Problem 24 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following has high vapour pressure at temperature below its melting point? (a) benzoic acid (b) citric acid (c) salicylic acid (d) all

Step-by-Step Solution

Answer

Benzoic acid has high vapour pressure below its melting point.

Step 1: Understanding Vapour Pressure

Vapour pressure is the pressure exerted by the vapour of a substance in equilibrium with its liquid or solid form. Substances with high vapour pressure at lower temperatures tend to sublimate easily or have significant volatility even below their melting points.

Step 2: Examine Melting Points and Volatility

Review the melting points of benzoic acid, citric acid, and salicylic acid. Benzoic acid sublimates at temperatures below its melting point (approximately 122.4 °C), indicating high vapour pressure. Citric acid and salicylic acid crystallize readily and are less volatile than benzoic acid.

Step 3: Compare Volatility Below Melting Points

Since benzoic acid is known to sublimate, it has a higher vapour pressure below its melting point compared to the other acids. Citric and salicylic acid are less volatile in solid form and have lower vapour pressures below their melting points.

Key Concepts

Sublimation

Sublimation is a fascinating process where a substance transitions directly from a solid to a gas phase, bypassing the liquid phase entirely. This process occurs under specific conditions, often involving low pressure or temperature.

This is usually seen in substances with high vapor pressure. Substances like benzoic acid can sublimate at temperatures below their melting points. Their molecules have enough energy to break free from the solid phase and transition into a gas.

Examples of common substances that exhibit sublimation include:

• Dry ice (solid carbon dioxide)
• Naphthalene (found in mothballs)
• Benzoic acid, as seen in our exercise example

Understanding sublimation is key to grasping the volatility of a substance and how it behaves under various conditions.

Volatility

Volatility refers to how easily a substance can vaporize. This is closely related to vapor pressure. A volatile substance has a high vapor pressure at a given temperature, indicating that it can transition into a gas easily.

Factors affecting volatility include:

• Temperature: Higher temperatures increase kinetic energy, facilitating vaporization.
• Intermolecular forces: Substances with weaker forces between molecules vaporize more readily.
• Molecular weight: Lighter molecules generally have higher volatility than heavier ones.

In our exercise, benzoic acid shows higher volatility below its melting point compared to citric acid and salicylic acid. This is because benzoic acid can sublimate easily, demonstrating its significant vapor pressure and volatility.

Melting Points

The melting point of a substance is the temperature at which it transitions from a solid to a liquid. This is when the kinetic energy of the molecules overcomes the intermolecular forces holding them in a solid structure.

Melting points are crucial for understanding a substance's behavior under different conditions:

• Materials with lower melting points generally have higher vapor pressures at those lower temperatures. This can indicate a tendency to sublimate.
• The melting point also provides insight into the strength of intermolecular bonds—the higher the melting point, the stronger the bonds.

In our exercise, analyzing the melting points helps differentiate the volatility and subliming properties of benzoic acid, citric acid, and salicylic acid. These factors explain the variances in vapor pressure at sub-melting-point temperatures.