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Q5.14:
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 ((H_{2}S(g))) is passed through this solution?

Solution :

Step 1: Chemical Reaction of Copper(II) Sulphate with Potassium Cyanide

When an aqueous solution of copper(II) sulphate ($\text{CuSO}_4$) is treated with excess potassium cyanide ($\text{KCN}$), the initial precipitate of copper(II) cyanide ($\text{Cu(CN)}_2$) is formed. Being unstable, it undergoes spontaneous reduction to form copper(I) cyanide ($\text{CuCN}$) and cyanogen gas ($\text{(CN)}_2$).

The chemical equations are as follows:

  • $\text{CuSO}_4(aq) + 2\text{KCN}(aq) \rightarrow \text{Cu(CN)}_2(s) + \text{K}_2\text{SO}_4(aq)$
  • $2\text{Cu(CN)}_2(s) \rightarrow 2\text{CuCN}(s) + (\text{CN})_2(g)$

With the addition of excess $\text{KCN}$, the $\text{CuCN}$ dissolves to form the highly stable coordination entity, tetracyanocuprate(I) ion:

$\text{CuCN}(s) + 3\text{KCN}(aq) \rightarrow \text{K}_3[\text{Cu(CN)}_4](aq)$

Step 2: Analysis of Coordination Entity Stability

The resulting complex is potassium tetracyanocuprate(I), represented as $\text{K}_3[\text{Cu(CN)}_4]$. Within this complex, copper exists in the $+1$ oxidation state. The complex ion $[\text{Cu(CN)}_4]^{3-}$ possesses an extremely high formation constant ($K_f$), which dictates the equilibrium concentration of free metal ions in the solution [Per the Principle of Coordination Equilibrium].

Step 3: Justification regarding $H_2S$ Precipitation

When hydrogen sulphide ($H_2S$) is passed through this solution, the copper ions are not available to react with sulphide ions ($S^{2-}$) because they are sequestered within the stable coordination sphere of the tetracyanocuprate(I) ion.

Aspect Explanation
Complex Stability The $[\text{Cu(CN)}_4]^{3-}$ ion is thermodynamically very stable.
Dissociation The dissociation equilibrium: $[\text{Cu(CN)}_4]^{3-} \rightleftharpoons \text{Cu}^+ + 4\text{CN}^-$ lies heavily to the left.
Ionic Product The concentration of $\text{Cu}^+$ ions produced is insufficient to exceed the solubility product ($K_{sp}$) of copper(II) sulphide or copper(I) sulphide.

[By the Common Ion Effect and Solubility Product Principle, precipitation occurs only when the ionic product exceeds the $K_{sp}$ of the salt]. Because the concentration of free $\text{Cu}^+$ is kept infinitesimally low by the strong cyano-ligands, the solubility product of $\text{Cu}_2\text{S}$ is not reached.

Complex: [Cu(CN)₄]³⁻ Strong Field Ligands prevent free Cu⁺ No Cu₂S Precipitate

Final Solution: The coordination entity formed is the tetracyanocuprate(I) ion, $[\text{Cu(CN)}_4]^{3-}$. No precipitate of copper sulphide is obtained because the complex is highly stable and provides an extremely low concentration of free $\text{Cu}^+$ ions, failing to reach the solubility product threshold required for the formation of copper sulphide.


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