Problem 66 - A Complete Resource Book in Chemistry for JEE Main

Question

In the extraction of silver, \(\mathrm{Ag}_{2} \mathrm{~S}\) is dissolved in (a) \(\mathrm{HNO}_{3}\) (b) \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (c) \(\mathrm{HCl}\) (d) \(\mathrm{KCN}\)

Step-by-Step Solution

Answer

Silver sulfide ( \mathrm{Ag}_{2} ~S ) is typically dissolved in potassium cyanide ( KCN ), choice (d).

Step 1: Understanding the Silver Ore

Silver is commonly found in the ore \(\mathrm{Ag}_{2} \mathrm{~S}\), also known as argentite or silver sulfide. In order to extract pure silver, the silver must be separated from sulfide.

Step 2: Determining the Best Reagent for Dissolution

The dissolution of \(\mathrm{Ag}_{2} \mathrm{~S}\) in a solution requires a reagent that can effectively break down the sulfide bonds without introducing unnecessary contaminants.

Step 3: Considering Chemical Reactions

Among the given options, \(KCN\) (potassium cyanide) is often used in the cyanide leaching process of silver because it effectively dissolves silver from the ore by forming a complex ion. The chemical reaction involved is:\(\mathrm{Ag}_{2}\mathrm{S}\, + \, 4\mathrm{KCN}\, \rightarrow \, 2\mathrm{K[Ag(CN)_{2}]} \, +\, \mathrm{K_{2}S}\).

Key Concepts

Argentite (Ag2S)

Argentite, scientifically represented as \(\mathrm{Ag}_{2} \mathrm{~S}\), is a mineral composed largely of silver sulfide. This mineral is one of the most important sources for the extraction of silver. The silver content in argentite is quite high, making it a favorable option for mining. The silver-sulfur bond in this mineral necessitates specific chemical processes to separate the valuable silver from sulfide. Understanding argentite's composition helps in determining the right method for extracting silver, ensuring that the process is both efficient and effective.

Cyanide Leaching

Cyanide leaching is a common method used to extract silver from minerals such as argentite. This chemical process involves the dissolution of silver ore in a cyanide solution.

• The cyanide solution facilitates the breakage of silver-sulfide bonds.
• It allows silver to separate from the sulfur and dissolve in the solution.
• This method is effective due to its ability to form a complex ion with silver.

This technique is preferred in industrial settings due to its high efficiency and ability to process large amounts of ore at once. Despite its effectiveness, strict environmental controls are required because cyanide is a highly toxic substance.

Complex Ion Formation

In the cyanide leaching process, the reaction involves forming a complex ion with silver. The chemical equation for this reaction is:\[\mathrm{Ag}_{2}\mathrm{S}\, + \, 4\mathrm{KCN}\, \rightarrow \, 2\mathrm{K[Ag(CN)_{2}]} \, +\, \mathrm{K_{2}S}\].During this reaction, silver forms a stable complex ion \(\mathrm{[Ag(CN)_{2}]}^-\) which is soluble in the mixture.

• This transformation from silver sulfide to complex ion increases the efficacy of silver recovery.
• The complex ion formation prevents the silver from reverting to a solid form, ensuring better extraction yield.

This process is key, as without complex ion formation, the efficiency of extracting silver would be drastically reduced.

Silver Sulfide Dissolution

Silver sulfide dissolution is a crucial part of silver extraction. The aim is to effectively break down the silver-sulfur bonds within \(\mathrm{Ag}_{2} \mathrm{~S}\) to free the silver for further processing. By using suitable reagents like potassium cyanide, the dissolution breaks these bond chains. Once dissolved, the silver can be isolated and extracted from the solution.

• This step is integral as it initiates the separation process.
• Proper dissolution ensures minimal silver loss and maximizes the quality and quantity of silver recovered.

Ensuring complete dissolution is essential to the overall success of silver extraction processes, making it a fundamental step in the industry.