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NCERT Class XII Chemistry
Chapter - Coordination Compounds
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Question : 14 of 43
Marks: +1, -0
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)\mathrm{H_2S_{(g)}} is passed through this solution?
Solution:  
[Cu(H2O)4]2++4CN−→[Cu(CN)4]2−highly stable+4H2O\mathrm{[Cu(H_2O)_4]^{2+}} + 4 \mathrm{CN}^{-} \rightarrow \underset{\text{highly stable}}{[\mathrm{Cu(CN)_4}]^{2-}} + 4 \mathrm{H_2O}
CN−^{-}is a strong field ligand, therefore, [Cu(CN)4]2−\mathrm{[Cu(CN)_4]^{2-}} is highly stable andhas large value of stability constant. On passing H2S\mathrm{H_2S}, CuS is not formedbecause this coordination entity does not give Cu2+\mathrm{Cu}^{2+} ion.
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