When an aqueous solution of \(\mathrm{KCN}\) is added to a solution containing \(\mathrm{Ni}^{2+}\) ions, a precipitate forms, which redissolves on addition of more KCN solution. Write reactions describing what happens in this solution. [Hint: \(\mathrm{CN}^{-}\) is a Brónsted-Lowry base \(\left(K_{\mathrm{b}}=10^{-5}\right)\) and a Lewis base. \(]\)

In the given exercise, we have an aqueous solution of \(\mathrm{KCN}\) and a solution containing \(\mathrm{Ni}^{2+}\) ions. We know that when \(\mathrm{KCN}\) dissolves in water, it forms \(\mathrm{K}^{+}\) and \(\mathrm{CN}^{-}\) ions. So, the ions in the solution are \(\mathrm{K}^{+}\), \(\mathrm{CN}^{-}\), and \(\mathrm{Ni}^{2+}\).

The initial reaction is the formation of a precipitate. Since \(\mathrm{CN}^{-}\) is acting as a Brónsted-Lowry base (with \(K_\mathrm{b}=10^{-5}\)) and a Lewis base, it will form a coordination complex with \(\mathrm{Ni}^{2+}\). The reaction can be represented as: \[Ni^{2+} + 2CN^{-} \rightarrow Ni(CN)_2\downarrow\] Here, a precipitate of \(Ni(CN)_2\) is formed.

Upon adding more KCN to the solution, the precipitate redissolves. This occurs as more \(\mathrm{CN}^{-}\) ions are available to form a more stable complex with \(\mathrm{Ni}^{2+}\). In this case, \(\mathrm{CN}^{-}\) acts as a Lewis base, coordinating with \(\mathrm{Ni}^{2+}\). The redissolution reaction can be represented as: \[Ni(CN)_2 + 2CN^{-} \rightarrow Ni(CN)_4^{2-}\] In this step, \(Ni(CN)_4^{2-}\) complex ion is formed, and the precipitate redissolves. So, the complete set of reactions describing what happens in this solution are: 1. Initial reaction forming precipitate: \(Ni^{2+} + 2CN^{-} \rightarrow Ni(CN)_2\downarrow\) 2. Redissolution of the precipitate: \(Ni(CN)_2 + 2CN^{-} \rightarrow Ni(CN)_4^{2-}\)