Problem 137 - 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) \(\mathrm{Cu}(\mathrm{CN})\), (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 \( \mathrm{K}_3\left[\mathrm{Cu(CN)}_4\right] \). Option (d).

Step 1: Understand the Reaction

When potassium cyanide (\( \mathrm{KCN} \)) is added to copper sulphate (\( \mathrm{CuSO}_4 \)), a displacement reaction occurs. Copper ions \( (\mathrm{Cu}^{2+}) \) from the copper sulfate are precipitated by \( \mathrm{CN}^- \) ions from potassium cyanide.

Step 2: Formation of Basic Complex

Initially, \( \mathrm{Cu}^{2+} \) ions react with \( \mathrm{CN}^- \) ions forming a copper cyanide complex \( \mathrm{Cu(CN)}_2 \), but in the presence of excess \( \mathrm{KCN} \), the complex undergoes further reaction.

Step 3: Complexation with Excess KCN

In excess \( \mathrm{KCN} \), the \( \mathrm{Cu}^{2+} \) from \( \mathrm{Cu(CN)}_2 \) further complexes to form \( \left[\mathrm{Cu(CN)}_4\right]^{3-} \), which, with potassium ions \((\mathrm{K}^+)\) from \( \mathrm{KCN} \), forms the compound \( \mathrm{K}_3\left[\mathrm{Cu(CN)}_4\right] \).

Step 4: Conclusion and Answer Selection

The correct compound formed is \( \mathrm{K}_3\left[\mathrm{Cu(CN)}_4\right] \), which corresponds to option (d).

Key Concepts

Displacement Reaction

In coordination chemistry, a displacement reaction is one where ions or atoms replace others within a compound. A classic example is when copper ions (\( \mathrm{Cu}^{2+} \)) within copper sulfate are replaced. This occurs when potassium cyanide (\( \mathrm{KCN} \)) is added to the copper sulfate solution. Here’s how it happens:

• The cyanide ions (\( \mathrm{CN}^- \)) added to the solution are highly reactive.
• The copper ions, which are positive, are attracted to the negative cyanide ions.
• This reaction results in the formation of a new compound where copper from the copper sulfate is no longer bonded to sulfate.

The displacement reaction initially forms copper cyanide but does not stop there, especially when there is an excess of \( \mathrm{KCN} \).This sets the stage for more complex chemistry to occur.

Complex Ions

Complex ions play a crucial role in the reactions involving copper sulfate and excess potassium cyanide. But what are complex ions?Complex ions consist of a central metal ion bonded to molecules or ions, known as ligands. In this specific reaction:

• The central metal ion is copper (\( \mathrm{Cu}^{2+} \)).
• The ligands are cyanide ions (\( \mathrm{CN}^- \)).

When excess \( \mathrm{KCN} \) is added, a special complex called \( \left[\mathrm{Cu(CN)}_4\right]^{3-} \) forms. This complex has four cyanide ions surrounding the copper ion.The charged complex ions need balanced opposite charges. Potassium ions, introduced by \( \mathrm{KCN} \), provide this balance as they pair up with the negatively charged complex to form \( \mathrm{K}_3\left[\mathrm{Cu(CN)}_4\right] \).This phenomenon shows how excess ligands can force further coordination, expanding the reaction beyond initial simple compounds.

Copper Sulfate reactions

Copper sulfate is a blue crystalline solid commonly used in chemistry for its distinct properties. It becomes particularly interesting in reaction stages involving complex ions:1. **Initial Formation**: - Copper sulfate dissolves in water to release copper ions (\( \mathrm{Cu}^{2+} \)) and sulfate ions (\( \mathrm{SO}_4^{2-} \)).2. **Reaction with \( \mathrm{KCN} \)**: - The addition of \( \mathrm{KCN} \) to the copper sulfate solution leads to a rearrangement. Initially, a compound \( \mathrm{Cu(CN)}_2 \), which is unstable, appears but rapidly changes.

• In scenarios where \( \mathrm{KCN} \) is in excess, the situation progresses further, leading to yet another transformation into \( \left[\mathrm{Cu(CN)}_4\right]^{3-} \).

3. **Product Formation**: - The presence of potassium ions from \( \mathrm{KCN} \) completes this transformation as they bind to \( \left[\mathrm{Cu(CN)}_4\right]^{3-} \) forming the final product: \( \mathrm{K}_3\left[\mathrm{Cu(CN)}_4\right] \).This remarkable series of reactions demonstrates copper sulfate’s ability to participate in highly coordinated, dynamic chemical changes.