Problem 97 - 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 \(\mathrm{Cu}\) 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 because the complex ion provides stability, preventing copper from displacing silver.

Step 1: Understanding the Question

The question is about the reason why \(\mathrm{K}\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]\) is used instead of \(\mathrm{AgNO}_{3}\) in silver plating of copper. We need to identify the option that correctly explains the reason for this choice.

Step 1: Analyze Option (a)

Option (a) suggests that \(\mathrm{K}\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]\) is used because \(\mathrm{Cu}\) cannot displace Ag from \(\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]^{-}\) due to less availability of \(\mathrm{Ag}^{+}\) ions. This implies stability in the complex ion present in \(\mathrm{K}\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]\) resulting in a stable silver plating process.

Step 2: Analyze Option (b)

Option (b) suggests that more voltage is required when using \(\mathrm{AgNO}_{3}\). This could be true, but it does not directly explain why \(\mathrm{K}\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]\) is preferred for silver plating.

Step 3: Analyze Option (c)

Option (c) suggests a thin layer of Ag is formed on Cu. While this could be a result of the silver plating process using \(\mathrm{K}\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]\), it doesn't specifically explain why this compound is chosen over \(\mathrm{AgNO}_{3}\).

Step 4: Analyze Option (d)

Option (d) suggests that \(\mathrm{Ag}^{+}\) ions are completely removed from the solution. This implies that \(\mathrm{K}\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]\) provides a controlled release of silver ions, making it a more stable and efficient source for silver plating compared to \(\mathrm{AgNO}_{3}\).

Step 6: Conclusion: Selection of the Correct Option

Among the options analyzed, option (a), which notes the stability of silver ions in the complex \(\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]^{-}\), is the most direct reason for using \(\mathrm{K}\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]\) in silver plating over \(\mathrm{AgNO}_{3}\).

Key Concepts

K[Ag(CN)2] usage

In the process of silver plating copper, the choice of chemical compounds can greatly affect the final outcome. One common choice is potassium dicyanoargentate(I), or \(\mathrm{K}[\mathrm{Ag}(\mathrm{CN})_{2}]\), instead of silver nitrate (AgNO3).
This choice is primarily due to the stable form of silver provided by \(\mathrm{K}[\mathrm{Ag}(\mathrm{CN})_{2}]\).
The use of \(\mathrm{K}[\mathrm{Ag}(\mathrm{CN})_{2}]\) ensures that the delicate balance of silver ions needed for a smooth, even plating is maintained.

• It limits the availability of free silver ions, avoiding excessive or rapid release.
• Helps achieve a uniform deposition without aggressive reactions.
• Minimizes the chances of irregular plating that can occur with other compounds.

By using this complex, the silver plating process becomes more controlled and efficient, leading to high-quality results.

Reduction Potential

Reduction potential is a key concept when considering which metals can be successfully plated on others.
This is because it determines the likelihood of a metal ion gaining electrons and being reduced to a solid metal. For silver plating, the reduction potential is crucial:

• Silver has a higher reduction potential than copper.
• This means silver ions are more likely to gain electrons and deposit onto a copper surface.
• The use of \(\mathrm{K}[\mathrm{Ag}(\mathrm{CN})_{2}]\) facilitates this transfer efficiently.

Thus, by understanding and leveraging the reduction potential, the silver ions in the complex are perfectly poised to coat the copper substrate in a stable manner.

Electroplating Process

Electroplating is a widely used technique that involves a chemical reaction controlled by electric current. The technique is pivotal for creating a metallic coat on an object, like silver plating on copper. Here's how it works:

• A solution containing metal ions, such as the complex \(\mathrm{K}[\mathrm{Ag}(\mathrm{CN})_{2}]\), is prepared.
• Copper objects are placed into the solution, connected as cathodes in an electrical circuit.
• When current passes through, silver ions move towards the copper object, reducing and depositing as a thin, even layer of metal.
• The reaction is smooth and manageable due to the controlled availability of silver ions from the complex.

This process allows precise control over the thickness and quality of the coating, making electroplating with this specific complex effective and reliable.

Complex Ion Stability

The stability of a complex ion, such as \(\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]^{-}\) in silver plating, is crucial for a successful and controlled process. The stability refers to how well the ion maintains its integrity without prematurely releasing its metal ions. Several factors influence this stability:

• The specific ligands, like cyanide (CN^{−}), create a highly stable complex with silver.
• This stability regulates ion availability, ensuring a gradual and uniform plating layer.
• Enhanced stability prevents unwanted premature reactions that can affect plating quality.

The complex \(\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]^{-}\) allows the metal ion release to be smooth and consistent, resulting in high-quality finishes and longevity in the plated item.