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General Chemistry Principles, Patterns, and Applications, 2011

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Chapter 17<br />

Solubility <strong>and</strong> Complexation Equilibriums<br />

Although focused exclusively on acid–base equilibriums in aqueous solutions, equilibrium concepts can also be<br />

applied to many other kinds of reactions that occur in aqueous solution. In this chapter, we describe the equilibriums<br />

involved in the solubility of ionic compounds <strong>and</strong> the formation of complex ions.<br />

Solubility equilibriums involving ionic compounds are important in fields as diverse as medicine, biology, geology,<br />

<strong>and</strong> industrial chemistry. Carefully controlled precipitation reactions of calcium salts, for example, are used by many<br />

organisms to produce structural materials, such as bone <strong>and</strong> the shells that surround mollusks <strong>and</strong> bird eggs. In<br />

contrast, uncontrolled precipitation reactions of calcium salts are partially or wholly responsible for the formation of<br />

scale in coffee makers <strong>and</strong> boilers, “bathtub rings,” <strong>and</strong> kidney stones, which can be excruciatingly painful. The<br />

principles discussed in this chapter will enable you to underst<strong>and</strong> how these apparently diverse phenomena are<br />

related. Solubility equilibriums are also responsible for the formation of caves <strong>and</strong> their striking features, such as<br />

stalactites <strong>and</strong> stalagmites, through a long process involving the repeated dissolution <strong>and</strong> precipitation of calcium<br />

carbonate. In addition to all of these phenomena, by the end of this chapter you will underst<strong>and</strong> why barium sulfate is<br />

ideally suited for x-ray imaging of the digestive tract, <strong>and</strong> why soluble complexes of gadolinium can be used for<br />

imaging soft tissue <strong>and</strong> blood vessels using magnetic resonance imaging (MRI), even though most simple salts of both<br />

metals are toxic to humans.<br />

17.1 Determining the Solubility of Ionic Compounds<br />

L E A R N I N G O B JE C T I V E<br />

1. To calculate the solubility of an ionic compound from its K sp.<br />

We begin our discussion of solubility <strong>and</strong> complexation equilibriums—those associated with the formation of<br />

complex ions—by developing quantitative methods for describing dissolution <strong>and</strong> precipitation reactions of ionic<br />

compounds in aqueous solution. Just as with acid–base equilibriums, we can describe the concentrations of ions in<br />

equilibrium with an ionic solid using an equilibrium constant expression.<br />

The Solubility Product<br />

When a slightly soluble ionic compound is added to water, some of it dissolves to form a solution,<br />

establishing an equilibrium between the pure solid <strong>and</strong> a solution of its ions. For the dissolution of<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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