Ncert Exercises & Solutions

9.14 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?

Step 1:

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].

Step 2:

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.

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