9.14What is the coordination entity formed when excess of aqueous KCN is added to an aqueous solution of copper sulphate? Why is it that no precipitate of copper sulphide is obtained when H2S(g) is passed through this solution?
When CuSO4 reacts with an excess of KCN then complex formation takes place. The reaction is as follows:
CuSO4(aq) + 4KCN(aq) → K2[Cu (CN)4](aq) + K2SO4
i.e. [Cu(H2O)4]2+ + 4CN- → [Cu(CN)4]2- + 4H2O
Thus, the coordination entity formed in the process is K2[Cu(CN)4].
K2[Cu(CN)4] is a very stable complex that does not ionize to give Cu2+ ions when added to water. Hence Cu2+ ions are not precipitated when H2S(g) is passed through the solution.