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).