Cryoscopy freezing point depression of the solvent

Calculation of the freezing-point depression of the solvent and hence the molecular weight of the solute by this method proceeds exactly the same way as for the boiling-point elevation. For cryoscopy of ideal solutions, equations corresponding to those for Ar and ke are AT/ = -kftnz and = (RT Mi)l(l000Lf), where ATf = T - Tf Hhe freezing-point depression. Tf is the... 

In the next section we shall discuss the physical properties and structure of the more important of the organic solvents mentioned in later chapters. In Chapter 2, the thermodynamic properties of solutions are discussed. Firstly, C. M. Criss discusses solubility and calorimetric measurements. In planning new work with unfamiliar solvents the extent of solubility of compounds is a primary consideration. Viscosity is also considered under this heading since although it is a transport property it is often used in the same way as thermodynamic measurements, to infer structural properties. An important technique for obtaining thermodynamic quantities of certain solutions is cryoscopy, the measurement of freezing point depressions, and this is discussed by R. Garnsey and J. E. Prue. Measurement of the e.m.f.s of suitable cells... 

When you dissolve something in water, for example, salt, the solution has slightly different properties than pure water. For example, a salty solution boils at a higher temperature and freezes at a lower temperature than pure water. Physical chemists have established that these changes, from pure solvent to solution, depend on the number of particles of the solute and not so much on the type of solute. The common name for these effects is colligative properties of mixtures and solutions. Outside of a physical chemical laboratory those of us who live in colder climates are well familiar with the phenomenon of freezing point depression or cryoscopy. This is also a physical chemical experiment actively practiced by the fish swimming in arctic seas. 

Cryoscopy. The freezing point of a solution is depressed below that of the pure solvent by an amount proportional to the mole fraction of solute. The value... 

Measurements of the elevation of solvent boiling point ebulliometry) and of the depression of solvent freezing point (cryoscopy) caused by the presence of a polymeric solute enable M to be evaluated in a similar way to VPO. J4owadays, however, such measurements are rarely used to evaluate M for polymers and so will be given only brief consideration here. 

If we consider the situation depicted in Figure 8.2 instead of that shown in Figure 8.1, then an analysis similar to the one carried out earlier leads to an expression for the depression in freezing point, which is identical to Eq. (8.3.11) except that AT is now Tf — T, where Tf and T are the freezing points of the pure solvent and the solution, respectively. Also, Ah becomes the molar latent heat of fusion of the pure solvent, and is replaced by Tf. This measurement is known as cryoscopy. 

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