Page 253 - Physical chemistry understanding our chemical world
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220 PHASE EQUILIBRIA
showing that the water will freeze at the lower temperature of
This value of T repre-
sents the depression of (273.16 − 0.32) K.
the freezing tempera-
ture, so it is negative SAQ 5.6 Pure water has a normal freezing point
of 273.15 K. What will be the new normal freezing point
3
of water if 11 g of KCl is dissolved in 0.9dm of water?
The cryoscopic constant of water is 1.86 K kg −1 mol −1 ; assume the density
of water is 1 g cm −3 , i.e. molality and molarity are the same.
An almost identical equation relates the elevation of boiling point to the molality:
T (elevation) = K (ebullioscopic) × 1000 × molality of the salt (5.18)
where K (ebullioscopic) relates to the elevation of boiling temperature. Table 5.3 con-
tains a few sample values of K (ebullioscopic) . It can be seen from the relative val-
ues of K (ebullioscopic) and K (cryoscopic) in Table 5.3 that dissolving a solute in a sol-
vent has a more pronounced effect on the freezing temperature than on the boiling
temperature.
Aside
The ice on a car windscreen will also melt when squirted with de-icer. Similarly, we add
anti-freeze to the water circulating in a car radiator to prevent it freezing; the radiator
would probably crack on freezing without it; see the note on p. 194.
Windscreen de-icer and engine anti-freeze both depress the freezing point of water
via the same principle as rock salt depressing the temperature at which ice freezes on a
road. The active ingredient in these cryoscopic products is ethylene glycol (II), which
is more environmentally friendly than rock salt. It has two physicochemical advantages
over rock salt: (1) being liquid, it can more readily enter between the microscopic
crystals of solid ice, thereby speeding up the process of cryoscopic melting; (2) rock
salt is impure, whereas II is pure, so we need less II to effect the same depression of
freezing point.
CH 2 CH 2
OH OH
(II)
Ethylene glycol is also less destructive to the paintwork of a car than rock salt is,
but it is toxic to humans.
