Solution nonelectrolyte, colligative properties

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... 

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. 

The relationships among colligative properties and solute concentration are best regarded as limiting laws. They are approached more closely as the solution becomes more dilute. In practice, the relationships discussed in this section are valid, for nonelectrolytes, to within a few percent at concentrations as high as 1 Af. At higher concentrations, solute-solute interactions lead to larger deviations. 

As noted earlier, colligative properties of solutions are directly proportional to the concentration of solute particles. On this basis, it is reasonable to suppose that, at a given concentration, an electrolyte should have a greater effect on these properties than does a nonelectrolyte. When one mole of a nonelectrolyte such as glucose dissolves in water, one mole of solute molecules is obtained. On the other hand, one mole of the electrolyte NaCl yields two moles of ions (1 mol of Na+, 1 mol of Cl-). With CaCl three moles of ions are produced per mole of solute (1 mol of Ca2+, 2 mol of Cl-). 

Because of electrostatic attraction, an ion in solution tends to surround itself with more ions of opposite than of like charge (Figure 10.12). The existence of this ionic atmosphere, first proposed by Peter Debye (1884-1966), a Dutch physical chemist in 1923, prevents ions from acting as completely independent solute particles. The result is to make an ion somewhat less effective than a nonelectrolyte molecule in its influence on colligative properties. 

In contrast to nonelectrolyte solutions, in the case of electrolyte solutions the col-ligative properties depart appreciably from the values following from the equations above, even in highly dilute electrolyte solutions that otherwise by all means can be regarded as ideal (anomalous colligative properties). 

Osmotic pressure is a colligative property and is dependent on the number of particles of solute(s) in a solution. The total number of particles of a solute in a solution is the sum of the undissociated molecules and the number of ions into which the molecule dissociates. The number of ions, in turn, depends on the degree of ionization. Thus, a chemical that is highly ionized contributes a greater number of particles to the solution than the same amount of a poorly ionized chemical. When a chemical is a nonelectrolyte such as sucrose or urea, the concentration of the solution depends only on the number of molecules present. The values of the osmotic pressure and other colligative properties are approximately the same for equal concentrations of different nonelectrolyte solutions. 

As we saw in Section 17.5, the activity coefficient of a nonelectrolyte solute can be calculated from the activity coefficient of the solvent, which, in turn, can be obtained from the measurement of colligative properties such as vapor pressure lowering, freezing point depression, or osmotic pressure. We used the Gibbs-Duhem equation in the form [Equation (17.33)]... 

According to modem theory, many strong electrolytes are completely dissociated in dilute solutions. The freezing-point lowering, however, does not indicate complete dissociation. For NaCl, the depression is not quite twice the amount calculated on the basis of the number of moles of NaCl added. In the solution, the ions attract one another to some extent therefore they do not behave as completely independent particles, as they would if they were nonelectrolytes. From the colligative properties, therefore, we can compute only the "apparent degree of dissociation" of a strong electrolyte in solution. 

Nonelectrolytes in aqueous solution Many molecular compounds dissolve in solvents but do not ionize. Such solutions do not conduct an electric current, and the solutes are called nonelectrolytes. Sucrose is an example of a nonelectrolyte. A Im sucrose solution contains only one mole of sucrose particles. Figure 15-16 compares the conductivity of a solution containing an electrolyte solute with one containing a nonelectrolyte solute. Which compound would have the greater effect on colligative properties, sodium chloride or sucrose ... 

The phrase concentration of solute particles in the definition of colligative property needs clarification. There are two kinds of solutes, those that exist in solution as neutral molecules and those that ionize when dissolved and exist in solution as ions. Compounds that ionize in water are called electrolytes. Those that do not ionize are nonelectrolytes. Glucose is a nonelectrolyte and exists as neutral molecules in solution. A 1.0 M solution of glucose is 1.0 M in solute particles. Sodium chloride, NaCl, is an electrolyte and exists in solution as separated sodium and chloride ions, NaClfo) — Na+(aq) + CV(aq). A 1.0 M solution of NaCl is 2.0 M in solute particles, 1.0 M Na+(aq) plus 1.0M CV(aq). The concentration of solute particles for compounds that ionize in solution will be some whole number multiple of the concentration of the compound itself. For those solutes that do not ionize when dissolved in water, the concentration of the compound and the concentration of the solute particles (molecules) will be the same. A listing of common nonelectrolytes and electrolytes in water appears in the following table. 

Why is the discussion of the colligative properties of electrolyte solutions more involved than that of nonelectrolyte solutions ... 

The osmotic pressure can be calculated from the solution concentration at any temperature. How do we determine "solution concentration" Recall that osmosis is a colligative property, dependent on the concentration of solute particles. Again, it becomes necessary to distinguish between solutions of electrolytes and nonelectrolytes. For example, a 1 M glucose solution consists of 1 mol of particles per liter glucose is a nonelectrolyte. A solution of 1 M NaCl produces 2 mol of particles per liter (1 mol of Na+ and 1 mol of CH). AIM CaClj solution is 3 M in particles (1 mol of Ca + and 2 mol of CH per liter). 

Boiling point elevation constant, A constant that corresponds to the change (increase) in boiling point produced by a one-molal ideal solution of a nonvolatile nonelectrolyte. Colligative properties Physical properties of solutions that depend on the number but not the kind of solute particles present. Colloid A heterogeneous mixture in which solute-like particles do not settle out also called colloidal dispersion. 

In Chapter 4, we classified solutes by their ability to conduct an electric current, which requires moving ions to be present. Recall that an electrolyte is a substance that dissociates into ions in aqueous solution strong electrolytes dissociate completely, and weak electrolytes dissociate very little. Nonelectrolytes do not dissociate into ions at all. To predict the magnitude of a colligative property, we refer to the solute formula to find the number of particles in solution. Each mole of nonelectrolyte yields 1 mol of particles in the solution. For example, 0.35 M glucose contains 0.35 mol of solute particles per liter. In principle, each mole of strong electrolyte dissociates into the number of moles of ions in the formula unit 0.4 M Na2S04 contains 0.8 mol of Na ions and 0.4 mol of S04 ions, or 1.2 mol of particles, per liter (see Sample Problem 4.1). 

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)... 

Distinguish between a strong electrolyte, a weak electrolyte, and a nonelectrolyte. How can colligative properties be used to distinguish between them What is the van t Hoff factor Why is the observed freezing-point depression for electrolyte solutions sometimes less than the calculated value Is the discrepancy greater for concentrated or dilute solutions ... 

We learn that physical properties such as the vapor pressure, melting point, boiling point, and osmotic pressure of a solution depend only on the concentration and not the identity of the solute present. We first study these colligative properties and their applications for nonelectrolyte solutions. (12.6)... 

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