6.3 Formation Constant
As seen with Acidic Metal Hydrides, some substances can form complex ions when in aqueous solution. The extent at which these complex ions can form is described with a new equilibrium constant called a formation constant written as Kf. Formation constants tend to be very large.
6.3.1 Silver(I) diammine cation, Ag(NH3)2+
Silver(I) chloride is not very soluble in water.
\[\mathrm{AgCl}(s) \rightleftharpoons \mathrm{Ag^+}(aq) + \mathrm{Cl^-}(aq) \quad K_{\mathrm{sp}} = 1.77\times 10^{-10}\]
However, the silver(I) cation readily forms a complex ion with ammonia in aqueous solution.
\[\mathrm{Ag^+}(aq) + 2\mathrm{NH_3}(aq) \rightleftharpoons \mathrm{Ag(NH_3)_2^+}(aq) \quad K_{\mathrm{f}} = 1.7\times 10^{7}\]
The presence of ammonia leads to the very favored formation of the silver(I) diammine complex ion and therefore increases the solubility of AgCl(s)! Summing the two reactions give
\[\begin{align*} \mathrm{AgCl}(s) + 2\mathrm{NH_3}(aq) \rightleftharpoons \mathrm{Ag(NH_3)_2^+}(aq) + \mathrm{Cl^-}(aq) \quad K = 3.01\times 10^{-3} \end{align*}\]
As AgCl dissociates, Ag+(aq) concentration is consumed as it forms a complex ion with ammonia. According to Le Chatelier’s principle, the removal of product in the first reaction (due to the second reaction) will drive the first reaction to the right to produce more product (thereby dissolving more AgCl)!
See Example 15.14 in the textbook for an equilibrium problem addressing Kf.