Problem 90 - A Complete Resource Book in Chemistry for JEE Main

Question

Which compound is formed when excess of \(\mathrm{KCN}\) is added to an aqueous solution of copper sulphate? (a) \(\mathrm{Cu}(\mathrm{CN})_{2}\) (b) \(\mathrm{K}_{2}\left[\mathrm{Cu}(\mathrm{CN})_{6}\right]\) (c) \(\mathrm{K}\left[\mathrm{Cu}(\mathrm{CN})_{2}\right]\) (d) \(\mathrm{K}_{3}\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]\)

Step-by-Step Solution

Answer

The compound formed is (d) \( \mathrm{K}_3[\mathrm{Cu}(\mathrm{CN})_4] \).

Step 1: Start with Reactants

The problem mentions the addition of excess potassium cyanide, \( \mathrm{KCN} \), to aqueous copper sulfate, \( \mathrm{CuSO}_4 \). The initial interaction is between the copper(II) ion \( \mathrm{Cu}^{2+} \) and the cyanide ion \( \mathrm{CN}^- \).

Step 2: Consider Direct Precipitation Reaction

During the reaction, copper(II) cyanide, \( \mathrm{Cu(CN)_2} \), can initially form as a white precipitate via:\[ \mathrm{Cu^{2+} + 2CN^- \rightarrow Cu(CN)_2} \] However, this compound is unstable in aqueous solution and can further react.

Step 3: Reacting in Excess of Cyanide

In the presence of excess cyanide, the unstable \( \mathrm{Cu(CN)_2} \) quickly dissolves as it forms a complex ion, \( [\mathrm{Cu(CN)_4}]^{3-} \), causing the complexation reaction:\[ \mathrm{Cu(CN)_2 + 2CN^- \rightarrow [Cu(CN)_4]^{3-}} \]

Step 4: Formation of the Stable Complex Compound

The stable complex ion \( [\mathrm{Cu(CN)_4}]^{3-} \) combines with potassium ions from \( \mathrm{KCN} \) to form the compound \( \mathrm{K_3[Cu(CN)_4]} \). This can be represented as:\[ 3\mathrm{K^+} + [\mathrm{Cu(CN)_4}]^{3-} \rightarrow \mathrm{K_3[Cu(CN)_4]} \] Therefore, the final product is a stable complex, potassium tetra-cyanocuprate(II).

Key Concepts

Complex Ion Formation

In coordination chemistry, complex ion formation is a fundamental concept where central metal ions are surrounded by molecules or anions, termed ligands. This process results in the creation of complex ions through coordinate bonds. These bonds form when a ligand donates a pair of electrons to an empty orbital on a metal ion, creating a coordination compound.

In the given exercise, the crucial step is the formation of the complex ion that stabilizes the otherwise precipitating copper ion in solution. The copper(II) cyanide initially precipitates but then dissolves as it forms a more stable coordination entity by bonding with additional cyanide ions. This reaction can happen under conditions where there is an excess of ligand, in this case, the cyanide ions. The compound formed is \[\left[ \text{Cu(CN)}_4 \right]^{3-}\] which involves copper ion fully coordinating with four cyanide ions, leading to a stable tetrahedral complex structure.

Ligand Exchange

Ligand exchange, also known as substitution, is a process where one ligand in a complex ion is replaced with another. This activity is significant in the reactions among complex ions and primarily occurs due to several available donor atoms of the potential ligands around the central atom.

In our example, ligand exchange is evident as the unstable \[ \text{Cu(CN)}_2 \] changes with more cyanide ligands. This exchange results in the creation of the \[ \left[ \text{Cu(CN)}_4 \right]^{3-} \], accommodating four cyanide ions instead of two, thus stabilizing the copper ion within the new coordination environment. Such ligand exchange reactions often depend on the kinetics and thermodynamics of the involved species, where equilibrium typically favors the formation of the most stable complex.

Copper Complexes

Copper complexes refer to coordination compounds where copper acts as the central metal ion. These complexes are known for their versatile chemistry and unique properties. Copper can exhibit multiple oxidation states, though the two most common are +1 and +2, affecting its behavior in forming complex ions.

In the scenario of the exercise, copper is (II) oxidation state initially noted as \( \mathrm{Cu}^{2+} \). The reaction progresses through complex formation, facilitated by the addition of cyanide ions, resulting in the stable complex ion \( \left[ \text{Cu(CN)}_4 \right]^{3-} \). This compound further reacts with potassium ions present in \( \mathrm{KCN} \) to ultimately yield \( \mathrm{K_3[Cu(CN)_4]} \), a coordination complex termed as tetra-cyanocuprate(II). These complex ions are recognized for their stability and distinct coordination geometries, which are significant in areas like coordination chemistry, analytical applications, and their potential uses in various industrial processes.