Freezing-point depression
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Freezing-point depression is the difference between the freezing points of a pure solvent and a solution mixed with a solute. It is directly proportional to the molal concentration of the solution, or more precisely, to the solute activity, according to the equation:
ΔTf = i · Kf · activity
- activity is in units of mol/kg, and is equal to an activity coefficient times the molality
- ΔTf, the freezing point depression is defined as T - Tf, where T is the freezing point of the solution and Tf is the freezing point of the pure solvent.
- Kf, the cryoscopic constant, is a colligative property, given by RTf2/ΔHf, where R is the gas constant, and Tf is the normal freezing point of the solvent and ΔHf is the heat of fusion per kilogram of the solvent
- Kf for water is 1.858 K·kg/mol (or more commonly used, 1.858 C/m) which means that per mole of solute dissolved in a kilogram of water the freezing point depression is 1.858 kelvins.
- i is the i factor or the van 't Hoff factor (see van 't Hoff), accounts for the number of individual ions formed by a compound in solution.
Examples:
- i = 1 for sugar in water.
- i = 2 for NaCl in water.
- i = 3 for CaCl2 in water.
- i = 2 for HCl in water. (complete dissociation)
- i = 1 for HCl in benzene. (no dissociation)
Freezing point depression can be used to measure the degree of dissociation of a solute or to measure its activity or to determine molar mass of the solute.
In Cohen's Practical Organic Chemistry of 1910 [1] the molar mass of napthalene is determined in a so-called Beckmann freezing apparatus at 128 g/mole.
[edit] See also
- Boiling-point elevation
- Eutectic point
- Colligative properties
- List of boiling and freezing information of solvents
[edit] References
- ^ Julius B. Cohen Practical Organic Chemistry 1910 Link to online text
