Problem 136 - A Complete Resource Book in Chemistry for JEE Main

Question

Two ores of the same metal \((\mathrm{M})\) are \(\left(\mathrm{A}_{1}\right)\) and \(\left(\mathrm{A}_{2}\right)\). (1) \(\mathrm{A}_{1} \stackrel{\text { calcination }}{\longrightarrow}\) Black residue \(\mathrm{C}+\mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O}\) (2) \(\mathrm{A}_{1} \stackrel{\mathrm{HCl}, \mathrm{Kl}}{\longrightarrow} \mathrm{I}_{2}+\mathrm{D}\) precipitate (3) \(\mathrm{A}_{2} \stackrel{\text { roasting }}{\longrightarrow} \mathrm{Gas}(\mathrm{G})+\operatorname{Metal}(\mathrm{M})\) (4) \(\mathrm{G} \stackrel{+\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \text { acidified }}{\longrightarrow}\) Green solution Identify \(\mathrm{A}_{1}\) and \(\mathrm{A}_{2}\) here? (a) \(\mathrm{A}_{1}=\mathrm{CuCO}_{3} \cdot \mathrm{Cu}(\mathrm{OH})_{2}, \mathrm{~A}_{2}=\mathrm{Cu}_{2} \mathrm{~S}\) (b) \(\mathrm{A}_{1}=\mathrm{Cu}_{2} \mathrm{~S}, \mathrm{~A}_{2}=\mathrm{CuCO}_{3} \cdot \mathrm{Cu}(\mathrm{OH})_{2}\) (c) \(\mathrm{A}_{1}=\mathrm{CuFeS}_{2}, \mathrm{~A}_{2}=\mathrm{Cu}_{2} \mathrm{~S}\) (d) none of these

Step-by-Step Solution

Answer

(a) \( A_1=\mathrm{CuCO}_{3} \cdot \mathrm{Cu(OH)}_{2}, \mathrm{~A}_2=\mathrm{Cu}_2 \mathrm{S} \).

Step 1: Understanding Calcination of A1

Calcination of \( A_1 \) produces black residue \( C \), \( CO_2 \), and \( H_2O \). This process suggests \( A_1 \) is likely a carbonate hydroxide compound of metal \( M \), such as copper carbonate hydroxide, \( \mathrm{CuCO}_{3} \cdot \mathrm{Cu(OH)}_{2} \), which breaks down to form black copper oxide.

Step 2: Reaction with HCl and Kl

Reaction (2) with \( HCl \) and \( Kl \) producing \( I_2 \) indicates \( A_1 \) contains a sulfate or thiosulfate component, supporting \( CuCO_{3} \cdot Cu(OH)_{2} \) since copper carbonate doesn't form precipitate \( D \) without forming other by-products.

Step 3: Roasting of A2

Roasting of \( A_2 \) releases gas \( G \) and leaves metal \( M \). This hints that \( A_2 \) might contain sulfide, as sulfide ores usually release sulfur dioxide when roasted. Hence, \( A_2 \) is likely \( \mathrm{Cu}_{2} \mathrm{S} \).

Step 4: Identifying Gas G Reaction

The gas \( G \) forms a green solution when reacted with acidified \( K_2Cr_2O_7 \), indicating \( G \) is probably sulfur dioxide (\( SO_2 \)), which turns the solution green due to chromium sulfate formation in an acidic environment. This supports \( A_2 \) as \( \mathrm{Cu}_{2} \mathrm{S} \).

Step 5: Conclusion from Steps

From all the observations, \( A_1 \) must be \( \mathrm{CuCO}_{3} \cdot \mathrm{Cu(OH)}_{2} \) since the reaction conditions match the formation of black copper oxide and fits with the role in reaction 2. \( A_2 \) is \( \mathrm{Cu}_{2} \mathrm{S} \) as roasting it produces \( SO_2 \). Thus, the correct choice is (a).

Key Concepts

Calcination

Calcination is a thermal treatment process applied to ores and other solid materials. During calcination, the material is heated to a high temperature in the absence of air or in a controlled atmosphere. The purpose of calcination is to bring about thermal decomposition, phase transition, or removal of a volatile fraction.
For example, in metallurgy, calcination is used to convert metal carbonates into metal oxides. When a compound like copper carbonate hydroxide (\(\mathrm{CuCO}_{3} \cdot \mathrm{Cu(OH)}_{2}\)) is calcined, it decomposes to form copper oxide, carbon dioxide, and water vapor. This process is indicated by the black residue formed, which is copper oxide.
Calcination plays a crucial role in preparing the ore for further processing like reduction, where the pure metal can be extracted.

Roasting

Roasting is another important thermal process used in metallurgy, often preceding the extraction of metal from its ore. Unlike calcination, roasting involves heating the ore in the presence of air or oxygen. This helps to oxidize the ore, which is usually a sulfide, into an oxide form, making the extraction of the metal easier.
In the context of the problem, roasting of copper sulfide (\(\mathrm{Cu}_{2}\mathrm{S}\)) releases sulfur dioxide gas and leaves behind the metal, copper, as residue. This illustrates the conversion and removal of sulfur elements in the form of gas.
Roasting not only aids in the purification of ore by removing undesirable elements like sulfur but also helps in altering the chemical structure of the ore to a form that is more amenable for reduction.

Copper Compounds

Copper compounds are an essential part of metallurgy, particularly in understanding the nuances of copper ore refining. Copper exists in various forms, including oxides, sulfides, and carbonates, each responding differently to calcination and roasting processes.
For copper carbonate hydroxide (\(\mathrm{CuCO}_{3} \cdot \mathrm{Cu(OH)}_{2}\)), calcination effectively converts it into copper oxide, a transformation evident by the black residue formed after heating. On the other hand, copper sulfide (\(\mathrm{Cu}_{2}\mathrm{S}\)) requires roasting for its conversion to copper oxide or metallic copper, accompanied by the emission of sulfur dioxide.
Understanding these compounds and their reactions under different thermal conditions is crucial for effective extraction and refining processes in metallurgy.

Chemical Reactions in Metallurgy

Chemical reactions play a pivotal role in metallurgy, transforming raw ores into refined metals. Calcination and roasting are key processes, each involving distinct chemical reactions depending on the ore's composition.
These reactions help in decomposing complex compounds into simpler oxides or elemental forms while releasing by-products like gases. For instance, calcination of copper carbonate hydroxide leads to the breakdown of carbonates, releasing carbon dioxide and water vapor while forming copper oxide.
Conversely, roasting involves reactions with oxygen, where sulfide ores are oxidized, emitting gases like sulfur dioxide while yielding the metal or its oxide form. Collectively, these chemical processes enhance the concentration and purity of the metal, facilitating subsequent extraction and utilization.