Slightly soluble ionic compounds

The Ksp expressions are based on balanced equations for saturated solutions of slightly soluble ionic compounds. The exponents in the K p expressions match the corresponding coefficients in the chemical equation. The coefficient 1 is not written, following chemical convention. 

Q A saturated solution of a slightly soluble ionic compound does not contain any of the solute in solid form. Is this system at equilibrium Explain your answer briefly. 

You calculate Qp by substituting the concentration of each ion into the expression. If Qp is larger than K p, the product of the concentrations of the ions is greater than it would be at equilibrium. For the system to attain equilibrium, some of the ions must leave the solution by precipitation. Conversely, if Qp is less than IQp, the product of the concentration of the ions is smaller than it is at equilibrium. Therefore, the solution is not yet saturated and more ions can be added to the solution without any precipitation. The relationship between Qsp and K p for the dissociation of a slightly soluble ionic compound is summarized on the next page. Use the following general equation as a reference. 

In the previous Sample Problems, you saw that combining solutions of soluble ionic compounds can cause the precipitation of a slightly soluble ionic compound. Analytical chemists use precipitation reactions to remove ions from solution or to identify ions in an unknown solution. 

For example, consider a solution that contains three halide ions Cl , Br , and l. Since these halides all come from the same group on the periodic table, they share many properties. When they are the anions in slightly soluble ionic compounds, however, they have different solubilities. (See Table 9.4.) Therefore, chemists can use fractional precipitation to separate them from solution. 

In general, the solubility of a slightly soluble ionic compound is decreased by the presence of a common ion in the solution, as illustrated in Figure 16.11. The quantitative aspects of the common-ion effect are explored in Worked Example 16.11. 

TABLE 16.2 Solubility Products of Some Slightly Soluble Ionic Compounds at 25°C... 

The dissolution in water of a slightly soluble ionic compound reaches an equilibrium characterized by a solubility-product constant, Kgp, that is much less than 1. Addition of a common ion lowers such a compound s solubility. Lowering the pH (adding HsO" ) increases the solubility if the anion of the ionic compound is that of a weak acid. 

A complex ion consists of a central metal ion bonded to molecules or anions called ligands. Complex ions form in a stepwise process characterized by a formation constant, K, that is much greater than 1. Adding a ligand increases the solubility of a slightly soluble ionic compound if the ligand forms a complex ion with the ionic compound s cation. 

In this section, we explore the aqueous equilibria of slightly soluble ionic compounds, which up to now we ve called insoluble. In a saturated solution at a particular temperature, equilibrium exists between the undissolved and dissolved solute (Chapter 13). Slightly soluble ionic compounds have a relatively low solubility, so they reach equilibrium with relatively little solute dissolved. At this point, it would be a good idea for you to review the solubility rules listed in Table 4.1. 

If we make the assumption that there is complete dissociation of a slightly soluble ionic compound into its component ions, then equilibrium exists between solid solute and aqueous ions. Thus, for example, for a saturated solution of lead(II) sulfate in water, we have... 

The form of Q p is identical to that of the other reaction quotients we have written each ion concentration is raised to an exponent equal to the coefficient in the balanced equation, which in this case also equals the subscript of each ion in the compound s formula. Thus, in general, for a saturated solution of a slightly soluble ionic compound, composed of the ions M" and X", the equilib-... 

Writing Ion-Product Expressions for Slightly Soluble Ionic Compounds Problem Write the ion-product expression for each of the following compounds ... 

The presence of a common ion decreases the solubility of a slightly soluble ionic compound. As we saw in the case of acid-base systems, Le Chatelier s principle helps explain this effect. Let s examine the equilibrium condition for a saturated solution of lead(II) chromate ... 

Solubility-Product Constants (fCjp) of Slightly Soluble Ionic Compounds... 

First, you would accurately measure the electrical conductance of pure water. The conductance of a solution of the slightly soluble ionic compound X should be greater than that of pure water. The increased conductance would indicate that some of the compound X had dissolved. 

FACTORS THAT AFFECT SOLUBILITY (SECTION 17.5) Several experimental factors, including temperature, affect the solubilities of ionic compounds in water. The solubility of a slightly soluble ionic compound is decreased by the presence of a second solute that furnishes a common ion (the common-ion effect). The solubility of compounds containing basic anions increases as the solution is made more acidic (as pH decreases). Salts with anions of negligible basicity (the anions of strong acids) are unaffected by pH changes. 

Equilibria of Slightly Soluble Ionic Compounds The Sobbility-Product Constant (K p) Calculations Involving The Effect of a Common Ion The Effect of pH Csp vs. Kgp... 

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