Problem 62 - A Complete Resource Book in Chemistry for JEE Main

Question

In the silver plating of copper, \(\mathrm{K}\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]\) is used instead of \(\mathrm{AgNO}_{3}\). The reason is (a) less availability of \(\mathrm{Ag}^{+}\)ions, as Cu cannot displace Ag from \(\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]^{-}\)ion (b) more voltage is required (c) a thin layer of \(\mathrm{Ag}\) is formed on \(\mathrm{Cu}\) (d) \(\mathrm{Ag}^{+}\)ions are completely removed from solution

Step-by-Step Solution

Answer

Option (a) is correct, as \([ ext{Ag}( ext{CN})_{2}]^{-}\) limits \( ext{Ag}^{+}\) ion availability, ensuring controlled deposition.

Step 1: Understanding the Question

The problem is asking why \( ext{K}[ ext{Ag}( ext{CN})_{2}]\) is used instead of \( ext{AgNO}_{3}\) for silver plating on copper. We have four options to choose from, each related to the behavior of silver ions (\( ext{Ag}^{+}\)) and their interaction with copper (Cu).

Step 2: Analyzing Option (a)

Option (a) suggests reduced availability of \( ext{Ag}^{+}\) ions because copper cannot displace silver from \([ ext{Ag}( ext{CN})_{2}]^{-}\). This implies \([ ext{Ag}( ext{CN})_{2}]^{-}\) is a stable complex, preventing \( ext{Ag}^{+}\) ions from being freely available in solution, which is favorable for controlled deposition.

Step 3: Analyzing Option (b)

Option (b) suggests that more voltage is required, indicating a greater potential difference is needed for deposition. However, the use of \([ ext{Ag}( ext{CN})_{2}]^{-}\) is typically chosen not for higher voltage requirements but for stability and controlled release of \( ext{Ag}^{+}\) ions.

Step 4: Analyzing Option (c)

Option (c) states that a thin layer of \( ext{Ag}\) forms on \( ext{Cu}\). This is a result of controlled availability of \( ext{Ag}^{+}\) to ensure gradual deposition, consistent with the use of \([ ext{Ag}( ext{CN})_{2}]^{-}\), supporting option (a) as a reason.

Step 5: Analyzing Option (d)

Option (d) suggests that \( ext{Ag}^{+}\) ions are completely removed from the solution. This isn't true because the silver ions are needed for plating; they are not removed but instead controlled through the complexation with cyanide.

Step 6: Determining the Correct Option

Based on the analysis, option (a) seems correct. The stable complex \([ ext{Ag}( ext{CN})_{2}]^{-}\) reduces the immediate availability of \( ext{Ag}^{+}\), ensuring controlled deposition and preventing copper from displacing silver.

Key Concepts

Complex Ions

Complex ions are an essential component of various chemical processes, including electroplating. A complex ion consists of a central atom or ion, often a metal, surrounded by molecules or anions called ligands. In the example of silver plating, \(\mathrm{K}[\mathrm{Ag}(\mathrm{CN})_{2}]\), we have a complex ion where silver (Ag) is the central metal ion and cyanide (CN) acts as the ligand. This complex formation is significant because:

• It stabilizes the silver ion in solution, preventing premature reactions.
• Complexes are usually more soluble, allowing for more precise control over metal ions in solutions.
• Increases the effectiveness of silver plating by maintaining silver ions in a desirable state for gradual deposition.

Understanding complex ions helps one grasp why certain compounds, like \(\mathrm{K}[\mathrm{Ag}(\mathrm{CN})_{2}]\), are preferred over simpler compounds such as \(\mathrm{AgNO}_{3}\) in applications like plating. By forming a stable complex ion, the deposition of silver onto copper can be controlled more efficiently, preventing unwanted reactions and ensuring a consistent coating.

Redox Reactions

Redox reactions, short for reduction-oxidation reactions, involve the transfer of electrons from one substance to another. A key aspect of electroplating is the redox reaction between the metal ions in solution and the substrate being plated. In silver plating, the reaction can be summarized as:
1. Silver ions (\(\mathrm{Ag}^{+}\)) are reduced to metallic silver (\(\mathrm{Ag}\)).
2. Simultaneously, another element, such as copper, may be oxidized.
This electron transfer is crucial because:

• It ensures that the metal ions in the solution are deposited onto the substrate, forming a metal layer.
• The controlled release of electrons helps form a smooth and even coating.
• Understanding redox reactions allows for better manipulation of plating processes, optimizing outcomes like adhesion and thickness of the deposit.

The balanced exchange in redox reactions is why complex ions such as \(\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]^{-}\) are used, ensuring that the reduction part of the reaction is gradual and controlled.

Stability of Complexes

The stability of a complex plays a pivotal role in its effectiveness for applications like electroplating. A stable complex like \(\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]^{-}\) holds onto its metal ions more tightly. This stability is important because:

• It prevents the easy displacement of the metal ion by other metals, such as copper in this case.
• Offers a controlled and sustained release of metal ions into the solution.
• Enhances the deposition process by ensuring metal ions are released only when needed, leading to a more uniform layer.

Cyanide (\(\mathrm{CN}^{-}\)) as a ligand provides significant stabilization to the silver ion, thanks to its ability to form strong bonds. This means that silver ions are not readily available to react haphazardly, reducing the chance of uneven plating, and ensuring a smooth, consistent coating on the substrate. Understanding the importance of complex stability allows one to appreciate the deliberate choice of using compounds like \([\mathrm{Ag}(\mathrm{CN})_{2}]^{-}\) in practical applications, where precision and quality are paramount.