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) Ag ions are completely removed from solution

Step-by-Step Solution

Answer

(a) less availability of \(\mathrm{Ag}^{+}\) ions, as \(\mathrm{Cu}\) cannot displace Ag from \(\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]^{-}\) ion

Step 1: Understanding the Chemistry of Plating

Silver plating involves depositing a layer of silver onto copper using a chemical reaction. It's important to have a control over the silver ions (\(\mathrm{Ag}^+\)) to ensure a proper, even layer is formed.

Step 2: Role of the Complex Ion

The complex ion \(\mathrm{K}\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]\) is used instead of \(\mathrm{AgNO}_{3}\) because it releases \(\mathrm{Ag}^+\) ions in a controlled manner due to the complexation with cyanide. This provides stability and reduces the reactivity of the \(\mathrm{Ag}^+\) ions.

Step 3: Preventing Displacement Reactions

Using \([\mathrm{Ag}(\mathrm{CN})_{2}]^{-}\) ensures that copper does not displace silver ions easily. If \(\mathrm{AgNO}_{3}\) were used, copper could displace silver as per its position in the electrochemical series, which might lead to undesired reactions or deposits.

Step 4: Analysis of Options

Among the options, choice (a) corresponds to the controlled availability of \(\mathrm{Ag}^+\) ions. The complex ion form ensures \(\mathrm{Cu}\) does not displace the silver ions, thus maintaining the controlled deposition process and preventing unwanted reactions.

Key Concepts

Silver Plating

Silver plating is a fascinating process involving the deposition of a thin layer of silver metal onto another material, often copper. This is achieved through an electrochemical process called electroplating. In this method, a silver compound is dissolved in an electrolyte bath, and an electric current is passed through this solution. This causes the silver ions to migrate and deposit onto the surface of the copper article.

Silver plating is applied for various reasons:

• Decorative purposes: Silver is valued for its shiny, aesthetically pleasing appearance.
• Corrosion resistance: It helps protect metals like copper from oxidation.
• Conductivity: Silver's excellent electrical conductivity is beneficial for electronic components.

The choice of the silver source in the solution is crucial for achieving the desired plating quality.

Complex Ions

Complex ions play a critical role in the silver plating process. In particular, \(\mathrm{K}\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]\) is a complex ion that is preferable over simple silver salts such as \(\mathrm{AgNO}_{3}\).

This complex ion consists of silver ions bonded with cyanide ions, forming a stable compound. Some benefits of using complex ions include:

• Controlled Release: The complex structure allows a controlled release of \(\mathrm{Ag}^{+}\) ions into the solution, leading to a uniform silver layer.
• Stability: Cyanide ligands stabilize the silver, reducing its reactivity and preventing unwanted side reactions.
• Enhanced Control: Control over the concentration of \(\mathrm{Ag}^{+}\) ions aids in forming a smooth and defect-free plating.

Complex ions like \(\mathrm{K}[\mathrm{Ag}(\mathrm{CN})_{2}]\) thus ensure a high-quality silver plating process by modulating the electrochemical environment.

Displacement Reactions

Understanding displacement reactions is crucial in preventing unwanted chemical interactions during silver plating. A displacement reaction occurs when a more reactive metal displaces a less reactive metal from its compound. In electroplating, this can be problematic.

For example, when using \(\mathrm{AgNO}_{3}\) alone, copper might displace silver from its compound due to copper's higher reactivity, leading to an uneven or incomplete coating. This also results in silver precipitating out of the solution, which is inefficient.

Using \([\mathrm{Ag}(\mathrm{CN})_{2}]^{-}\) complex ions prevents this problem by:

• Reducing Reactivity: The strong bonds within the complex reduce the likelihood of copper easily displacing silver ions.
• Maintaining Chemistry: Ensuring consistent copper exposure without loss of silver plating material.
• Efficient Process: Providing a controlled and efficient plating process, minimizing waste.

Thus, avoiding displacement reactions ensures a more consistent and uniform silver plating result.