📘 2.4.1 Vapour Pressure of Liquid–Liquid Solutions

 Certainly! Here's a CBSE Class 12 Chemistry–friendly explanation of Section 2.4.1: Vapour Pressure of Liquid–Liquid Solutions, based on the NCERT Chapter 1 – Solutions. It’s simplified for better understanding and suitable for classroom presentation or notes.

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📘 2.4.1 Vapour Pressure of Liquid–Liquid Solutions

(NCERT Class 12 Chemistry – Chapter 1: Solutions)

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💧 What is a Liquid–Liquid Solution?

A liquid–liquid solution is formed when:

• Both solute and solvent are liquids

• The components may or may not be volatile

Examples:

• Water + ethanol

• Acetone + benzene

When both liquids are volatile, their vapour pressures combine, and their behaviour can be ideal or non-ideal.

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✅ Ideal Liquid–Liquid Solutions

An ideal solution is one that obeys Raoult’s Law at all concentrations and temperatures.

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📘 Raoult’s Law:

For two volatile liquids A and B in a binary solution:

$$p_A = x_A \cdot p_A^0 \quad \text{and} \quad p_B = x_B \cdot p_B^0$$ $$p_{\text{total}} = p_A + p_B = x_A p_A^0 + x_B p_B^0$$

Where:

• $p_A, p_B$: partial vapour pressures

• $x_A, x_B$: mole fractions of components A and B

• $p_A^0, p_B^0$: vapour pressures of pure components

📌 Note: $x_A + x_B = 1$

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📊 Graphical Representation (Ideal Solution):

• If plotted, partial vapour pressures and total vapour pressure vs mole fraction give straight lines.

• The graph is linear, showing no deviation from Raoult’s Law.

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🔎 Characteristics of Ideal Liquid–Liquid Solutions:

• No enthalpy change: $\Delta H_{\text{mix}} = 0$

• No volume change: $\Delta V_{\text{mix}} = 0$

• Intermolecular forces between A–B = A–A = B–B

🧪 Examples:

• Benzene + Toluene

• n-Hexane + n-Heptane

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📝 Board Exam Tips:

• Always write the Raoult’s Law formula

• Label the graph of vapour pressure vs mole fraction

• State the conditions for ideal solution

• Give examples of ideal liquid–liquid mixtures

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Would you like me to explain non-ideal solutions (positive/negative deviation) or prepare a diagram slide for Raoult’s Law as well?