The Effect of Complex Ion Equilibria on Solubility

Recall from Section 16.5 that the solubility of an ionic compound with a basic anion increases with increasing acidity because the acid reacts with the anion and drives the reaction to the right. Similarly, the solubility of an ionic compound containing a metal cation that forms complex ions increases in the presence of Lewis bases that complex with the cation. The most common Lewis bases that increase the solubility of metal cations are NH3, CN , and OH . For example, silver chloride is only slightly soluble in pure water  

However, adding ammonia increases its solubility dramatically because, as we saw previously in this section, the ammonia forms a complex ion with the silver cations  

The large value of Kf significantly lowers the concentration of Ag (a ) in solution and therefore drives the dissolution of AgCl(i). The two previous reactions can be added together  

As we learned in Section 14.3, the equilibrium constant for a reaction that is the sum of two other reactions is the product of the equilibrium constants for the two other reactions. Adding ammonia changes the equilibrium constant for the dissolution of AgCl(i) by a factor of 3.0 X 10 jl.ll X 10 = 1.7 X 10 (17 million), which makes the 

Formation Normally insoluble AgCl is made soluble by the addition of NH3, which forms a complex ion with Ag and dissolves the AgCl. 

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