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) \(\mathrm{Ag}^{+}\)ions are completely removed from solution (d) less availability of \(\mathrm{Ag}^{+}\)ions, as \(\mathrm{Cu}\) cannot displace Ag from \(\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]\) ion

Step-by-Step Solution

Answer

(d) Less availability of \( \mathrm{Ag}^{+} \) ions, as \( \mathrm{Cu} \) cannot displace Ag from \([\mathrm{Ag}(\mathrm{CN})_{2}]^-\).

Step 1: Understanding Silver Plating

In silver plating, a solution containing silver ions is used to coat a layer of silver on a metal, such as copper. The choice of solution influences the plating process and outcomes.

Step 2: Analyzing the Role of K[Ag(CN)2]

The compound \( \mathrm{K}[\mathrm{Ag}(\mathrm{CN})_{2}] \) is used because it provides complex ions \( [\mathrm{Ag}(\mathrm{CN})_{2}]^- \). These complex ions release silver ions very slowly into the solution.

Step 3: Comparing with AgNO3

If \( \mathrm{AgNO}_{3} \) is used, it would release Ag\(^{+}\) ions freely into the solution, leading to rapid and poor quality deposition due to quick reaction with copper.

Step 4: Reason for Complex Ion

In \( \mathrm{K}[\mathrm{Ag}(\mathrm{CN})_{2}] \), the \( [\mathrm{Ag}(\mathrm{CN})_{2}]^- \) forms a stable complex, preventing excessive availability of \( \mathrm{Ag}^{+} \) ions so that copper doesn't displace silver ions easily, improving the quality of plating.

Step 5: Conclusion

Thus, \( \mathrm{K}[\mathrm{Ag}(\mathrm{CN})_{2}] \) is chosen because it provides less availability of \( \mathrm{Ag}^{+} \) ions, ensuring copper cannot displace silver, and offering control over silver ion release. The correct option is (d).

Key Concepts

Complex Ions

When we talk about complex ions, we're referring to a special structure where a metal ion is surrounded by other molecules or ions, known as ligands. In the silver plating process, complex ions play an important role because they help control how silver is deposited onto a metal surface like copper.
In our original exercise, the compound \( \mathrm{K} \left[ \mathrm{Ag} (\mathrm{CN})_2 \right] \) is utilized. This compound forms a complex ion \( [\mathrm{Ag} (\mathrm{CN})_2]^− \). The presence of this complex ion stabilizes the silver ion, preventing it from being too reactive. This allows for a gentle and even deposition of silver on copper.

• The central metal ion, silver (Ag\(^{+}\)), is surrounded by cyanide ions (CN\(^−\)).
• This configuration prevents the immediate release of silver ions into the solution.

This slow release is crucial for creating a uniform layer of silver on the copper surface.

Electroplating Process

The electroplating process is a technique used to coat a metal object with a thin layer of another metal. It involves the use of an electric current to reduce dissolved metal ions onto a conductive surface. This method is frequently used for decorative purposes, corrosion protection, or to improve electrical conductivity.
In the context of silver plating, the process begins by immersing the object to be plated, like copper, into a solution containing metal ions. An external power source is used to apply a negative charge to the copper and a positive charge to a silver piece serving as the anode.

• The metal ions migrate towards the metal object, which absorbs the electrons and forms a layer of silver.
• Electroplating helps in achieving a smooth, shiny, and durable silver coating.

Choosing the correct chemical composition for the solution, such as incorporating complex ions, can significantly influence the final outcome of the electroplating process.

Metal Deposition

During metal deposition, ions in the solution are transformed into solid metal and are deposited onto a surface. This process involves multiple electrochemical reactions, where metal ions gain electrons and form a metal layer. The quality of deposited metal depends on various factors, including the rate of ion availability, current density, and electrolyte composition.
Slower deposition, as facilitated by complex ions, typically results in a smoother and more adherent coat. In silver plating on copper:

• Using \( [\mathrm{Ag} (\mathrm{CN})_2]^− \) ensures a controlled release of \( \mathrm{Ag}^{+} \).
• The slow deposition minimizes coating flaws, enhancing the quality and uniformity of the silver layer.

In practice, this improves the overall durability and appearance of the silver plating.

Ionic Compounds

Ionic compounds are formed by the electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions). They are a key component in electroplating, as they comprise the solutions used for metal deposition.
In the exercise, \( \mathrm{AgNO}_{3} \) is a simple ionic compound that releases \( \mathrm{Ag}^{+} \) ions quickly. On the contrary, \( \mathrm{K} \left[ \mathrm{Ag} (\mathrm{CN})_2 \right] \) acts differently:

• The presence of complex ions from \( \mathrm{K} \left[ \mathrm{Ag} (\mathrm{CN})_2 \right] \) ensures a steady but slow release of \( \mathrm{Ag}^{+} \).
• This prevents excessive availability of silver ions, avoiding rapid and uncontrolled plating reactions.

Understanding the nature of ionic compounds and their behavior under varied conditions is essential in optimizing plating processes for best results.