Question

What 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?

Answer 1

i. When excess of aqueous KCN is added to an aqueous solution of copper sulphate, the coordination entity formed is [Cu(CN)₄]^2 -.
[Cu(H₂O)₄]²⁺ + 4CN ^-  →   [Cu(CN)₄]^{2 -}  + 4H₂O
ii. [Cu(CN)₄]^2- is a coordination compound and hence, retains its identity in the aqueous solution. It does not dissociate to give free Cu²⁺ ions. Hence, no precipitate of copper sulphide is obtained when H₂S(g) is passed through this solution