Ksp

Note that the precipitate, which is a solid, does not appear in the Ksp expression. It is important to remember, however, that equation 6.6 is valid only if PbCbCs) is present and in equilibrium with the dissolved Pb + Values for selected solu-... 

Equilibrium constants for complexation reactions involving solids are defined by combining appropriate Ksp and K expressions. Eor example, the solubility of AgCl increases in the presence of excess chloride as the result of the following complexation reaction... 

This reaction can be separated into three reactions for which equilibrium constants are known—the solubility of AgCl, described by its Ksp... 

Before accepting this answer, we first check our assumptions. Using the Ksp equation we calculate the [Ag+] to be... 

The true thermodynamic equilibrium constant, Ksp, for the solubility of AglOa, therefore, is... 

However, if we wish to follow the change in concentration for Ag+ then we must first calculate its concentration. To do so we use the Ksp expression for AgCl... 

Strategy The first step is to write down the Ksp expression ... 

Kgp values can be used to make predictions as to whether or not a precipitate will form when two solutions are mixed. To do this, we follow an approach very similar to that used in Chapter 12, to determine the direction in which a system will move to reach equilibrium. We work with a quantity Q, which has the same mathematical form as K. The difference is that the concentrations that appear in Q are those that apply at a particular moment. Those that appear in are equilibrium concentrations. Putting it another way, the value of Q is expected to change as a precipitation reaction proceeds, approaching Ksp and eventually becoming equal to it. 

If Q > K, the solution contains a higher concentration of ions than it can hold at equilibrium. A precipitate forms, decreasing the concentrations until the ion product becomes equal to Ksp and equilibrium is established. 

If Q < Ksp, the solution contains a lower concentration of ions than is required for equilibrium with the solid. The solution is unsaturated. No precipitate forms equilibrium is not established. 

If Q = Ksp, the solution is just saturated with ions and is at the point of precipitation. 

Equilibrium curve for silver chloride. Silver chloride (s) is in contact with Ag+ and Cl- ions in aqueous solution. The product Q of the concentration of ions [Ag+] X [Cl-] is equal to Ksp (curved line)when equilibrium exists. If 0 > K,p, AgCI(s) tends to precipitate out until equilibrium is reached. If 0 < Ksp, additional solid dissolves. 

In the laboratory, strontium chromate (Ksp = 3.6 X 10-5) can be prepared by mixing solutions containing strontium ions and chromate ions, where the concentrations of these ions are high enough to form a precipitate. Decide whether a precipitate will form when... 

One way to establish equilibrium between a slightly soluble solid and its ions in solution is to stir the solid with water to form a saturated solution. As you might expect, the solubility of the solid, s, in moles per liter, is related to the solubility product constant, Ksp. In the case of barium sulfate dissolving in water we have... 

When an ionic solid consists of anions and cations of different charges, the relation between Ksp and s takes a different form, but the principle is the same (Example 16.4). 

BaF2----- 2 moles F ). Substitute into the Ksp expression and solve for s. Remember the... 

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