Freezing-point depression

Freezing point depression ( GPE ) (or melting point depression (SPE) or melting point depression (SPD ) ) refers to the phenomenon that the melting point of solutions is lower than that of the pure liquids.

Definition

The freezing point depression is proportional to the molality of the solute particles B for dilute solutions:

Here, the freezing point per mole of solute per kilogram of solvent reduces to a solvent- specific value. b is the concentration of all dissolved particles of the material in moles per kg of solvent and the initial concentration of the substance. the cryoscopic constant that depends only on the solvent and the solute (with water this value is 1.86 (K · kg) / mol). It can be from the Raoult's law and the Clausius- Clapeyron equation to derive

In which

  • Is the gas constant = 8.314472 J / (mol · K),
  • The freezing point of the solvent in K and
  • The specific heat of fusion of the solvent in J / kg

Is. This relationship is valid only for very dilute solutions (concentrations < 0.1 mol / L ) in highly concentrated solutions, the activity of the ions and the water is observed. Very highly concentrated solutions also have a triple point at which freezes the salt solution, previously only freezes water from the solution, the solution becomes more and more concentrated.

Since the freezing point of each exactly 1.86 K decreases when dissolving one mole of particles in a kilogram of water, the corresponding temperature difference is also called molar freezing point depression. This effect is independent of the nature of the solute, it is a colligative property.

The calculation of the particle is to be noted that salts dissociate in aqueous solution. Sodium chloride (NaCl) is divided, for example into the ions Na and Cl-. From 1 mole of sodium chloride so arise 2 moles of particles, and this value is taken into account for the calculation of n in the above mentioned equation.

As dependent on the molality or the concentration of the solute is the boiling point. One speaks of Molar boiling point elevation. Cause of these effects is a reduction of the chemical potential of the solution relative to the pure solvent through the entropy of mixing.

Examples

Background

In addition to the elevation of boiling point, the freezing point depressant is a further consequence that is related to the reduced vapor pressure of solutions.

If a liquid mixture of the solid substance A and solvent B in equilibrium with the solid substance A, then the amount posted ( with a chemical potential ). It continues to apply to the differentials.

Here, the index refers to the liquid phase, while the solid phase features. From the above equation, the total differentials be applied:

Wherein the mole fraction of solute in solvent. Is carried out at constant pressure, the equation takes the simplified form:

The difference between the entropy of the solid and the liquid state ( ) corresponds to the molar entropy of fusion of the substance A. This size can be described as.

Than the melting point temperature of the pure solid phase is referred to. Substituting this relation into the above equation and integrating between the limits of temperatures and T or 1 and the mole fraction, we obtain:

With and and we obtain the equation

Replacing by, and which is or leads upon insertion of molalities on the following equations ( with T · T ≈ T):

With

By forming the equation, one can determine the molecular weight of the solute from the observed reduction in freezing point. The following applies:

Applications

  • A practical application of freezing-point depression there in the winter when road salt (sodium chloride) is applied in freezing temperatures on ice and snow surfaces. A saturated salt solution having a freezing point of -21 ° C, the dissolution of the salt results in a liquid brine, the ice thaws.
  • To produce a freezing mixture ( in a chemical laboratory or earlier if the ice cream production) ice -salt mixtures. The melting heat is removed from the salt - ice -water mixture, the mixture cools.
  • The addition of cryolite reduces the melting temperature of alumina in the production of aluminum according to the Hall - Héroult process.
  • From the freezing point depression, the molecular mass of the solute can be determined. The measurement procedure for this purpose is called the cryoscopic method. This method of molecular weight determination was mainly used to wide application in organic and inorganic chemistry, when it was necessary to determine the molar masses of newly - synthesized or isolated compounds. For this purpose, if possible solvents are chosen, which have a large cryoscopic constant. Nowadays, the molar mass is usually done by mass spectrometry.
Cyclohexane
363868
de