiting the separation of the individual components via normal distillation.
• The inability to change the proportion through simple distillation classifies this mixture as azeotropic.

Understanding the behavior of such mixtures is crucial in fields like chemistry and chemical engineering, especially when attempting to purify substances.

Phase Equilibrium

Phase equilibrium refers to a state where multiple phases of a substance exist in balance without net change. In the context of water and ethanol azeotrope, phase equilibrium is achieved at the boiling point when the liquid and vapor compositions are identical.

• The mixture reaches this equilibrium at a specific temperature and pressure, leading to a stable composition.
• This equilibrium state is critical in understanding why azeotropes resist separation in distillation.
• At equilibrium, the properties such as density, refractive index, and specific heat of liquid and vapor phases closely match.

This phenomenon is essential for comprehending distillation limitations and exploring alternative methods for separating azeotropic mixtures.

Boiling Point

The boiling point of a liquid is the temperature at which its vapor pressure equals the surrounding pressure, causing it to transition into vapor. The water and ethanol azeotrope has a specific boiling point of 78.15°C.

• This temperature is lower than the boiling point of pure water (100°C) and close to that of pure ethanol (78.37°C).
• At this precise point, the water-ethanol mixture boils, and its vapor mirrors the liquid mixture in composition.
• Understanding boiling points is imperative for distillation and provides insight into why certain mixtures like azeotropes demand advanced separation techniques.

By studying boiling points, scientists and engineers can develop strategies such as pressure-swing distillation to effectively separate components in azeotropic mixtures.