Problem 91 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following pair of compounds can be separated by using fractional distillation (a) Acetone and methyl alcohol (b) Benzene and toluene (c) Ethyl alcohol \((95.5 \%)\) and water \((4.5 \%)\) (d) Crude petroleum (gasoline and kerosene)

Step-by-Step Solution

Answer

Benzene and toluene can be separated by fractional distillation.

Step 1: Understanding Fractional Distillation

Fractional distillation is a separation process used to separate liquids with different boiling points. It works best for mixtures where the components have a significant difference in boiling points, allowing them to vaporize at different temperatures and be collected separately.

Step 2: Analyzing Each Pair

(a) Acetone and methyl alcohol have boiling points of 56°C and 65°C respectively. (b) Benzene and toluene have boiling points of 80°C and 111°C respectively. (c) Ethyl alcohol and water, even at 95.5% purity, are notoriously difficult to separate by fractional distillation because they form an azeotrope. (d) Crude petroleum components like gasoline and kerosene have a wide range of boiling points.

Step 3: Identifying Closest Boiling Points

For effective fractional distillation, closer boiling point differences between liquids increase effectiveness of separation. However, forming an azeotropic mixture (like in option c) prevents easier separation. Benzene and toluene have a significant difference in boiling points, 31°C, which is relatively suitable for fractional distillation compared to others.

Step 4: Selecting the Correct Pair

Given that benzene (80°C) and toluene (111°C) have a suitable boiling point difference and do not form azeotropes, they can be effectively separated using fractional distillation. The other options either have components too close in boiling point without significant differences or form azeotropic mixtures, making separation difficult.

Key Concepts

Boiling Points

Boiling points are crucial in determining how substances behave when heated. The boiling point of a liquid is the temperature at which it changes from a liquid to a gas. This concept is essential in processes like fractional distillation.

• Each substance has a unique boiling point, determined by its molecular structure and intermolecular forces.
• Liquids with lower boiling points vaporize more easily than those with higher boiling points.

In fractional distillation, substances in a mixture are separated based on their differing boiling points. The greater the difference in boiling points, the easier it is to separate the substances.

• For example, benzene and toluene have boiling points of 80°C and 111°C respectively. They can be separated effectively because there is a significant temperature difference.
• A mixture of acetone and methyl alcohol is harder to separate because their boiling points (56°C and 65°C) are closer together.

Azeotropes

Azeotropes complicate the process of separating mixtures by boiling. These are mixtures of two or more liquids that maintain a constant boiling point and composition during distillation. That means they behave as a single pure substance.

• In some cases, this boiling point can be either lower or higher than the individual components’ boiling points.
• Common examples include mixtures like ethyl alcohol and water, which form an azeotrope at 95.5% alcohol purity.

Azeotropes are problematic in fractional distillation because they do not separate into their components at their boiling points.

• The homogeneous nature of an azeotropic mixture during boiling complicates attempts to extract pure components.
• Special techniques or additives are often needed to break the azeotropic behavior and enable separation.

Separation Techniques

Separation techniques are methods used in chemistry to divide components of a mixture. Fractional distillation is one of the most common techniques for liquids, especially when they have differing boiling points.

• It involves heating the mixture to vaporize the component with the lowest boiling point first.
• The vapor is then condensed back into a liquid and removed from the mixture, resulting in partial separation after each cycle.

However, not all mixtures are suitable for fractional distillation. Azeotropes, for example, require more advanced separation techniques. Other separation methods include:

• Simple distillation, used when the boiling point difference is very large.
• Filtration, typically used for separating solids from liquids.
• Chromatography, for separating components based on differing affinities to the stationary phase.

Understanding the properties of the mixture components is vital to selecting the most appropriate method.