Q72 / 200 · NEET 2002 · Chemistry

When $\mathrm{CuSO}_4$ reacts with $\mathrm{KCN}$ it forms $\mathrm{CuCN}$, which is insoluble in water. It is soluble in excess of $\mathrm{KCN}$ due to the formation of the following complex:

  • A
    $\mathrm{K}_2\left[\mathrm{Cu}(\mathrm{CN})_4\right]$
  • Correct answer
    B
    $\mathrm{K}_3\left[\mathrm{Cu}(\mathrm{CN})_4\right]$
  • C
    $\mathrm{CuCN}_2$
  • D
    $\mathrm{Cu}\left[\mathrm{KCu}(\mathrm{CN})_4\right]$

Solution

$\mathrm{CuSO}_4$ reacts with $\mathrm{KCN}$ to give a white precipitate of cuprous cyanide and cyanogen gas. The cuprous cyanide dissolves in excess of $\mathrm{KCN}$ forming $\mathrm{K}_3\left[\mathrm{Cu}(\mathrm{CN})_4\right]$ $\begin{gathered} \mathrm{CuSO}_4+2 \mathrm{KCN} \rightarrow \mathrm{K}_2 \mathrm{SO}_4+ \mathrm{Cu}(\mathrm{CN})_2 \\ 2 \mathrm{Cu}(\mathrm{CN})_2 \rightarrow 2 \mathrm{CuCN}+\mathrm{CN}-\mathrm{CN} \\ \mathrm{CuCN}+3 \mathrm{KCN} \rightarrow \mathrm{K}_3\left[\mathrm{Cu}(\mathrm{CN})_4\right] \end{gathered}$

Hence, the correct answer is option: 'B'.

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