Problem 25 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

In the cyanide extraction process of silver from argentite ore, the oxidising and reducing agents used are (a) \(\mathrm{O}_{2}\) and \(\mathrm{CO}\) respectively [2012] (b) \(\mathrm{O}_{2}\) and \(\mathrm{Zn}\) dust respectively (c) \(\mathrm{HNO}_{3}\) and \(\mathrm{Zn}\) dust respectively (d) \(\mathrm{HNO}_{3}\) and \(\mathrm{CO}\) respectively

Step-by-Step Solution

Answer

(b) \(\mathrm{O}_{2}\) and \(\mathrm{Zn}\) dust respectively.

Step 1: Understanding the Problem

The exercise asks us to identify the oxidizing and reducing agents in the cyanide extraction process of silver from argentite ore. Argentite is primarily silver sulfide (\(\mathrm{Ag}_2\mathrm{S}\)). The process involves leaching of \(\mathrm{Ag}_2\mathrm{S}\) with a dilute solution of NaCN in the presence of oxygen and then recovering silver using a reducing agent.

Step 2: Outline of the Process

In the cyanide process, silver sulfide is treated with sodium cyanide and in the presence of oxygen, it forms a soluble silver-cyanide complex. The reaction involves the oxidation of sulfide. The addition of a reducing agent, like zinc dust, precipitates the silver from the complex.

Step 3: Identifying Oxidizing Agent

In the leaching reaction, oxygen is involved as the oxidizing agent. Oxygen converts silver sulfide into silver-cyanide complex by oxidizing sulfide (\(\mathrm{S}^{2-}\) to \(\mathrm{SO}_{4}^{2-}\)).

Step 4: Identifying Reducing Agent

Once the silver is in solution as a complex compound, zinc dust is added to precipitate the silver. Zinc acts as the reducing agent by donating electrons to silver ions, reducing \(\mathrm{Ag}^{+}\) to \(\mathrm{Ag}\).

Step 5: Final Answer

The oxidizing and reducing agents in the cyanide process are \(\mathrm{O}_{2}\) (oxidizing agent) and \(\mathrm{Zn}\) dust (reducing agent), respectively.

Key Concepts

Oxidizing Agent

In the context of the cyanide extraction process of silver, the term "oxidizing agent" refers to a substance that helps facilitate the conversion of silver from its sulfide ore form into a soluble complex form. Let's break down how this works in the specific case of silver extraction.

During the cyanide process, silver is extracted from argentite, which is actually silver sulfide (\(\mathrm{Ag}_2\mathrm{S}\)). An oxidizing agent is needed to convert the sulfide ions into sulfate ions, thereby enabling the formation of a soluble complex with the silver. This complexation is achieved by leaching the ore with a dilute sodium cyanide solution, but only in the presence of oxygen.

Oxygen (\(\mathrm{O}_2\)) serves as this crucial oxidizing agent:

• It reacts directly with the silver sulfide.
• It facilitates the oxidation of sulfide ions (\(\mathrm{S}^{2-}\)) to sulfate ions (\(\mathrm{SO}_{4}^{2-}\)).

The role of \(\mathrm{O}_2\) is essential as it allows the silver to form a stable and soluble compound, paving the way for further processing and eventual recovery.

Reducing Agent

In chemical reactions, a reducing agent is responsible for donating electrons to another substance. In the cyanide extraction process of silver, zinc dust plays this vital role.

After successfully forming a soluble silver-cyanide complex, the silver is still dissolved within this complex. Here, we need to precipitate silver out of the solution, which is where a reducing agent comes into play. By using zinc dust, the silver ions in the solution receive electrons from the zinc, causing them to revert to solid metallic silver (\(\mathrm{Ag}\)).

Steps of Reduction with Zinc:

• Zinc (\(\mathrm{Zn}\)) releases electrons.
• The silver ions (\(\mathrm{Ag}^{+}\)) in the complex gain these electrons and are reduced to metallic silver.

This transformation, driven by the reducing power of zinc, allows the silver to precipitate out from the solution, thus making it possible to recover pure silver from the ore.

Silver Cyanide Complex

The formation of a silver-cyanide complex is a key step in the cyanide extraction process. Understanding this complex is crucial as it transforms silver into a state that can be efficiently processed.

When silver sulfide (\(\mathrm{Ag}_2\mathrm{S}\)) is treated with sodium cyanide in the presence of oxygen, it results in the formation of a soluble silver-cyanide complex. The chemical reaction involves the components:

• Silver ions (\(\mathrm{Ag}^{+}\)) combine with cyanide ions (\(\mathrm{CN}^{-}\)).
• This forms a stable coordination compound often written as \(\mathrm{[Ag(CN)_2]^{-}}\).

The creation of this complex is crucial as it allows the silver to be mobilized in solution, separating it from its sulfide ore. Without this complex formation, extracting metallic silver in an economically feasible manner from the ore would be very challenging. This silver-cyanide complex is then subjected to further processing where the metallic silver is recovered by reduction.