Problem 21 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which series of reactions correctly represents chemical reactions related to iron and its compound? (a) \(\mathrm{Fe} \stackrel{\mathrm{dil} \cdot \mathrm{H}_{2} \mathrm{SO}_{4}}{\longrightarrow} \mathrm{FeSO}_{4} \stackrel{\mathrm{H}_{2} \mathrm{SO}_{4}, \mathrm{O}_{2}}{\longrightarrow}\) \(\mathrm{Fe}_{2}\left(\mathrm{SO}_{4}\right)_{3} \stackrel{\text { heat }}{\longrightarrow} \mathrm{Fe}\) (b) \(\mathrm{Fe} \stackrel{\mathrm{O}_{2}, \text { heat }}{\longrightarrow} \mathrm{FeO} \stackrel{\text { dil. } \mathrm{H}_{2} \mathrm{SO}_{4}}{\longrightarrow}\) \(\mathrm{FeSO}_{4} \stackrel{\text { heat }}{\longrightarrow} \mathrm{Fe}\) (c) \(\mathrm{Fe} \stackrel{\mathrm{Cl}_{2}, \text { heat }}{\longrightarrow} \mathrm{FeCl}_{3} \stackrel{\text { heat, air }}{\longrightarrow} \mathrm{FeCl}_{2} \stackrel{\mathrm{Zn}}{\longrightarrow} \mathrm{Fe}\) (d) \(\mathrm{Fe} \stackrel{\mathrm{O}_{2} \text {, heat }}{\longrightarrow} \mathrm{Fe}_{3} \mathrm{O}_{4} \stackrel{\mathrm{CO}, 600^{\circ} \mathrm{C}}{\longrightarrow}\) \(\mathrm{FeO} \stackrel{\mathrm{CO}, 700^{\circ} \mathrm{C}}{\longrightarrow} \mathrm{Fe}\)

Step-by-Step Solution

Answer

Series (d) is correct: it properly converts iron to iron oxide and back to iron.

Step 1: Understand the Chemical Reactions Involved

To determine the correct series of reactions, we need to understand each chemical process described. The main processes include reactions of iron with other substances to convert it into various compounds and back to iron.

Step 2: Analyze Reaction Series (a)

In series (a), iron reacts with dilute sulfuric acid to form ferrous sulfate (FeSO₄). Then, further oxidation with sulfuric acid and oxygen converts FeSO₄ to ferric sulfate (Fe₂(SO₄)₃). The final step involves heating, which is supposed to lead back to iron, but ferric sulfate decomposes into different compounds, not pure iron. Hence, this series is incorrect.

Step 3: Analyze Reaction Series (b)

In series (b), iron reacts with oxygen under heat to form FeO (iron(II) oxide). FeO reacts with dilute sulfuric acid to form FeSO₄. Heating FeSO₄ typically decomposes it into iron oxide and gases, rather than pure iron. Thus, reaction series (b) does not correctly lead back to iron.

Step 4: Analyze Reaction Series (c)

In series (c), iron reacts with chlorine to form FeCl₃. This should be an oxidation process rather, but proceeding to convert FeCl₃ to FeCl₂ with air and heat suggests a reduction process which is incorrect. Typically, heating and air do not reduce FeCl₃ to FeCl₂ straightforwardly, and finally using Zn does not convert FeCl₂ to iron directly. Hence, series (c) has inconsistencies.

Step 5: Analyze Reaction Series (d)

In series (d), iron reacts with oxygen and heat to form Fe₃O₄ (magnetite). Heating with carbon monoxide at 600°C reduces Fe₃O₄ to FeO, and further reduction with CO at 700°C reduces FeO to iron. This sequence is valid, as iron oxide can be reduced to metallic iron through controlled reduction with carbon monoxide.

Step 6: Conclusion

After analyzing all series, series (d) correctly represents the series of reactions related to iron and its compounds: oxidation to Fe₃O₄, followed by reduction sequences that bring it back to iron.

Key Concepts

Iron Compounds

Iron compounds are formed when iron reacts with various chemical elements. These compounds are essential in many industrial processes and everyday products.

• Ferrous Sulfate (\( \text{FeSO}_4 \)): Often formed when iron reacts with dilute sulfuric acid. It is commonly known as green vitriol and is used in water treatment and as a fertilizer.


• Ferric Sulfate (\( \text{Fe}_2(\text{SO}_4)_3 \)): Iron in a higher oxidation state typically formed through further oxidation of ferrous sulfate. It is used in dyeing and as a coagulant in water purification.


• Iron Oxides: Various oxides, such as \( \text{FeO} \) (iron(II) oxide), \( \text{Fe}_3\text{O}_4 \) (magnetite), and \( \text{Fe}_2\text{O}_3 \) (hematite), are formed when iron reacts with oxygen. These are used in pigments and as precursor materials in steelmaking.

Understanding the formation and decomposition of these compounds helps in comprehending larger processes like oxidation-reduction and metal extraction.

Oxidation-Reduction

Oxidation-reduction (redox) reactions are fundamental chemical processes involving the transfer of electrons between substances. These reactions are crucial in transforming and utilizing iron.In a redox reaction involving iron:

• Oxidation: Iron loses electrons and can be transformed from a lower to a higher oxidation state. For example, iron reacts with oxygen to form various iron oxides, such as iron(II) oxide (\( \text{FeO} \)) or iron(III) oxide (\( \text{Fe}_2\text{O}_3 \)).


• Reduction: This process involves gaining electrons, converting iron compounds back to metallic iron. In industry, reducing agents like carbon monoxide (CO) are used to extract pure iron from its oxides.

Iron's natural propensity to change oxidation states makes it versatile but also challenging to keep in its metallic form. This principle is key in many metallurgical processes, including metal extraction.

Metal Extraction

Metal extraction involves separating metals from their oxides and refining them into pure forms. In the context of iron, this process includes reducing iron oxidized states back to metallic iron.The prominent method for extracting iron involves:

• Reduction with Carbon Monoxide: High-temperature reactions in a blast furnace reduce iron oxides. For instance, magnetite \( \text{Fe}_3\text{O}_4 \) can be reduced to iron(II) oxide by CO, which is further reduced to iron by extra CO at higher temperatures.


• Use of a Blast Furnace: Industrially, large-scale extraction of iron occurs in blast furnaces, where raw materials like iron ore, coke, and limestone are combined to produce molten iron.

Understanding these extraction processes is fundamental to grasping how raw iron ore is transformed into usable metal. Proper manipulation of temperature and reactants ensures efficient recovery and refinement of iron, vital for producing steel and other products.