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 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) Ag ions are completely removed from solution

Step-by-Step Solution

Answer

The reason is (a) less availability of Ag+ ions, as Cu cannot displace Ag from the [Ag(CN)2]− ion.

Step 1: Understanding the Function of the Complex Ion

In electroplating, \(\mathrm{K}[\mathrm{Ag}(\mathrm{CN})_2]\) is chosen over \(\mathrm{AgNO}_3\) because the complex ion \(\left[\mathrm{Ag}(\mathrm{CN})_2\right]^-\) ensures controlled deposition of silver. This is because cyanide complexes significantly reduce the concentration of free \(\mathrm{Ag}^+\) ions, making silver deposition orderly and fine.

Step 2: Investigating Option (a)

Option (a) states that less \(\mathrm{Ag}^+\) ions are available, preventing copper from displacing silver from the complex. This is true because the complex formation stabilizes silver ions, reducing \(\mathrm{Ag}^+\) availability for displacement by copper.

Step 3: Checking Option (b)

Option (b) suggests that more voltage is required. Since the complex ion stabilizes \(\mathrm{Ag}^+\), the process does not primarily require more voltage but stability in Ag ion deposition.

Step 4: Evaluating Option (c)

Option (c) says that a thin layer of \(\mathrm{Ag}\) is formed on \(\mathrm{Cu}\). While true that controlled deposition occurs, this doesn't distinctly agree with the rationale for using the complex over \(\mathrm{AgNO}_3\).

Step 5: Analyzing Option (d)

Option (d) claims that \(\mathrm{Ag}^+\) ions are completely removed from the solution. This is incorrect since the solution still contains \(\mathrm{Ag}^+\) ions, but they are part of the complex and not free ions.

Step 6: Conclusion

Option (a) is the best explanation. The complex formation with cyanide ensures a low concentration of free silver ions, which prevents copper from displacing silver, facilitating controlled silver plating.

Key Concepts

Electroplating

Electroplating is a process where a metal is coated onto another material's surface using electrical current. This method is commonly used to enhance resistance to wear and corrosion, improve appearance, and provide specific surface properties. In the case of silver plating on copper, electroplating ensures a consistent and high-quality finish. Current is passed through an electrolytic solution, where the metal ions are reduced and deposited onto the material to be plated.

• The object to be plated is made the cathode (negative electrode).
• The metal that provides the ions for plating is the anode (positive electrode).
• An electrolyte containing metal ions completes the circuit, allowing the transfer of metal atoms.

In silver plating, we often use a specially formulated solution to manage the ion concentration and deposition properly, ensuring a uniform layer of silver is deposited onto the copper surface.

Complex Ion

A complex ion is a structure formed when metal ions combine with other molecules or ions, creating a stable complex. In the context of silver plating on copper, this concept plays a crucial role due to the creation of the complex ion \([Ag(CN)_2]^-\), which involves silver ions coordinated with cyanide ions.

Complex ions have distinct advantages in controlled reaction environments:

• They reduce the concentration of free metal ions.
• Their stability prevents undesirable side reactions.
• They allow more controlled and gradual deposition of metals.

The use of complex ions like \([Ag(CN)_2]^-\) in electroplating helps maintain the desired rate of silver ion deposition, ensuring a precise silver layer on the substrate.

Controlled Deposition

Controlled deposition refers to the careful management of metal ion reduction and placement during the electroplating process. It is essential to achieve a uniform and defect-free metal layer on the substrate. Through complex ions, controlled deposition becomes feasible by regulating the availability of metal ions in the solution.

The benefits of controlled deposition are numerous:

• It ensures the thickness of the metal layer is consistent.
• It minimizes waste and overuse of materials.
• It enhances the mechanical and aesthetic properties of the plating.

By employing complex ions, like \([Ag(CN)_2]^-\), the process maintains a low concentration of free ions, thus creating optimal conditions for a gradual and uniform deposition of silver.

Cyanide Complexes

Cyanide complexes involve a fascinating interplay of chemistry where cyanide ions form stable associations with metal ions. In the electroplating context, \([Ag(CN)_2]^-\) is a primary example.

Cyanide enhances metal processing for several reasons:

• Stability: It creates strong complexes that hold the metal ions firmly, ensuring controlled deposition.
• Solubility: Cyanide complexes in solution maintain a stable ion concentration.
• Efficiency: It enables fine control over the plating process, reducing energy consumption and improving results.

Using cyanide complexes in electroplating of silver over copper, the reduced concentration of free \([Ag]^+\) ions secures the delicate surface finish and avoids unwanted bulk deposition, demonstrating their indispensable role in modern metalwork.