Slightly soluble ionic compounds equilibria

For a slightly soluble ionic compound, equilibrium exists between solid solute and aqueous ions. Thus, for example, for a saturated solution of lead(II) fluoride, we have... 

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. 

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. 

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. 

Writing the ion-Product Expression The form of is identical to that of other reaction quotients each 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. At saturation, the concentration terms have their equilibrium values. Thus, in general, for a slightly soluble ionic compound, M X, composed of the ions M"+ and, the ion-product expression at equilibrium is... 

Describe the equilibrium of a slightly soluble ionic compound in water, and explain the meaning of understand how a common ion and pH affect solubility and how to predict precipitate formation from the values of... 

When an ionic compound is dissolved in water, it usually goes into solution as the ions. When an excess of a slightly soluble ionic compound is mixed with water, an equihbrium occurs between the solid compound and the ions in the saturated solution. For the salt calcium oxalate, CaC204, you have the following equilibrium ... 

K the symbol for the kelvin, the absolute temperature unit the symbol for an equilibrium constant. is the constant for the ionization of a weak acid is the constant for the equilibrium between a slightly soluble ionic compound and a saturated solution of its ions is the constant for the ionization of water. 

The solubility product constant (Kgp) is the equilibrium constant that indicates to what extent a slightly soluble ionic compound dissociates in water. 

The addition of the common ion shifts the equilibrium of a slightly soluble ionic compound toward the undissolved compound, causing more to precipitate. Thus, the solubility of the compound is reduced. 

The solubility of a substance is the concentration of its saturated solution. The solubility product constant, Kgp, is the equilibrium constant that describes the formation of a saturated solution of a slightly soluble ionic compound. It is the product of ionic concentrations, with each term raised to an appropriate power. 

For part (a), we write the appropriate chemical equations and solubility product expression, designate the equilibrium concentrations, and then substitute into the solubility product expression. For part (b), we recognize that NaF is a soluble ionic compound that is completely dissociated into its ions. MgF2 is a slightly soluble compound. Both compounds produce F ions so this is a common ion effect problem. We write the appropriate chemical equations and solubility product expression, represent the equilibrium concentrations, and substitute into the solubility product expression. For part (c), we compare the molar solubilities by calculating their ratio. 

Like most salts, calcium fluoride is an ionic compound that dissociates into ions when it dissolves in water. Calcium fluoride is also one of a large class of salts that are said to be slightly soluble in water. The ions in solution and any solid salt are at equilibrium, as the following equation for CaF2 and water shows. 

As an approximation, the dissolved portion of a slightly soluble salt dissociates completely into ions. In a saturated solution, the ions are in equilibrium with the solid, and the product of the ion concentrations, each raised to the power of its subscript in the compound s formula, has a constant value (Qsp = K p). The value of K p can be obtained from the solubility, and vice versa. Adding a common ion lowers an ionic compound s solubility. Adding HgO" (lowering the pH) increases a compound s solubility if the anion of the compound is that of a weak acid. If Qsp > K p for an ionic compound, a precipitate forms when two solutions, each containing one of the compound s ions, are mixed. Lakes bounded by limestone-rich soils form buffer systems that prevent harmful acidification by acid rain. 

Whether and to what extent a salt precipitates is characterized by the solubility product Aisp. The solubility product should be recalled from general chemistry texts as the equilibrium constant describing the formation of a slightly soluble (or nearly insoluble) ionic compound from its component ions in solution. Consider... 

To deal quantitatively with an equilibrium, you must know the equihbrium constant. In the next section, we will look at the equilibria of slightly soluble, or nearly insoluble, ionic compounds and show how you can determine their equilibrium constants. Once you find these values for various ionic compounds, you can use them to answer solubihty or precipitation questions. 

In general, the solubility product constant (Aq,) is the equilibrium constant for the solubility equilibrium of a slightly soluble (or nearly insoluble) ionic compound. It equals the product of the equilibrium concentrations of the ions in the compound, each concentration raised to a power equal to the number of such ions in the formula of the compound. Like any equilibrium constant, K p depends on the temperature, but at a given temperature it has a constant value for various concentrations of the ions. ... 

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