Freezing Point Depression:

Freezing Point of a Liquid:

• Freezing point of a liquid is a temperature at which the vapour pressure of solid is equal to the vapour pressure of liquid.

Depression of Freezing Point of a Liquid:

• The lowering of vapour pressure of a solution causes a lowering of the freezing point compared to that of the pure solvent.
• We know that at the freezing point of a substance, the solid phase is in dynamic equilibrium with the liquid phase. A solution will freeze when its vapour pressure equals the vapour pressure of the pure solid solvent as is clear from the graph.

• According to Raoult’s law, when a non-volatile solid is added to the solvent its vapour pressure decreases and now it would become equal to that of solid solvent at lower temperature. Thus, the freezing point of the solvent decreases.
• Let Tfo be the freezing point of pure solvent and Tf be the freezing point of solution. The increase in the freezing point ΔTf = Tf  – Tfo  is known as depression of freezing point.
The depression of freezing point (ΔTf) is directly proportional to the lowering of vapour pressure (Δp).
Thus, ΔTf  α  Δp.

4. Experiments have shown that for dilute solutions the depression of freezing point (ΔTf) is directly proportional to the molal concentration of the solute in a solution. Thus, the depression of freezing point also depends on the number of solute molecules rather than their nature.

ΔTf  α  m

ΔTf =  Kf  m       ………….. (1)

• Here m (molality) is the number of moles of solute dissolved in 1 kg of solvent and the constant of proportionality,  Kf is called Freezing Point Elevation Constant or Molal Elevation Constant (cryoscopic Constant). The unit of  Kf is K kg mol-1.
• The molal elevation of freezing point constant is defined as the depression of freezing point produced, when one mole of solute is dissolved in 1 kg of solvent.
• Now,

• The experimental method to determine molecular mass of non volatile solute by determining freezing points of pure solvent and solution of known concentration is called cryoscopy.

Assignment 2.4 B

1. Define the following a) Freezing point of a liquid, b) Molal depression of freezing point constant
2. Explain phenomenon of depression of freezing point of a solution on addition of non volatile solute. OR What is depression of freezing point of a liquid?
3. Prove that depression of freezing point of a solution having non volatile solute is a colligative property.
4. Derive an expression for molecular mass of a non volatile solute in a solution using concept of depression of freezing point. OR derive relation between depression of freezing point and molar mass of solute.
5. Does molal depression of freezing point constant depend on the nature of solute.