FREEZING-POINT DEPRESSION

'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:
''ΔT_f'' = ''i'' · ''K''_f · activity

★ activity is in units of mol/kg, and is equal to an activity coefficient times the molality

★ ''ΔT_f'', the ''freezing point depression'' is defined as ''T - T_f'', where ''T'' is the freezing point of the solution and ''T_f'' is the freezing point of the pure solvent.

★ ''K''_f, the cryoscopic constant, is a colligative property, given by RT_f²/ΔH_f, where R is the gas constant, and T_f is the normal freezing point of the solvent and ΔH_f is the heat of fusion per kilogram of the solvent

★
★ ''K''_f 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, the activity, or the molar mass of the solute, although this particular process, called 'cryoscopy' (Greek "freeze-viewing"), is not as common as it once was. It was still taught as a useful analytic procedure in Cohen's ''Practical Organic Chemistry '' of 1910,^[1] in which the molar mass of napthalene is determined in a so-called 'Beckmann freezing apparatus'.
Freezing-point depressions occur whenever a solute is added to a pure solution, such as water. This is due to solute molecules disrupting the ability of the solvent to form crystals during the freezing process. Because of this, the liquid range of solvent is increased resulting in a freezing point depression.

Contents

See also

References

See also

★ Boiling-point elevation

★ Eutectic point

★ Colligative properties

★ List of boiling and freezing information of solvents

References

1. Julius B. Cohen ''Practical Organic Chemistry'' '1910' Link to online text