Problem 38 - A Complete Resource Book in Chemistry for JEE Main

Question

In steam distillation, the vapour pressure of the volatile organic compound is (a) more than atmospheric pressure (b) equal to atmospheric pressure (c) less than atmospheric pressure (d) none of these

Step-by-Step Solution

Answer

In steam distillation, the vapour pressure of the volatile organic compound is equal to atmospheric pressure.

Step 1: Understanding Steam Distillation

In steam distillation, a mixture of steam and a volatile organic compound is heated. The compound is distilled over together with water, often resulting in lower boiling points than the compound would usually have.

Step 2: Concept of Vapour Pressure

Vapour pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system. When a volatile compound is exposed to steam, its vapour pressure contributes to reaching total atmospheric pressure.

Step 3: Relating Vapour Pressure and Atmospheric Pressure in Steam Distillation

In steam distillation, the sum of the vapour pressures of the water (steam) and the volatile organic compound equals atmospheric pressure. This allows the distillation process to occur at temperatures lower than the boiling point of the pure compound.

Step 4: Conclusion of Analysis

Based on the principles of steam distillation, the correct answer is that the vapour pressure of the volatile organic compound in a steam distillation setup will combine with the steam to reach atmospheric pressure. Hence, the correct choice is that the vapour pressure of the volatile organic compound is 'equal to atmospheric pressure'.

Key Concepts

Vapour Pressure in Steam Distillation

Vapour pressure is a key concept in understanding how steam distillation works. It refers to the pressure exerted by a gas above its liquid form in a closed system at a certain temperature. This pressure is significant in steam distillation because it affects the boiling point of liquids. When steam distillation occurs, heat is applied to a mixture of steam and a volatile organic compound. This heat causes the compound to vaporize even at lower temperatures than its normal boiling point. This is important because the vapour pressure of the volatile compound sums up with that of the steam.

• The **total vapour pressure** is the sum of the vapour pressures of water and the compound.
• This sum must equal the **atmospheric pressure** for boiling to occur.

By lowering the boiling point, steam distillation allows many compounds to be distilled at temperatures that prevent decomposition, making vapour pressure an essential factor in this process.

Understanding Volatile Organic Compounds

Volatile organic compounds (VOCs) play a crucial role in steam distillation. These are chemical compounds that can easily vaporize at room temperature. Their volatility makes them suitable for steam distillation, contributing to their vapour pressure significantly. VOCs have unique characteristics:

• They usually have **high vapour pressures**, which means they turn into vapour more easily.
• They are often used in the distillation of essential oils or fragrance extraction.

Steam distillation leverages the volatility of these compounds to effectively separate them from non-volatile substances. By utilizing their ability to vaporize easily, the process efficiently extracts compounds that would otherwise require high temperatures, preventing potential degradation of sensitive materials.

The Role of Atmospheric Pressure

Atmospheric pressure is the pressure exerted by the weight of the air in the Earth's atmosphere. It's a crucial factor in steam distillation because it sets the threshold for boiling to occur. In a steam distillation setup, the process relies on achieving this external pressure condition for both water and volatile organic compounds.

• **At sea level** the average atmospheric pressure is about 101.3 kPa or 1 atmosphere.
• In steam distillation, the vapour pressures of water and the VOCs combine to equal atmospheric pressure, enabling distillation.

Achieving atmospheric pressure allows the distillation to occur at reduced temperatures. By reaching this equilibrium, steam distillation becomes possible without heating substances to their higher boiling points. This makes atmospheric pressure a pivotal element in allowing the safe and efficient distillation of temperature-sensitive materials.