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Colligative Properties

Chapter 16.3

Learning Objectives

Know the definition of colligative properties

Know what the important colligative properties are

Be able to think through logically the lowering or raising of these properties due to colligative processes

Colligative Properties: Definition

Colligative properties depend only on the number of solute particles present (concentration), not on the identity of the solute particles.

Among colligative properties are Vapor pressure lowering Boiling point elevation Freezing point depression Osmotic pressure (book does not cover)

Vapor Pressure Lowering

As solute molecules are added to a solution, the solvent become less volatile ( = decreased vapor pressure).

Solute-solvent interactions contribute to this effect. Shells of water of solvation reduce the availability of solvent molecules for vaporization.

Vapor Pressure Lowering

Boiling Point Elevation

Adding a solute, reduces vapor pressure, more energy is needed to make the solutions boil, raising the boiling point.

Boiling point elevation = BP of the solution - BP of the solvent.

Directly proportional to the number of solute molecules.

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What is freezing point depression?

Freezing point depression is the temperature difference between the freezing point of a solution and the pure solvent

Presence of a solute interferes with solid formation, requires more kinetic energy be withdrawn for solution to freeze

Freezing Point Depression

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How is water affected?

Boiling point of water increases 0.512C per mol of solute particles in 1000 g water

Freezing point of water decreases 1.86C per mol of solute particles in 1000 g water

Ionic substances have a greater affect than molecular solutes

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Why Ionic Better Than Covalent?

Osmotic Pressure

Osmotic Pressure