Properties of Nonelectrolyte Solutions

Colligative properties (or collective properties) are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles. These properties are bound together by a common origin— they all depend on the number of solute particles present, regardless of whether they are atoms, ions, or molecules. The colligative properties are vapor-pressure lowering, boUing-point 

If a solute is nonvolatile (that is, it does not have a measurable vapor pressure), the vapor pressure of its solution is always less than that of the pure solvent Thus, the relationship between solution vapor pressure and solvent vapor pressure depends on the concentration of the solute in the solution. This relationship is expressed by Raouh s law, which states that the vapor pressure of a solvent over a solution. Pi, is given by the vapor pressure of the pure solvent. Pi, times the mole fraction of the solvent in the solution, Xi. 

To review the concept of equilibrium vapor pressure as it applies to pire liquids, see Section 11.8. 

In a solution containing only one solute, Xi = I X2, where X2 is the mole fraction of the solute. Equation (12.4) can therefore be rewritten as 

We see that the decrease in vapor pressure, AP, is directly proportional to the solute concentration (measured in mole fraction). 

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Acree W. E. Jr. (1984). Thermodynamic Properties of Nonelectrolyte Solutions. New York Academic Press. 

In the previous chapter, we described the thermodynamic properties of nonelectrolyte solutions. In this chapter, we focus on electrolytes as solutes. Electrolytes behave quite differently in solution than do nonelectrolytes. In Chapter 11, we described the strong electrolyte standard state and summarized relationships between the activity of the solute ai, the mean activity coefficient 7 , and the molality m in Table 11.3. 

Acree, W. E. Thermodynamic properties of nonelectrolyte solutions Academic Press Orlando, FL, 1984. 

In this chapter, we apply some of the general principles developed heretofore to a study of the bulk thermodynamic properties of nonelectrolyte solutions. In Sec. 11-1 we discuss conventions for the description of chemical potentials in nonelectrolyte solutions and introduce the concept of an ideal component. In Sec. 11-2, we demonstrate how the concept of solution molecular weight can be introduced into thermodynamics in a natural fashion. Section 11-3 is devoted to a study of the properties of ideal solutions. In Sec. 11-4, we discuss the properties of solutions that can be considered to be ideal when they are dilute but are not necessarily ideal when they are more concentrated. In Sec. 11-5, regular solutions are defined and some of their properties are derived. Section 11-6 is devoted to a study of some of the approximations that prove useful in the derivation of the properties of real solutions. Finally, in Sec. 11-7, some of the experimental techniques utilized for the measurement of chemical potentials and activity coefficients of components in solution are described. 

The colligative properties of nonelectrolyte solutions provide a means of determining the molar mass of a solute. Theoretically, any of the four colligative properties... 

Figure 25.26 Liquid-vapor equilibria for mixtures of acetone in cyclohexane. Source WE Acree Jr, Thermodynamic Properties of Nonelectrolyte Solutions, Academic Press, Orlando, 1984. From KVK Rao and CV Rao, Chem Eng Sci 7, 97 (1957).

So far we have discussed the colligative properties of nonelectrolyte solutions. Because electrolytes undergo dissociation when dissolved in water [W Section 4.1], we must consider them separately. Recall, for example, that when NaCl dissolves in water, it dissociates into Na Co ) and C aq). For every mole of NaCl dissolved, we get two moles of ions in solution. Similarly, when a formula unit of CaCL dissolves, we get three ions one Ca ion and two Cl ions. Thus, for every mole of CaCl2 dissolved, we get three moles of ions in solution. Colligative properties depend only on the number of dissolved particle.s—not on the type of particles. This means that a 0.1 m solution of NaCl will exhibit a freezing point depression twice that of a 0.1 m solution of a nonelectrolyte, such as sucrose. Similarly, we expect a 0.1 m solution of CaCL to depress the freezing point of water three times as much as a 0.1 m sucrose solution. To account for this effect, we introduce and define a quantity called the van t Hoff factor (i), which is given by... 

For example, when 1 mol of NaCl dissolves in water, it forms 1 mol of dissolved Na" ions and 1 mol of dissolved CF ions. Therefore, the resulting solution has 2 mol of dissolved particles. The colligative properties of electrolyte solutions reflect this higher concentration of dissolved particles. In this section we examine colligative properties of nonelectrolyte solutions we then expand the concept to include electrolyte solutions in Section 12.7. 

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