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 \(\mathrm{H}_{2} \mathrm{O}\) nor \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) in their pure state.

Step 1: Understanding Azeotropes

An azeotrope is a mixture of two or more liquids that has the same composition in vapor and liquid phases at a certain temperature and pressure. This means that during distillation, the mixture boils and condenses at a specific temperature without changing its composition. In this case, the azeotropic mixture of water and ethanol boils at 78.15°C.

Step 2: Analyzing the Problem

Since this is an azeotropic mixture, when distillation is attempted, the vapor will have the same composition as the liquid mixture. This property prevents the separation of the components using simple distillation techniques.

Step 3: Determining the Feasibility of Obtaining Pure Components

Given that the azeotropic mixture cannot change composition upon boiling and condensing, it is not possible to obtain pure water or pure ethanol directly from this mixture through simple distillation. Additional methods like adding another component (special techniques) are required to break the azeotrope.

Key Concepts

Distillation

Distillation is a common method used to separate mixtures into their individual components. It works based on the principle that different substances have different boiling points. When a mixture is heated, the substance with the lowest boiling point will vaporize first. This vapor is then cooled in a condenser, turning it back into liquid form and allowing it to be collected separately.
In the case of a simple distillation, the process involves:

• Heating the mixture to the boiling point of the more volatile component.
• Vaporizing this component.
• Condensing the vapor back to a liquid.
• Collecting the liquid separately from the original mixture.

Distillation works well for separating components with significantly different boiling points. However, it becomes more complicated with mixtures like azeotropes where components boil together, making simple separation nearly impossible.

Ethanol-Water Azeotrope

An azeotrope is a specific type of mixture where the components have the same composition in both liquid and vapor phases at a particular temperature and pressure. The ethanol-water azeotrope is an example where simple distillation won't separate its components.

• The ethanol-water azeotrope occurs at 78.15°C with approximately 95.6% ethanol and 4.4% water by volume.
• At this point, the mixture behaves as if it were a single pure substance.
• During distillation of the azeotrope, both components vaporize and condense together, maintaining the same ratio.

Because of these properties, special techniques such as azeotropic distillation or pressure-swing distillation may be needed to separate ethanol and water in their pure forms.

Liquid-Vapor Equilibrium

Liquid-vapor equilibrium refers to the balance between a liquid and its vapor in a closed system. This occurs when the rate of evaporation of the liquid equals the rate of condensation of the vapor.
In an azeotropic mixture, such as the ethanol-water azeotrope, liquid-vapor equilibrium plays a crucial role. The unique boiling point at which the azeotropic mixture vaporizes happens due to:

• The equal distribution of components between the liquid and vapor phases.
• The prevention of simple distillation from altering the component's proportions within the mixture.
• The creation of a challenge in achieving pure separation of components in simpler distillation setups.

Understanding liquid-vapor equilibrium is essential for grasping why azeotropes require alternative methods for separation. These methods disrupt the equilibrium, allowing for a change in composition in either the liquid or vapor phase, thus enabling separation.