Colligative properties of strong electrolytes

Unlike weak electrolytes, solutions of strong ones have a far higher specific conductance the rise of the latter with rising concentration is also much more rapid. There is another difference the anomalies ascertained in the colligative properties of strong electrolytes cannot be ascribed to partial dissociation of molecules to ions as in the case of weak electrolytes. 

To calculate the colligative properties of strong electrolyte solutions, we incorporate the van t Hoff factor into the equation ... 

For colligative properties of strong electrolyte solutions, the solute formula tells us the number of particles. For instance, the boiling point elevation iAT ) of 0.050 m NaCl should be 2 X of 0.050 m glucose (C(5Hj20(5), because NaCl dissociates into two particles per formula unit. Thus, we use a multiplying factor called the van t Hoff factor (i), named after the Dutch chemist Jacobus van t Hoff (1852-1911) ... 

Not only the conductance but also the colligative properties of strong electrolytes show deviations from the values to be expected on the basis of complete ionization. The freezing-point depression of, for example, a NaCl solution is less than we would expect for 2 moles of ions per mole of NaCl the van t Hoff factor approaches 2 only in very dilute solutions (Table 12.2, page 217). These diminutions in the colliga-... 

The colligative properties of an electrolyte solution can be used to determine percent dissociation. Percent dissociation is the percentage of dissolved molecules (or formula units, in the case of an ionic compound) that separate into ions in solution. For a strong electrolyte such as NaCl, there should be complete, or 100 percent, dissociation. However, the data in Table 13.4 indicate that this is not necessarily the case. An experimentally determined van t Hoff factor smaller than the corresponding calculated value indicates less than 100 percent dissociation. As the experimentally determined van t Hoff factors for NaCl indicate, dissociation of a strong electrolyte is more complete at lower concentration. The percent ionization of a weak electrolyte, such as a weak acid, also depends on the concentration of the solution. 

Salt is a strong electrolyte that produces two ions, Na+ and Cl, when it dissociates in water. Why is this important to consider when calculating the colligative property of freezing point depression ... 

In all other solutions the so called degree of dissociation, as determined from the measurement of some colligative property, merely indicates the magnitude of interionic forces, it cannot, however, be taken as a measure of the quantity of dissociated and undissociated molecules of the solute. A complete theory of strong electrolytes, at least of their diluted solutions, has been developed by Debye and Hiickel, this theory is the basis of modern electrochemistry. 

Strong Electrolytes. Solutes of this type, such as HCl, are completely dissociated in ordinary dilute solutions. However, their colligative properties when interpreted in terms of ideal solutions appear to indicate that the dissociation is a little less than complete. This fact led Arrhenius to postulate that the dissociation of strong electrolytes is indeed incomplete. Subsequently this deviation in colligative behavior has been demonstrated to be an expected consequence of interionic attractions. 

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 of Solutions Nonvolatle Nonelectrolyte Solutions Solute Molar Mass Vdatle Nonelectrolyte Solutions Strong Electrolyte Solutions... 

The colligative properties of solutions depend on the total concentration of solute particles, regardless of whether the particles are ions or molecules. Thus, we would ejqject a 0.100 in solution of NaCl to have a freezing-point depression of (0.200 n/)(1.86 C/n/) = 0.372 C because it is 0.100///in Na (c<7) and 0.100 in in Cl (aq). The measured freezing-point depression is only 0.348°C, howevei and the situation is similar for other strong electrolytes. A 0.100 in solution of KCl, for example, freezes at —0.344°C. 

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

It is important to realise that whilst complete dissociation occurs with strong electrolytes in aqueous solution, this does not mean that the effective concentrations of the ions are identical with their molar concentrations in any solution of the electrolyte if this were the case the variation of the osmotic properties of the solution with dilution could not be accounted for. The variation of colligative, e.g. osmotic, properties with dilution is ascribed to changes in the activity of the ions these are dependent upon the electrical forces between the ions. Expressions for the variations of the activity or of related quantities, applicable to dilute solutions, have also been deduced by the Debye-Hiickel theory. Further consideration of the concept of activity follows in Section 2.5. 

A—Freezing-point depression is a colligative property, which depends on the number of particles present. The solution with the greatest concentration of particles will have the greatest depression. The concentration of particles in E (a non electrolyte) is 0.10 m. All other answers are strong electrolytes, and the concentration of particles in these may be calculated by multiplying the concentration by the van t Hoff factor. 

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. 

Also the so called degree of dissociation, determined from the colligative properties, does not agree with the result obtained from the measurement of the electrical conductance. Finally the law of chemical equilibrium, applicable to the dissociation of weak electrolytes, cannot be applied to the strong ones. 

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