Problem 69 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

(a) Write the chemical equations involved in the extraction of lead from galena by self reduction process. (b) Match the following extraction processes with the appropriate metals listed below : (i) Silver (A) Fused salt electrolysis (ii) Calcium (B) Carbon reduction (iii) Zinc (C) Carbon monoxide reduction (iv) Iron (D) Amalgamation (v) Copper (E) Self reduction

Step-by-Step Solution

Answer

(i) - (D), (ii) - (A), (iii) - (B), (iv) - (C), (v) - (E). Lead extraction involves roasting and self-reduction.

Step 1: Understand the Self-Reduction Process for Lead

Lead is extracted from its ore, galena (PbS), by a process known as self-reduction or air oxidation. This process involves roasting galena in the presence of oxygen to form lead oxide (PbO) and sulfur dioxide (SO₂). The formed lead oxide is then reduced by the remaining unreacted galena to yield lead metal and more sulfur dioxide.

Step 2: Write the Equations for Lead Extraction

The first step is roasting: \[2 PbS + 3 O_2 \rightarrow 2 PbO + 2 SO_2\]The second step is self-reduction: \[2 PbO + PbS \rightarrow 3 Pb + SO_2\]This set of reactions shows how metallic lead is obtained from galena.

Step 3: Match Extraction Processes with Metals

Given the extraction methods and metals, match them correctly: - Silver is often extracted using Amalgamation: (i) - (D) - Calcium is extracted through Fused Salt Electrolysis: (ii) - (A) - Zinc is extracted by Carbon reduction: (iii) - (B) - Iron's extraction involves Carbon monoxide reduction: (iv) - (C) - Copper can be extracted by Self reduction: (v) - (E)

Step 4: Confirm the Matches

Check and confirm each match: (i) Silver - Amalgamation (D) (ii) Calcium - Fused Salt Electrolysis (A) (iii) Zinc - Carbon reduction (B) (iv) Iron - Carbon monoxide reduction (C) (v) Copper - Self reduction (E) The matching process tasks are correct and reflect standard methods used in metallurgy.

Key Concepts

Lead Extraction

Lead extraction is a fascinating process where metallic lead is obtained from its naturally occurring ore, known as galena (PbS). This process usually includes two main steps: roasting and self-reduction. Galena is first roasted in air, which means it is heated in the presence of oxygen. This results in the formation of lead oxide (PbO) and sulfur dioxide (SO_2). This reaction can be represented as:\[2 \text{PbS} + 3 \text{O}_2 \rightarrow 2 \text{PbO} + 2 \text{SO}_2\]After roasting, a self-reduction process takes place. Here, some of the lead oxide reacts with the remaining unreacted galena, leading to the production of metallic lead and more sulfur dioxide:\[2 \text{PbO} + \text{PbS} \rightarrow 3 \text{Pb} + \text{SO}_2\]These chemical reactions are crucial for converting galena into usable metallic lead. Understanding these steps is essential for anyone studying metallurgy or metal extraction methods.

Self-Reduction Process

The self-reduction process is a unique method in metallurgy primarily used for extracting metals like lead and copper without the need for additional reducing agents. In this process, the ore itself or a deliberately formed intermediary substance is utilized to perform the reduction.
For lead extraction, after roasting galena to form lead oxide, the remaining galena acts as the reducing agent. This means lead oxide is reduced to metallic lead without any external entities. This internal mechanism is known as self-reduction.
Similarly, in copper extraction, the self-reduction process involves cuprite or similar intermediaries that aid the reduction process. This method aids in minimizing costs and simplifies the metallurgy process as it relies heavily on the inherent properties of the ore and formed intermediaries.

Roasting in Metallurgy

Roasting is a crucial metallurgical process often carried out before the actual reduction phase. It involves heating the ore in the presence of excess oxygen, leading to its partial or complete oxidation. This is particularly important for sulfide ores like galena, which consists largely of metal sulfides.
During roasting, for galena, lead sulfide (PbS) is turned into lead oxide (PbO), emitting sulfur dioxide (SO_2) as a byproduct, as shown by:\[2 \text{PbS} + 3 \text{O}_2 \rightarrow 2 \text{PbO} + 2 \text{SO}_2\]Roasting serves several purposes: it removes volatile impurities, changes the physical or chemical state of the ore to make them easier to reduce, and sometimes aids in separating the metal from other unwanted minerals.
This process is essential for preparing the ore for subsequent phases like self-reduction, making the metal extraction process more efficient.

Metals and Their Extraction Methods

Different metals are extracted using various methods, depending on the properties of the ore and the metal itself. Metal extraction techniques can differ greatly:

• Amalgamation: Commonly used for gold and silver extraction, this process involves dissolving the metal into mercury to form an amalgam, which is then refined.
• Fused Salt Electrolysis: Metals like calcium and aluminum are often extracted using this method, which involves passing an electric current through a molten salt to obtain the metal.
• Carbon Reduction: A method suitable for extracting metals like zinc, where carbon acts as a reducing agent to extract the metal from its oxide.
• Carbon Monoxide Reduction: Used for metals like iron, this method relies on carbon monoxide to reduce metal oxides into metal.
• Self Reduction: Utilized in the extraction of metals like lead and copper, where the ore can reduce itself when heated, without external reducing agents.

Each method has its own significance based on economic factors, purity requirements, and the nature of the ore. Understanding these methods provides insights into how raw ores transform into valuable metals used in everyday products.