Problem 15 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the statements given below concerning properties of solutions, describe a colligative effect? (a) boiling point of pure water decreases by the addition of ethanol (b) vapour pressure of pure water decreases by the addition of nitric acid (c) vapour pressure of pure benzene decreases by the addition of naphthalene (d) boiling point of pure benzene increases by the addition of toluene

Step-by-Step Solution

Answer

Statements b, c, and d describe colligative effects.

Step 1: Understand Colligative Properties

Colligative properties are properties of solutions that depend on the number of solute particles in a given amount of solvent, but not on the identities of the solute particles. They include vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure.

Step 2: Analyze Each Statement

Review each statement in the context of colligative properties: (a) Boiling point of pure water decreases - This is not a colligative effect as dilution typically raises boiling point. (b) Vapor pressure of pure water decreases - Vapor pressure lowering is a colligative property. (c) Vapor pressure of pure benzene decreases - Vapor pressure lowering is a colligative property. (d) Boiling point of pure benzene increases - Boiling point elevation is a colligative property.

Step 3: Identify the Correct Options

Colligative effects occur in (b), (c), and (d) where the addition of a solute to a solvent decreases vapor pressure or increases boiling point. These changes are due to the number of solute particles, not their chemical nature.

Key Concepts

Vapor Pressure Lowering

When you add a solute to a solvent, the vapor pressure of the solvent decreases. This phenomenon is known as vapor pressure lowering. It's an important colligative property of solutions. The reason this happens is that solute particles occupy space at the surface of the liquid, where evaporation occurs. They essentially "block" the solvent molecules from escaping into the vapor phase. As a result, fewer solvent molecules can evaporate, leading to a lower vapor pressure.

The decrease in vapor pressure is directly proportional to the concentration of solute particles. It doesn't matter what type of particles they are; only the number counts. This is why it's categorized as a colligative property.

• In a scenario where nitric acid is added to water, as in option (b) from the exercise, the vapor pressure of the water decreases because of this very property.
• Similarly, adding naphthalene to benzene as in option (c) causes a decrease in benzene's vapor pressure, demonstrating the concept yet again.

Boiling Point Elevation

Boiling point elevation occurs when you add a non-volatile solute to a solvent. This addition increases the boiling point of the solution compared to the pure solvent. The increase happens because the solute particles disrupt the solvent molecules at the surface, requiring a higher temperature to allow the solvent molecules to escape as vapor. Simply put, you need more heat energy to make the liquid boil.

This is another classic colligative property, relying on the number of solute particles rather than their identity. For each mole of solute particles added, the boiling point rises by a certain amount, depending on the solvent. Moreover, the formula used to calculate this change is:\[\Delta T_b = i \cdot K_b \cdot m\]where:

• \(\Delta T_b\) is the boiling point elevation
• \(i\) is the van't Hoff factor, representing the number of particles the solute splits into
• \(K_b\) is the ebullioscopic constant of the solvent
• \(m\) is the molality of the solution

In the exercise, adding toluene to benzene, as mentioned in option (d), leads to an increase in benzene's boiling point, illustrating this concept.

Solute-Solvent Interactions

In the framework of colligative properties, solute-solvent interactions play a crucial role. These interactions dictate how the solute particles influence the properties of the solvent. When a solute is introduced to a solvent, it doesn't just sit there; it interacts uh with the solvent molecules. These interactions affect how solvent molecules escape as vapor (lowering vapor pressure) and impact the energy needed to transition to the gas phase (elevating boiling point).

Solute particles can be any type of dissolved substance: ions, molecules, or atoms. Their common characteristic concerning colligative properties is that their number, not their chemical makeup, governs the effect on the solvent. For example:

• In vapor pressure lowering, solute molecules occupy space at the solvent surface, reducing escape into gas due to reduced surface area available for evaporation.
• In boiling point elevation, solute particles disrupt the orderly phase change of solvent molecules, requiring more heat energy to reach the boiling point.

Therefore, the introduction of any solute, whether an acid like nitric acid or a hydrocarbon like toluene and naphthalene, would interact with solvents like water and benzene to create these colligative effects as discussed.