Problem 9 - 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) a thin layer of \(\mathrm{Ag}\) is formed on \(\mathrm{Cu}\) (b) more voltage is required (c) \(\mathrm{Ag}^{\prime}\) ions are completely removed from solution (d) less availability of \(\mathrm{Ag}^{+}\)ions, as Cu cannot displace Ag from \(\left[\mathrm{Ag}(\mathrm{CN})_{2}^{-}\right]\)ion

Step-by-Step Solution

Answer

(d) less availability of Ag⁺ ions, as Cu cannot displace Ag from [Ag(CN)₂]⁻ ion.

Step 1: Understanding the Reaction

In silver plating, silver ions are deposited on a copper substrate. To understand why \(\mathrm{K[Ag(CN)}_{2}]\) is used over \(\mathrm{AgNO}_{3}\), we must analyze the ion availability and the chemical reactions involved.

Step 2: Comparing Silver Sources

With \(\mathrm{AgNO}_{3}\), silver ions \(\mathrm{Ag}^{+}\) are freely available in solution. However, using \(\mathrm{K[Ag(CN)}_{2}]\), the silver is in complex form as \(\mathrm{[Ag(CN)}_{2}]^{-}\), reducing the free \(\mathrm{Ag}^{+}\) ions in the solution.

Step 3: Copper's Reactivity

Copper has a tendency to displace less noble metals from solutions. In the presence of free \(\mathrm{Ag}^{+}\) ions, copper can displace silver from solution, leading to inefficient plating. The complex ion in \(\mathrm{K[Ag(CN)}_{2}]\) prevents copper from displacing silver.

Step 4: Analyzing Voltage Requirement and Ag Deposit

The reduced availability of \(\mathrm{Ag}^{+}\) ions from the complex also affects the voltage required for the plating process, as a moderate concentration of ions is involved. However, it mostly aims to prevent Cu from displacing Ag, ensuring a smooth layer of silver deposits.

Step 5: Conclusion Choice

The correct choice should reflect the prevention of undesired reactions due to control over available \(\mathrm{Ag}^{+}\) ions. Thus, \(\mathrm{K[Ag(CN)}_{2}]\) is used because Cu cannot displace Ag from the \(\mathrm{[Ag(CN)}_{2}]^{-}\) ion, not directly for voltage or thickness reasons.

Key Concepts

Complex Ion Formation

In silver plating, complex ions play a key role, ensuring a controlled deposition process. A complex ion is a type of charged species, where a central metal atom is surrounded by molecules or anions, which are known as ligands. For instance, in the silver plating process, silver forms a complex with cyanide ions, \(\mathrm{Ag(CN)_2^-}\). Here, cyanide ions act as ligands, binding to the silver cation and forming a stable ion in solution.

This formation is crucial because it controls the availability of free silver ions (\(\mathrm{Ag^+}\)) in the solution. \[\mathrm{K[Ag(CN)_2]} \rightarrow \mathrm{K^+} + \mathrm{[Ag(CN)_2]^-}\] This equation shows that in the dissociation of \(\mathrm{K[Ag(CN)_2]}\), silver remains part of the complex, not freely available as \(\mathrm{Ag^+}\). This results in a reduced chance for uncontrolled silver deposits, providing a smooth and well-defined silver layer over copper.

Overall, complex ion formation allows for refined applications in processes like silver plating, offering desired outcomes by carefully managing the metal ions involved.

Copper Reactivity

Copper is known for its reactivity, especially with ions of less noble metals, such as silver. This means that when copper comes into contact with free silver ions (\(\mathrm{Ag^+}\)), it can readily displace these ions from the solution because copper has the ability to donate electrons to reduce \(\mathrm{Ag^+}\) to elemental silver. This potential reaction can interfere with the silver plating process by precipitating silver undesirably onto the copper surface.

### Copper's Protective EnvironmentWith the use of complex ions such as \(\mathrm{[Ag(CN)_2]^-}\), the reactivity of copper within the plating solution is managed. The complexed form of silver reduces the amount of \(\mathrm{Ag^+}\) ions directly in solution, making it challenging for copper to displace them. As a result, copper's ability to spontaneously strip silver from the solution is inhibited, leading to controlled and efficient silver deposition.

This strategy ensures the preservation of the plating integrity by allowing silver to only deposit through designated electrochemical processes rather than unwanted chemical reactions.

Reduction of Free Silver Ions

The reduction of free silver ions within the plating solution is an essential consideration for achieving high-quality plating results. Free silver ions (\(\mathrm{Ag^+}\)) in the solution tend to be reactive, easily getting reduced and precipitated as metallic silver, generally contributing to unwanted outcomes during plating.

### Mitigating Unwanted ReactionsTo minimize such occurrences, complex ions such as \(\mathrm{[Ag(CN)_2]^-}\) are utilized. The primary advantage lies in their capacity to reduce the direct availability of \(\mathrm{Ag^+}\) in the solution, stabilizing the system. In this form, silver benefits from controlled release throughout the plating process, accomplished via the slow dissociation of the complex at the cathode.

This gradual release is vital for ensuring consistent and uniform silver deposition because it avoids rapid reduction and plating of silver, which could otherwise lead to patchy or coarse coatings. By regulating the availability of free silver ions, precision in silver plating over copper is successfully attained, which is essential for technical applications demanding fine layers with smooth finishes.