Problem 30 - A Complete Resource Book in Chemistry for JEE Main

Question

Aniline is insoluble in water and possesses a vapour pressure of \(10-15 \mathrm{~mm} \mathrm{Hg}\) at \(373 \mathrm{~K}\). It can be conveniently purified by (a) simple distillation (b) steam distillation (c) crystallization (d) sublimation

Step-by-Step Solution

Answer

Aniline can be conveniently purified by steam distillation.

Step 1: Understanding the Physical Properties of Aniline

Aniline is insoluble in water and has a relatively high vapor pressure at a given temperature. This suggests that it can exist as a vapor at lower temperatures when mixed with steam.

Step 2: Exploring Purification Methods

The methods provided are simple distillation, steam distillation, crystallization, and sublimation. We need to choose a method that aligns with the properties of aniline: insolubility in water and has a moderate vapor pressure.

Step 3: Evaluating Simple Distillation

Simple distillation is suitable for volatile liquids with high purity when the compounds differ significantly in boiling points. Aniline's moderate vapor pressure allows some volatility, but its insolubility in water isn't a primary consideration for simple distillation.

Step 4: Evaluating Steam Distillation

Steam distillation leverages the insolubility of compounds in water and relies on the formation of a vapor with water at lower temperatures. This method is ideal for purifying compounds like aniline that are volatile and form immiscible mixtures with water.

Step 5: Finalizing the Purification Method

Crystallization is for solid compounds, while sublimation is for solid-to-vapor transitions bypassing the liquid state. Aniline, being a liquid at room temperature, is best purified by vaporizing with steam.

Key Concepts

Steam Distillation

Steam distillation is an effective method for purifying substances like aniline, which are insoluble in water and have notable vapor pressures. This process involves distilling water along with the compound to form a mixture of vapors. When heated, steam and the compound's vapors rise together and condense back to liquid form later. This allows the removal of impurities without exposing the compound to high temperatures.

Advantages of Steam Distillation

• Purifies heat-sensitive compounds
• Works well with immiscible liquids
• Minimizes thermal decomposition

By using steam distillation, you can effectively purify aniline while maintaining its integrity.

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid form. It measures the tendency of particles to escape from the liquid surface into the gaseous phase. Aniline has a vapor pressure of 10-15 mmHg at 373 K, indicating its readiness to vaporize under certain conditions. The higher the vapor pressure at a given temperature, the more volatile the compound is.

Why Vapor Pressure Matters

• Influences boiling point
• Indicates volatility
• Affects distillation efficiency

Knowing aniline's vapor pressure helps in choosing effective purification methods that involve phase transitions.

Insolubility

When a substance is insoluble, it does not dissolve in a given solvent. Aniline is insoluble in water, meaning it forms a separate phase when mixed with it. This property plays a crucial role in purification methods like steam distillation, which capitalizes on the substance's inability to mix with water.

Impact on Purification

Insolubility allows steam distillation to be effective as it prevents aniline from dissolving in the water. Instead, it forms a distinct phase that can be selectively distilled. Through understanding insolubility, one can successfully choose the correct purification technique.

Boiling Point

The boiling point of a compound is the temperature at which it changes from a liquid to a gas. During distillation processes, the boiling point is an essential factor because it determines the conditions under which a compound will vaporize. Aniline's boiling point is relatively high, but by employing steam distillation, it can vaporize at a lower temperature. This protects the compound from potential degradation.

Factors Affecting Boiling Point

• Intermolecular forces
• Molecular weight
• Vapor pressure

Understanding these factors helps in determining the suitable temperature range for purifying aniline.

Volatile Liquids

Volatile liquids are substances that easily vaporize due to their low boiling points or high vapor pressures. Aniline is considered a volatile liquid because of its ability to turn into vapor at moderate temperatures when paired with steam distillation.

Characteristics of Volatile Liquids

• High vapor pressure
• Low boiling point relative to ambient temperatures
• Readily form vapors at lower temperatures with heat

The volatility of aniline aids in its efficient purification via steam distillation, leveraging its propensity to vaporize without needing extreme heat.