Freezing point depression
Freezing point depression ( GPE ) (also melting point depression ( SPE ) or melting point depression ( SPD )) describes the phenomenon that the melting point of solutions is lower than the melting point of pure liquid solvents .
For dilute solutions, the depression of the freezing point is proportional to the molality b of the particles of the dissolved substance (i.e. the concentration of all dissolved particles in the substance in moles per kilogram of solvent and not the initial concentration of the substance):
The freezing point per mole of dissolved substance per kilogram of solvent is reduced by a solvent-specific value.
When calculating the molality of the dissolved substances, it should be noted that e.g. B. Dissociate salts in aqueous solution . Table salt (NaCl) breaks down e.g. B. into the ions Na + and Cl - . 1 mol of sodium chloride produces 2 mol of particles, and this value must be taken into account in the above equation. This is done by multiplying by the Van 't Hoff factor , which indicates how many particles the dissolved substance dissociates into on average:
is the cryoscopic constant , which only depends on the solvent and not on the dissolved substance (with water as the solvent, this value is 1.86 ( K kg) / mol). It can be from the Raoult's law and the Clausius-Clapeyron equation derived to
- the gas constant = 8.314472 J / (mol K)
- the freezing point of the solvent in K
- the specific enthalpy of fusion of the solvent in J / kg.
This relationship only applies to very dilute solutions (concentrations <0.1 mol / L ); the activity of the ions and the water must be taken into account for more concentrated solutions . Very highly concentrated solutions also have a triple point at which the salt solution freezes, before only water freezes out of the solution, the solution is increasingly concentrated.
Since the freezing point drops by exactly 1.86 K every time you dissolve one mole of particle in one kilogram of water, the associated temperature difference is also called the molar lowering of the freezing point . This effect is independent of the type of solute, it is a colligative property .
The boiling point of solutions also depends on the molality or the concentration of the dissolved substances; it increases . One speaks here of a molar boiling point increase . The cause of these effects is a lowering of the chemical potential of the solution compared to the pure solvent due to the entropy of mixing .
|solvent||Freezing point in ° C||Freezing point depression
in K kg / mol
In addition to the increase in the boiling point, the lowering of the freezing point is another consequence that is related to the reduced vapor pressure of solutions.
If a liquid mixture of the solid substance A and the solvent B is in equilibrium with the solid substance A , this results in the approach (with as chemical potential ). It still applies to the differentials .
The index denotes the liquid phase, while the solid phase denotes. The total differentials can be set up from the above equation :
As is the melting point temperature refers to the pure solid phase. If you insert this relationship into the above equation and integrate between the limits of the temperatures and T or 1 and the mole fraction , the result is:
with and and you get the equation
Replacing by , where and or is, leads to the following equations when introducing molalities (with T · T ≈ T ):
By transforming the equation, one can determine the molar mass of the dissolved substance from the observed lowering of the freezing point. The following applies:
- There is a practical application of the freezing point lowering in winter, when de-icing salt ( sodium chloride ) is applied to ice and snow surfaces when the temperature is below zero . A saturated saline solution has a freezing point of −21 ° C, dissolving the salt results in a liquid saline solution, the ice thaws.
- Ice and salt mixtures are used to produce a cold mixture (in a chemical laboratory or earlier in ice cream production). The heat of fusion is withdrawn from the table salt, ice and water mixture, and the mixture cools down.
- The addition of cryolite reduces the melting temperature of aluminum oxide in aluminum production using the Hall-Héroult process .
- The molecular mass of the dissolved substance can be determined from the lowering of the freezing point . The measurement method for this is known as cryoscopy . This method of determining molar mass used to be widely used in organic and inorganic chemistry when it came to determining the molar mass of newly synthesized or isolated compounds . For this purpose, solvents are selected, if possible, which have a large cryoscopic constant. Nowadays, molar mass is usually determined by mass spectrometry .
- Video: Cryoscopy and Ebullioscopy as Colligative Properties - A solution boils and freezes later than the pure solvent . Jakob Günter Lauth (SciFox) 2013, made available by the Technical Information Library (TIB), doi : 10.5446 / 15677 .