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) \(\mathrm{Ag}^{+}\)ions are completely removed from solution

Step-by-Step Solution

Answer

(a) less availability of \( \text{Ag}^+ \) ions, as Cu cannot displace Ag from \([\text{Ag}( ext{CN})_2]^-\) ion.

Step 1: Understand the Role of Complex Ions

The complex ion \( ext{Ag}( ext{CN})_2^-\) is formed to stabilize silver ions in solution. This stabilization is important during silver plating as it allows controlled deposition of silver onto copper.

Step 2: Evaluate Silver Ion Availability

In the case of \( ext{AgNO}_3\), dissociation is full, releasing \( ext{Ag}^+\) ions which can be displaced by copper. In \( ext{K[Ag(CN)}_2\text{]}\), silver ions are partially unavailable due to complexation.

Step 3: Consider Displacement Reaction

Copper can displace silver from \( ext{Ag}NO_3\) because \( ext{Ag}^+\) ions are free. However, from \([ ext{Ag}( ext{CN})_2]^-,\) copper cannot easily displace silver due to the stability of the complex ion.

Step 4: Reason for Using Complex Ions

Using \([ ext{Ag}( ext{CN})_2]^-,\) prevents copper from displacing silver during deposition, hence forming a stable and uniform silver layer. This is due to the reduced free \( ext{Ag}^+\) ion concentration in solution.

Key Concepts

Complex Ions and Their Role in Silver Plating

Complex ions play a significant role in chemical reactions, especially in processes like silver plating. When it comes to silver plating, the complex ion \[\text{Ag}(\text{CN})_2^-\] is particularly important. This ion forms when silver ions (\(\text{Ag}^+\)) combine with cyanide ions (\(\text{CN}^-\)). This combination creates a stable structure, allowing silver to be more controlled during the plating process.

• Complex ions stabilize metal ions in solution.
• They prevent immediate precipitations or reactions with other elements in the solution.
• Important for controlling reaction rates and deposition processes.

In the context of silver plating copper, complex ions ensure that silver ions remain in solution without being easily displaced, which is crucial for achieving a smooth, even coat of silver over the copper surface. The use of \[\text{K[Ag}(\text{CN})_2]\] instead of \(\text{AgNO}_3\) is deliberate, as these complex ions make fewer free \(\text{Ag}^+\) ions available, thus preventing premature silver deposition.

Understanding Displacement Reactions

Displacement reactions are a type of chemical reaction where an element displaces another in a compound. These reactions are fundamental in understanding how certain elements interact with each other, particularly metals. In the case of silver plating,

• A more reactive metal can displace a less reactive metal from its compounds.
• In \(\text{AgNO}_3\), silver ions are free and more susceptible to being displaced by copper (a more reactive metal).
• However, \([\text{Ag}(\text{CN})_2]^-\) forms a stable complex, which copper cannot easily disrupt.

This stability means that copper cannot displace silver when it is part of a complex ion. This protects the silver, allowing it to instead slowly deposit onto the copper surface.
Chemists take advantage of this property to control how metals are deposited in metal coatings and other similar applications, ensuring that the final product has the desired characteristics.

The Benefits of Controlled Deposition

Controlled deposition is crucial in processes where the quality of the coating is of utmost importance. By understanding and using chemistry properly, like in the case of silver plating copper, we achieve the controlled and even application of silver.

• Controlled deposition refers to the planned and steady addition of a substance onto another surface.
• It helps in forming a uniform layer, avoiding clumping or uneven coating.
• In silver plating, controlled deposition ensures a smooth, lustrous finish, minimizing material wastage.

The use of the complex ion \([\text{Ag}(\text{CN})_2]^-\)plays a pivotal role in achieving controlled deposition during silver plating on copper. With fewer free \(\text{Ag}^+\) ions, the deposition is slow and steady, allowing for precision and accuracy in the thickness and quality of the silver layer. This approach is essential not only for aesthetic purposes but also for ensuring the longevity and resilience of the plated layer.