Explain the concept and cause of freezing point depression E

Explain the concept and cause of freezing point depression. Elaborate on how it is related to colligative properties and melting points.

Solution

Freezing point depression:

It is the process of adding solute substance in solvent to decrease freezing point. Sodium chloride-water solution decreases their temperature below the freezing point. This phenomenon is practically used in the radiator fluid. Ethylene glycol-water mixture is useful for a radiator in cold season.

Formula For Depression Of Freezing Point:

TF = KF · b · i

Where,

TF is freezing point depression (TF (pure solvent) - T F(solution))

KF is the cryoscopic constant.

b is the molality.

i is the vant’t Hoff factor.

The freezing point depression can also be explained in terms of vapor pressure. Adding solute to a solvent will essentially dilute the solvent molecules, and according to Raoult’s law, this leads to a decrease in vapor pressure. Considering the fact that the vapor pressure of the solid and liquid forms must be the same at freezing point, because otherwise the system would not be at equilibrium, the lowering of the vapor pressure leads to the lowering of the temperature at which the vapor pressures of the liquid and frozen forms of the solution will be equal.

What causes freezing point depression?

When a solute and solvent are mixed, there is increase in entropy and the liquid finds it thermodynamically more preferable to stay in the liquid state due to more randomness. Hence the added solute increases the ‘desire’ for the solvent to stay in the liquid state thus decreasing the freezing point.

What is the reason why freezing point depression is a colligative property

Freezing point depression is a colligative property observed in solutions that results from the introduction of solute molecules to a solvent. The freezing points of solutions are all lower than that of the pure solvent and is directly proportional to the molality of the solute.(so its a colligative property because it depends on number of moles of solute present)

Colligative properties are properties of solutions that depend on the number of molecules in a given volume of solvent and not on the properties (e.g. size or mass) of the molecules. Boiling point and melting point (freezing point) depend only on the the number of particles dissolved in the solution, not the identity of the particles.