Colligative Properties

Colligative Properties of Solutions

What Is a Colligative Property?

• Definition: A property of solutions that is dependent only on the number of solute particles, not their identity.

• Importance of Ionic vs. Covalent Compounds: Ionic compounds significantly affect the properties compared to covalent ones.

Three Important Colligative Properties:

1. Vapor-Pressure Lowering

2. Freezing-Point Depression

3. Boiling-Point Elevation

Vapor Pressure Lowers

• A solution containing a nonvolatile solute always exhibits a lower vapor pressure than the pure solvent.

• Adding a nonvolatile solute diminishes the vapor pressure of the solution.

• Shell of Solvation Molecules: Nonvolatile solutes create a shell around solvation molecules, hindering evaporation and reducing vapor pressure.

Colligative Properties: Examples

Vapor Pressure Lowering

• Concept: Pressure is arising from the molecules that have escaped from the liquid phase.

• Effect of Nonvolatile Solutes: Nonvolatile solutes inhibit as many water molecules from exiting the liquid, thus lowering vapor pressure.

Vapor Pressure, cont’d

• Volatile Substances:

  • These substances increase vapor pressure and evaporate easily; e.g., acetone:

  • Mixed with water, they facilitate other molecules’ escape, resulting in higher vapor pressure.

Ionic Solutes

Sodium Chloride (NaCl)

• Decrease in vapor pressure relates to the number of solute particles in solution.

  • Dissociation:

    • NaCl(s) → Na⁺(aq) + Cl⁻(aq)

  • 1 mol of NaCl yields 2 moles of particles in solution.

Calcium Chloride (CaCl2)

• Similar to NaCl, the decrease in vapor pressure is also proportional to solute particle count.

  • Dissociation:

    • CaCl2(s) → Ca²⁺(aq) + 2Cl⁻(aq)

  • 1 mol of CaCl2 results in 3 moles of particles in solution.

Molecular Solutes

Glucose (C6H12O6)

• As a non-electrolyte, the decrease in vapor pressure is based solely on the number of particles present.

  • Dissociation:

    • C6H12O6(s) → C6H12O6(aq)

  • 1 mol results in just 1 mole of particles in solution, having minimal impact on colligative properties.

Freezing Point Lowers

• Definition: The difference between the freezing point of a solution and that of a pure solvent.

• Adding a solute decreases the freezing point.

  • Example Calculation:

    • For 1 L of water + 1 mol solute: FPD = -1.86°C

    • For 1 L of water + 1 mol NaCl: FPD = 1 L water + 2 mol solute:

      • FPD = 2 × -1.86°C = -3.72°C

Real-Life Examples of Freezing Point Depression

• Airplane De-icing: Use of solutions to prevent ice formation on aircraft.

• Salting Roads: Application of salt to melt ice on roads.

• Ethylene Glycol in Car Radiators: Used to prevent freezing in colder temperatures.

Boiling Point Increases

• Definition: The difference between the boiling point of a solution and that of a pure solvent.

• Adding a solute raises the boiling point.

  • Example Calculation:

    • For 1 L of water + 1 mol solute: BPE = +0.51°C

    • For 1 L of water + 2 mol NaCl: BPE = 1 L water + 2 mol solute:

      • BPE = 2 × +0.51°C = +1.02°C (resulting in a total boiling point elevation of +2.4°C).