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 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

(a) 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 onto the surface of another metal, such as copper. We usually have AgNO3 in solution for easy availability of Ag+ ions, but in this scenario, K[Ag(CN)2] is used. We need to understand why this complex is preferable.

Step 2: Role of Complex Ion

The complex ion [Ag(CN)2]⁻ is stable and does not readily release Ag+ ions compared to AgNO3, where Ag+ ions are more freely available. This leads to slower deposition because copper (Cu) does not easily displace silver (Ag) from this stable complex.

Step 3: Comparing Ionic Availability

The key difference is the availability of Ag+ ions. In K[Ag(CN)2], the availability of free Ag+ ions is significantly reduced, meaning that Ag+ ions are not readily available for reaction, which is different from the scenario in the presence of AgNO3.

Step 4: Evaluating Options

- Option (a): Correct, as it aligns with the fact that the free availability of Ag+ ions is reduced, preventing copper from displacing Ag in the complex. - Option (b): Incorrect, as the voltage requirement isn't the main reason for the use of K[Ag(CN)2]. - Option (c): Incorrect, as both solutions can form thin layers of Ag. - Option (d): Incorrect, as Ag+ ions are not completely removed but are less available.

Key Concepts

Complex Ion Stability

In the realm of chemistry, stability isn't just about sitting still; it's about how components in a solution can withstand changes without easily breaking down.
When we talk about the complex ion \([\text{Ag}( ext{CN})_2]^ -\), we refer to a silver ion that is bound tightly with two cyanide ions. This binding creates a more stable structure compared to having free Ag⁺ ions floating in the solution, like in AgNO₃.

• Stable structures do not release their ions easily.
• This stability is crucial during silver plating because it controls the rate at which silver ions get deposited on copper.
• Even though the [Ag(CN)₂]⁻ complex holds the Ag⁺ ions firmly, it allows controlled and gradual release during the plating process, ensuring a smoother layer of silver.

Such stability is particularly important in industrial applications where uniform plating is desired for the durability and appearance of the plated object.

Ionic Availability

Ionic availability is a measure of how readily ions are available to participate in a chemical reaction.
In the context of silver plating, using \( \text{K}[\text{Ag}( ext{CN})_2] \) significantly affects the presence of active Ag⁺ ions.
While AgNO₃ provides a high concentration of available Ag⁺ ions, \([\text{Ag}( ext{CN})_2]^- \) holds them back effectively.

• Fewer free Ag⁺ ions means that the plating happens more slowly, allowing for controlled deposition.
• This slower process is beneficial for producing a thin and even coating of silver on the copper surface.
• Within the plating bath, the reduction in free Ag⁺ ions helps to maintain a steady and continuous release, preventing rapid and uneven metal deposition.

By understanding the availability of ions, manufacturers can adjust the plating process to either speed it up or slow it down, tailoring the end result to specific requirements.

Electrochemical Reactions

An electrochemical reaction involves a process where chemical reactions produce electrical changes, or electricity causes chemical changes.
In silver plating, these reactions are fundamental because they involve the transfer of electrons to achieve metal deposition.
In the case of silver plating with copper, electrons from copper interact with silver ions to form solid silver on the copper surface. When using \(\text{K}[\text{Ag}( ext{CN})_2]\), the stable complex affects the electrochemical process by limiting the sudden release of Ag⁺ ions, creating a smoother transition of silver onto copper.

• The copper acts as a reducing agent, giving electrons to silver ions, reducing them to metallic silver.
• The overall electrochemical reaction needs careful control to ensure the final product is consistent in quality.
• Any imbalance in the reaction can result in poor plating, including irregular thickness or adhesiveness of the silver layer.

Electrochemical reactions are intricately linked to the plating process's success, emphasizing the importance of stability and ionic availability.