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

(d) \( \mathrm{K}_{3}[\mathrm{Cu(CN)}_{4}] \)

Step 1: Identify the Reactants

The reactants in the problem are copper sulfate (\( \mathrm{CuSO}_4 \)) and potassium cyanide (\( \mathrm{KCN} \)). We are adding an excess of \( \mathrm{KCN} \) to \( \mathrm{CuSO}_4 \).

Step 2: Initial Reaction Formation

\( \mathrm{KCN} \) is a source of \( \mathrm{CN}^- \) ions which can initially react with \( \mathrm{CuSO}_4 \) to precipitate \( \mathrm{Cu(CN)}_{2} \). However, \( \mathrm{Cu(CN)}_{2} \) is unstable and decomposes into \( \mathrm{CuCN} \) and \( \mathrm{C}_2\mathrm{N}_2 \).

Step 3: Complexation with Excess CN^-

The \( \mathrm{CuCN} \) forms a complex with excess \( \mathrm{CN}^- \) ions in the solution, leading to the formation of a complex ion, \([\mathrm{Cu(CN)_4}]^{3-}\). This is because cyanide is a good ligand and forms stable complexes with copper.

Step 4: Final Compound Formation

The formed complex \([\mathrm{Cu(CN)_4}]^{3-}\) binds with potassium ions from the \( \mathrm{KCN} \) solution, resulting in the formation of \( \mathrm{K}_3[\mathrm{Cu(CN)_4}] \), which can be identified as one of the options provided.

Step 5: Select the Correct Option

Comparing arguments from our analysis, the compound that matches the chemical formation is \( \mathrm{K}_{3}[\mathrm{Cu(CN)}_{4}] \). This corresponds to option (d).

Key Concepts

Complex Ion Formation

In the realm of coordination chemistry, complex ions are formed when central metal ions bond with molecules or ions, called ligands. This process occurs because these metal ions typically have empty d-orbitals capable of accepting electron pairs from the ligands. A great example is the reaction between copper sulfate (\( \mathrm{CuSO}_4 \)) and potassium cyanide (\( \mathrm{KCN} \)), where the cyanide ions (\( \mathrm{CN}^- \)) act as ligands.

• Initially, copper ions form a complex with available cyanide ions to create \( \mathrm{Cu(CN)_2} \).
• However, this initially formed complex is unstable and requires further interaction with additional cyanide ions to reach stability.

Thus, in the presence of excess cyanide ions from the excess \( \mathrm{KCN} \), the copper ions are enclosed by four cyanide ligands forming a stable complex entity \([\mathrm{Cu(CN)_4}]^{3-}\). This advanced bonding scenario involves coordination bonds where cyanides use their lone pairs to bond with copper, a typical feature of complex ion formation.

Ligand Exchange

Ligand exchange is a crucial concept in coordination chemistry as it relates to the swapping of ligands around a central metal ion. This phenomenon allows for the formation and transformation of complex ions as observed in our exercise. When excess \( \mathrm{KCN} \) is added to a \( \mathrm{CuSO}_4 \) solution, the cyanide ions (\( \mathrm{CN}^- \)) initially bond with copper, forming a slightly unstable compound, \( \mathrm{Cu(CN)_2} \). The magic happens when more \( \mathrm{CN}^- \) ions in the solution facilitate a ligand exchange:

• The unstable \( \mathrm{Cu(CN)_2} \) decomposes into \( \mathrm{CuCN} \) and \( \mathrm{C}_2\mathrm{N}_2 \)
• Subsequently, the \( \mathrm{CuCN} \) undergoes ligand exchange with excess \( \mathrm{CN}^- \), forming the complex ion \([\mathrm{Cu(CN)_4}]^{3-}\)

This exchange ensures a more stable structure, and the ability to achieve such stability is integral for the further application of coordination compounds in various fields like catalysis and medicine.

Stability of Complexes

The stability of complex ions is a cornerstone concept in coordination chemistry. Stability is significantly influenced by the nature of the ligands and their ability to donate electron pairs. When dealing with copper ions, cyanide \( \mathrm{CN}^- \) ions are exceptional at stabilizing the complex.

• Cyanide is a strong field ligand meaning it can donate electrons effectively, stabilizing the positive charge of the metal ion.
• This results in the formation of stronger coordination bonds, due to the Ï%s pairing of the electron clouds between the ligand and the metal ion as seen in \([\mathrm{Cu(CN)_4}]^{3-}\)

Such a stable complex formation minimizes energy and maximizes stability, rendering the ion less reactive towards other possible ligand exchanges. Stable complexes, such as \( \mathrm{K}_3[\mathrm{Cu(CN)_4}] \), therefore tend to be crucial in applications that require long-term chemical stability. They find usage in various industries including electroplating and medical imaging, where resilience of the compound is essential.