Lecture Notes on Colligative Properties

Introduction to Colligative Properties

• Solutions differ from pure solvents due to solute particles interfering with physical processes.
• Colligative properties depend on solute concentration, not chemical identity.

Key Colligative Properties

• Vapor Pressure Lowering
• Boiling Point Elevation
• Freezing Point Depression

Concentration Measurement

• Molality (m):
  • Defined as moles of solute per kilogram of solvent.
  • Example: 1.31 molal solution of iodine in dichloromethane.
  • Denoted by a lowercase italic 'm'.

Effect of Solute Particles

• Solute particles interfere at the interface between phases.

Vapor Pressure Lowering

• Liquid has specific vapor pressure due to surface particles evaporating.
• Solute at surface reduces area for solvent to evaporate, lowering vapor pressure.
• New vapor pressure = vapor pressure of pure solvent x mole fraction of solvent.

Boiling Point Elevation

• Solute blocks solvent molecules from becoming gas, requiring more heat.
• Boiling point change = molality x solvent-specific constant (Kb).

Freezing Point Depression

• Solute prevents efficient lattice formation in freezing.
• Freezing point change = molality x another constant (Kf).
• Application: Salt on icy streets lowers freezing point of water.

Constants

• Kb and Kf: Found in textbooks or online, specific to solvents.

Practical Implications

• Boiling Point Adjustment: Add change to original boiling point as solute raises it.
• Freezing Point Adjustment: Subtract change from original freezing point as solute lowers it.

Conclusion

• Understanding colligative properties helps in practical applications like de-icing.
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