Lecture on Colligative Properties and Vapour Pressure

Introduction

Host: Welcome to the Solution Chapter.
Topic: One-shot study on Pollution and Air Pressure (Colligative Properties and Vapour Pressure).
Checking: Ensuring everyone can hear and see properly before starting.
Topics to be covered: Vapour pressure, Raoult's Law, colligative properties, and van't Hoff factor.

Concept: The pressure exerted by vapors of a liquid at equilibrium with its liquid phase.
Condition: Rate of evaporation equals rate of condensation.
Explanation: When liquid molecules vaporize and exert pressure on the surface; this is called vapour pressure.
Illustration: Closed container with liquid and vapor forming above, where vapour pressure develops.

Temperature: Vapour pressure is directly proportional to temperature. As temperature increases, more molecules have enough energy to escape into the vapor phase, increasing vapour pressure.
Intermolecular Forces: Vapour pressure is inversely proportional to intermolecular forces. Stronger intermolecular forces in a liquid mean fewer molecules escape into the vapor phase, lowering vapour pressure.

Boiling Point Relation: Vapour pressure is inversely related to boiling point. Higher vapour pressure corresponds to a lower boiling point.
Example: Comparing different liquids’ boiling points and vapour pressures using specific substances like HBr, HI, HF, etc.

Raoult's Law: For a solution of volatile liquids, the total vapour pressure is the sum of partial pressures of each component, proportional to their mole fractions.
Mathematical Form: P_total = P_A + P_B; we can also write P_total = P^0_A * X_A + P^0_B * X_B for volatile components.
Non-Volatile Solute Scenario: When a non-volatile solute is added, vapour pressure decreases.
Example: Mixing two volatile liquids and deriving the total vapour pressure using their individual vapour pressures and mole fractions.

Raoult's Law Graphs: Linear plots with positive or negative deviations depending on the interactions between molecules (ideal, positive deviation, negative deviation).
Ideal Solutions: Exhibit a linear relationship where actual vapour pressures match calculated ones.

Colligative Properties: Properties depending on the number of solute particles, not their identity. Includes relative lowering of vapour pressure, elevation of boiling point, depression of freezing point, and osmotic pressure.
Lowering of Vapour Pressure: Addition of non-volatile solute decreases the surface area for solvent molecules to vaporize, lowering the vapour pressure. Mathematically: ΔP = P^0 - P = X_B * P^0.
Elevation of Boiling Point: Boiling point elevation due to non-volatile solute addition which raises boiling point. Mathematically: ΔT_b = i * K_b * m.
Depression of Freezing Point: Adding solutes extends the freezing point lower than that of pure solvent. Mathematically: ΔT_f = i * K_f * m.
Osmotic Pressure: The pressure required to prevent solvent from passing into the solution through a semipermeable membrane. Mathematically: Π = i * M * R * T.

Boiling Point Elevation (ΔT_b): ΔT_b = K_b * m (where K_b is the ebullioscopic constant, m is molality)
Freezing Point Depression (ΔT_f): ΔT_f = K_f * m (where K_f is the cryoscopic constant, m is molality)
Osmotic Pressure (Π): Π = M * R * T (where M is molarity, R is the gas constant, T is absolute temperature in Kelvin)

Isoosmotic and Hyper/Hypotonic Solutions: Comparisons based on osmotic pressure. Isotonic solutions have equal osmotic pressure, hypertonic have higher osmotic pressure compared to another, and hypotonic have lower.
Azeotropes: Mixtures with constant boiling points, either showing minimum or maximum boiling points and cannot be separated by distillation.

Purpose: Accounts for the degree of dissociation or association of solute particles in a solution, affecting colligative properties.
Calculation: i = normal molar mass/calculated molar mass or i = total no. of particles after dissociation/initial no. of particles before dissociation.
Cases: Association: For example, dimerization where multiple units associate reducing the number of particles.
Cases: Dissociation: For example, electrolyte dissociation increasing the number of particles.

Calculations: Examples using strong and weak electrolytes, impacts of dissociation on boiling points, freezing points, and osmotic pressures.

Review: Summarizing key topics such as vapour pressure, colligative properties, Raoult's Law, and the Van't Hoff factor.
Closing: Encouragement to practice more problems to solidify understanding.