Freezing point depression, and

The properties of a solution differ considerably from those of the pure solvent Those solution properties that depend primarily on the concentration of solute particles rather than their nature are called colligative properties. Such properties include vapor pressure lowering, osmotic pressure, boiling point elevation, and freezing point depression. This section considers the relations between colligative properties and solute concentration, with nonelectrolytes that exist in solution as molecules. 

These problems can be minimized by the use of polymeric flow improvers and freeze-point depressants, similar to those employed in fuel oils. 

Boiling-Point Elevation and Freezing-Point Depression... 

Boiling-point elevation and freezing-point depression measurements have demonstrated that at all concentrations in THF and at low concentrations in ether (up to 0.1M) Grignard reagents prepared from alkyl bromides and iodides are monomeric, that is, there are few or no molecules with two magnesium atoms. Thus, part of the Schlenk equilibrium is operating... 

The depression of the activity may be measured in various ways. The most obvious would involve a measurement of the vapor pressure lowering, but this method is superseded by others both in accuracy and in simplicity of execution. The boiling point elevation and freezing point depression methods relegated vapor pressure measurement... 

TABLE 8.2. Sodium Chloride Equivalents (E) and Freezing Point Depression (ATI" ") Values of Selected Compounds. 

Raoult s law, osmotic pressure, and freezing point depression calculations use, without conversion, which of the following respective concentration units... 

Cohesion intermolecular attractive force between particles within a substance Colligative Property a property dependent on the number of particles in solution and not on the type of particles, for example, boiling point elevation and freezing point depression... 

Adding an impurity to a solvent makes its liquid phase more stable through the combined effects of boiling point elevation and freezing point depression. That s why you r irely see bodies of frozen salt water. The salt in the oceans lowers the freezing point of the water, making the liquid phase more stable and able to sustain temperatures slightly below 0°C. 

The extensive application of N204 in pure form to rocketry is a fairly recent development however, its use as a stabilizer and freezing point depressant for HN03 dates back more than 15 years. Mixtures of HN03, N204, and small amounts of water and HF are referred to as inhibited red fuming nitric acid (IRFNA) or without the HF (a corrosion inhibitor) as merely red fuming nitric acid (RFNA). 

Electrolytes, depending upon their strength, dissociate to a greater or less extenl in polar solvents. The extent to which a weak electrolyte dissociates may be determined by electrical conductance, electromotive force, and freezing point depression methods. The electrical conductance method is the most used because of its accuracy and simplicity. Arrhenius proposed that the degree of dissociation, a. of a weak electrolyte at any concentration in solution could be found from the rutio of the equivalent conductance. A. of the electrolyte at the concentration in question to (he equivalent conductance at infinite dilution A0 of the electrolyte. Thus... 

X V iution), the determination of the molar mass of a solute requires a measurement of mass, volume, temperature, and osmotic pressure. Osmotic pressures are generally large and can be determined quite accurately, thus yielding accurate molar masses. Boiling-point elevations and freezing-point depressions are usually small and not very accurate, so molar mass determinations based on those measures often are not accurate. 

As with vapor-pressure lowering (Section 11.6), the actual amount of boiling-point elevation and freezing-point depression observed for solutions of ionic substances depends on the amount of dissociation, as given by a van t Hoff factor. The formulas for both boiling-point elevation and freezing-point depression can be modified to take dissociation into account ... 

The fundamental cause of boiling-point elevation and freezing-point depression in solutions is the same as the cause of vapor-pressure lowering (Section 11.6) the entropy difference between the pure solvent and the solvent in a solution. Let s take boiling-point elevations first. We know that liquid and vapor phases are in equilibrium at the boiling point (Tb) and that the free-energy difference between the two phases (AGvap) is therefore zero (Section 8.14). 

The Boiling-Point Elevation and Freezing-Point Depression activity (eChapter 11.7) illustrates how the boiling point and freezing point of water are affected by the addition of different solutes. 

Vapor-pressure lowering, boiling-point elevation, and freezing-point depression are very similar thermodynamically. For example, the increase in boiling point ATh is interpreted thermodynamically by using the boiling-point elevation constant Kb to obtain the molality of the solution, as stated in the equation... 

It was formulated as [Hg(SCN2H4)2Cl2]1 8 after A. Werner, before P. C. R y s work on the mono-derivative.4 Although this wholly covalent formulation is not impossible, especially in view of the production of mercury (II) sulfide (not oxide) by addition of soda solution,2 it does not fit in with the tables of electrical conductivities and freezing-point depressions (Tables I and II). 

Solutions of arsenic pentafluoride have conductivities and freezing point depressions (Dean et al., 1970) that are only approximately one-half those of SbF5 at the same concentration and it has been shown that even in dilute solutions there is essentially complete formation of the As2Ff1 ion according to equation (14). 

Mercury Thermometers. In the past, mercury thermometers were by far the most common type of laboratory thermometer. Concerns about health hazards associated with mercury have now reduced their role. However, when high precision is required (calorimetry and freezing-point depressions, for example), oil-in-glass thermometers are not suitable since the use of fine capillaries is not feasible with oil. Furthermore, the safety hazards involved in the laboratory use of mercury thermometers are quite low, as discussed at the end of this section. For these reasons, a description of the use of mercury thermometers is still pertinent. 

Table 14.1 Common Boiling Point Elevation (Kb) and Freezing Point Depression (Kf) Constants...

Be sure to distinguish between freezing point and freezing-point depression. 

---