Nonelectrolytes volatile nonelectrolyte solutions

Nonvolatile Nonelectrolyte Solutions Solute Molar Mass Volatile Nonelectrolyte Solutions Strong Electrolyte Solutions... 

In 1888, the French physical chemist Francois Raoult published his finding that when a dilute liquid solution of a volatile solvent and a nonelectrolyte solute is equilibrated with a gas phase, the partial pressure pA of the solvent in the gas phase is proportional to the mole fraction xa of the solvent in the solution ... 

Volatile Nonelectrolyte Solutions 414 Strong Electrolyte Solutions 415... 

In about 1886, the French chemist Fran9ois Marie Raoult observed that the partial vapor pressure of solvent over a solution of a nonelectrolyte solute depends on the mole fraction of solvent in the solution. Consido- a solution of volatile solvent, A, and nonelectrolyte solute, B, which may be volatile or nonvolatile. According to Raoult s law, the partial pressure of solvent, P, over a solution equals the vapor pressure of the pure solvent, Pf, times the mole fraction of solvent, Xj, in the solution. 

Small amounts of a nonvolatile, nonelectrolyte solute and a volatile solute are each dissolved in separate beakers containing 1 kg of water. If the number of moles of each solute is equal ... 

In principle the activity coefficients yb of solute substances B in a solution can be directly determined from the results of measurements at ven temperature of the pressure and the compositions of the liquid (or solid) solution and of the coexisting gas phase. In practice, this method fails unless the solutes have volatilities comparable with that of the solvent. The method therefore usually fails for electrolyte solutions, for which measurements of ye in practice, much more important than for nonelectrolyte solutions. Three practical methods are available. If the osmotic coefficient of the solvent has been measured over a sufficient range of molalities, the activity coefficients /b can be calculated the method is outlined below under the sub-heading Solvent. The ratio yj/ys of the activity coefficients of a solute B in two solutions, each saturated with respect to solid B in the same solvent but with different molalities of other solutes, is equal to the ratio m lm of the molalities (solubilities expressed as molalities) of B in the saturated solutions. If a justifiable extrapolation to Ssms 0 can be made, then the separate ys s can be found. The method is especially useful when B is a sparingly soluble salt and the solubility is measured in the presence of varying molalities of other more soluble salts. Finally, the activity coefficient of an electrolyte can sometimes be obtained from e.m.f. measurements on galvanic cells. The measurement of activity coefficients and analysis of the results both for solutions of a single electrolyte and for solutions of two or more electrolytes will be dealt with in a subsequent volume. Unfortunately, few activity coefficients have been measured in the usually multi-solute solutions relevant to chemical reactions in solution. 

Vapor Pressures of Solutions Containing a Volatile (Nonelectrolyte) Solute... 

Now interpret phase X as pure solute then Cs and co become the equilibrium solubilities of the solute in solvents S and 0, respectively, and we can apply Eq. (8-58). Again the concentrations should be in the dilute range, but nonideality is not a great problem for nonelectrolytes. For volatile solutes vapor pressure measurements are suitable for this type of determination, and for electrolytes electrode potentials can be used. 

Equilibrium concentrations describe the maximum possible concentration of each compound volatilized in the nosespace. Despite the fact that the process of eating takes place under dynamic conditions, many studies of volatilization of flavor compounds are conducted under closed equilibrium conditions. Theoretical equilibrium volatility is described by Raoulf s law and Henry s law for a description of these laws, refer to a basic thermodynamics text such as McMurry and Fay (1998). Raoult s law does not describe the volatility of flavors in eating systems because it is based upon the volatility of a compound in a pure state. In real systems, a flavor compound is present at a low concentration and does not interact with itself. Henry s law is followed for real solutions of nonelectrolytes at low concentrations, and is more applicable than Raoult s law because aroma compounds are almost always present at very dilute levels (i.e., ppm). Unfortunately, Henry s law does not account for interactions with the solvent, which is common with flavors in real systems. The absence of a predictive model for real flavor release necessitates the use of empirical measurements. 

A study of the acid-base properties of solutes in nonaqueous solvents must include consideration of hydrogen ion activities and in particular a comparison of their activities in different solvents. Attempting to transpose interpretations and methods of approach from aqueous to nonaqueous systems may lead to diflSculty. The usual standard state (Section 2-2) for a nonvolatile solute is arbitrarily defined in terms of a reference condition with activity equal to concentration at infinite dilution. Comparisons of activities are unsatisfactory when applied to different solvents, because different standard states are then necessarily involved. For such comparisons it would be gratifying if the standard state could be defined solely with reference to the properties of the pure solute, as it is for volatile nonelectrolytes (Section 2-7). Unfortunately, for ionic solutes a different standard state is defined for every solvent and every temperature. 

In this section, we focus most of our attention on the simplest case, the colligative properties of solutes that do not dissociate into ions and have negligible vapor pressure even at the boiling point of the solvent. Such solutes are called nonvolatile nonelectrolytes sucrose (table sugar) is an example. Later, we briefly explore the properties of volatile nonelectrolytes and of strong electrolytes. 

Colligative properties are related to the number of dissolved solute particles, not their chemical nature. Compared with the pure solvent, a solution of a nonvolatile nonelectrolyte has a lower vapor pressure (Raoult s law), an elevated boiling point, a depressed freezing point, and an osmotic pressure. Colligative properties can be used to determine the solute molar mass. When solute and solvent are volatile, the vapor pressure of each is lowered by the presence of the other. The vapor pressure of the more volatile component is always higher. Electrolyte solutions exhibit nonideal behavior because ionic interactions reduce the effective concentration of the ions. 

Describe electrolyte behavior and the four colligative properties, explain the difference between phase diagrams for a solution and a pure solvent, explain vapor-pressure lowering for nonvolatile and volatile nonelectrolytes, and discuss the van t Hoff factor for colligative properties of electrolyte solutions ( 13.5) (SPs 13.6-13.9) (EPs 13.59-13.83)... 

FIGURE 9.7 Experimental (symbols) and fitted (lines) results for Henry s constants (H21) for Hydrogen sulfide (2) in water (1) from Equation 9.37 through Equation 9.41. (Reprinted with permission from A. Plyasunov, J. P. O Connell, R. H. Wood, and E. L. Shock, 2000, Infinite Dilution Partial Molar Properties of Aqueous Solutions of Nonelectrolytes. II. Equations for the Standard Thermodynamic Functions of Hydration of Volatile Nonelectrolytes over Wide Ranges of Conditions Including Subcritical Temperatures, Geochimica Et Cosmochimica Acta, 64, 2779, With permission from Elsevier.)... 

Plyasunov, A. V., J. P. O Connell, and R. H. Wood. 2000. Infinite dilution partial molar properties of aqueous solutions of nonelectrolytes. I. Equations for partial molar volumes at infinite dilution and standard thermodynamic functions of hydration of volatile nonelectrolytes over wide ranges of conditions. Geochimica et Cosmochimica Acta. 64,495. 

Raoult s law predicts that when we increase the mole fraction of nonvolatile solute particles in a solution, the vapor pressure over the solution will be reduced. In fact, the reduction in vapor pressure depends on the total concentration of solute particles, regardless of whether they are molecules or ions. Remember that vapor-pressure lowering is a colligative property, so it depends on the concentration of solute particles and not on their kind. In our applications of Raoult s law, however, we will limit ourselves to solutes that are not only nonvolatile but nonelectrolytes as well. We consider the effects of volatile substances on vapor pressure in the "Closer Look" box in this section, and we will consider the effects of electrolytes in our discussions of freezing points and boiling points. 

If we treat the liquid mixture as a binary solution in which solute B is a volatile nonelectrolyte, Henry s law behavior occurs in the limit of infinite dilution ... 

The vapor pressure above an ideal solution of a nonvolatile nonelectrolyte is lowered by an amount proportional to the mole fraction of the solute (Raoult s law). For a volatile nonelectrolyte, the vapor has a higher proportion of the more volatile solute than the solution does. (Section 13.5)... 

In this section, we discuss colligative properties of three types of solute—nonvolatile nonelectrolytes, volatile nonelectrolytes, and strong electrolytes. 

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