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Chemistry of Iron (Fe) - Practice Questions

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Multiple Choice Questions (MCQs)

Question 1

Which of the following statements about iron ions is INCORRECT? (A) Fe3+ (aq) has a d5 electronic configuration. (B) Fe2+ (aq) acts as a reducing agent. (C) Fe3+ (aq) is generally more stable than Fe2+ (aq) in acidic aqueous solution due to its half-filled d-orbital configuration. (D) Fe3+ (aq) is easily reduced to Fe2+ (aq) in the presence of strong oxidizing agents.

Solution: The correct answer is (D).

Explanation:

  • (A) The electronic configuration of Fe is [Ar] 3d6 4s2. Fe3+ loses three electrons (two from 4s and one from 3d), resulting in [Ar] 3d5. This is correct.
  • (B) Fe2+ (d6) readily loses an electron to form the more stable Fe3+ (d5) configuration. Therefore, Fe2+ acts as a reducing agent. This is correct.
  • (C) The d5 configuration of Fe3+ represents a half-filled d-orbital, which imparts extra stability. Thus, Fe3+ is generally more stable than Fe2+ in acidic aqueous solutions. This is correct.
  • (D) Fe3+ itself is an oxidizing agent (it gets reduced). Therefore, it would be reduced to Fe2+ in the presence of reducing agents, not oxidizing agents. Strong oxidizing agents would tend to keep iron in its higher oxidation state (Fe3+) or even oxidize Fe2+ to Fe3+. This statement is incorrect.

Question 2

In the extraction of iron, which of the following reactions primarily occurs in the ‘combustion zone’ (Bosh) of the blast furnace? (A) CaCO3 → CaO + CO2 (B) Fe2O3 + 3CO → 2Fe + 3CO2 (C) C + O2 → CO2 (D) CaO + SiO2 → CaSiO3

Solution: The correct answer is (C).

Explanation: The blast furnace operates with distinct temperature zones:

  • (A) CaCO3 → CaO + CO2: This is the calcination of limestone, which occurs in the upper, cooler zones (around 800-900 °C).
  • (B) Fe2O3 + 3CO → 2Fe + 3CO2: This is the main reduction reaction of iron oxide by carbon monoxide, occurring in the middle and lower zones (400-700 °C, and up to 1200 °C).
  • (C) C + O2 → CO2: This reaction, the combustion of coke with hot air, is highly exothermic and provides the high temperatures required for the furnace operation. It occurs in the ‘bosh’ or combustion zone (1500-1900 °C), directly above the tuyeres where hot air is blown in. The CO2 produced further reacts with hot coke to form CO, which acts as the primary reducing agent.
  • (D) CaO + SiO2 → CaSiO3: This is the slag formation reaction, where acidic impurities (like SiO2) react with the basic flux (CaO) to form molten slag. This occurs in the slag formation zone (around 1000-1200 °C).

Question 3

When a solution of ferric chloride (FeCl3) is treated with potassium ferrocyanide [K4Fe(CN)6], a deep blue precipitate is formed. This precipitate is known as: (A) Prussian Blue (B) Turnbull’s Blue (C) Mohr’s Salt (D) Blue Vitriol

Solution: The correct answer is (A).

Explanation:

  • (A) The reaction between ferric salts (Fe3+) and potassium ferrocyanide (K4[Fe(CN)6]) produces a deep blue precipitate known as Prussian Blue, with the general formula KFe[Fe(CN)6] or Fe4[Fe(CN)6]3. 4FeCl3 + 3K4[Fe(CN)6] → Fe4[Fe(CN)6]3(s) + 12KCl This reaction is used as a test for Fe3+ ions.
  • (B) Turnbull’s Blue is formed by the reaction of ferrous salts (Fe2+) with potassium ferricyanide (K3[Fe(CN)6]). Its exact structure is complex but is often considered to be similar to Prussian Blue, sometimes even identical (Fe3[Fe(CN)6]2).
  • (C) Mohr’s Salt is ammonium ferrous sulfate hexahydrate, (NH4)2Fe(SO4)2·6H2O, a stable double salt of ferrous iron.
  • (D) Blue Vitriol is the common name for copper(II) sulfate pentahydrate, CuSO4·5H2O.

Assertion-Reason Questions

Directions: In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R). Mark the correct choice as: (A) If both A and R are true and R is the correct explanation of A. (B) If both A and R are true but R is not the correct explanation of A. (C) If A is true but R is false. (D) If A is false but R is true. (E) If both A and R are false.

Question 1

Assertion (A): Ferrous salts are readily oxidised to ferric salts on exposure to air. Reason (R): Ferric ions (Fe3+) are more stable than ferrous ions (Fe2+) due to the half-filled d5 electronic configuration.

Solution: The correct answer is (A).

Explanation:

  • Assertion (A) is true. Ferrous salts (containing Fe2+) are strong reducing agents and are easily oxidized by atmospheric oxygen to ferric salts (containing Fe3+), especially in the presence of moisture or in solution. For example, a clear solution of FeSO4 gradually turns yellowish-brown upon standing in air due to the formation of Fe2(SO4)3 or Fe(OH)SO4.
  • Reason (R) is true. The electronic configuration of Fe2+ is [Ar] 3d6, and that of Fe3+ is [Ar] 3d5. The d5 configuration is a half-filled d-subshell, which possesses extra stability due to symmetry and exchange energy. This inherent stability drives the oxidation of Fe2+ to Fe3+.
  • R is the correct explanation of A. The greater stability of Fe3+ (d5) is the fundamental reason why Fe2+ (d6) tends to lose an electron and get oxidized to Fe3+.

Question 2

Assertion (A): Iron becomes passive when treated with concentrated nitric acid. Reason (R): Concentrated nitric acid forms a protective, non-porous layer of iron oxide on the surface of iron.

Solution: The correct answer is (A).

Explanation:

  • Assertion (A) is true. When iron is treated with concentrated nitric acid, it becomes passive, meaning it loses its characteristic chemical reactivity (e.g., it no longer reacts with dilute acids or displacing copper from copper sulfate solution).
  • Reason (R) is true. Concentrated nitric acid is a strong oxidizing agent. It oxidizes the surface of iron to form a thin, adherent, non-porous, and impervious layer of iron oxide (such as Fe3O4 or Fe2O3). This oxide layer acts as a protective barrier, preventing further reaction of the underlying metal with the acid or other reagents.
  • R is the correct explanation of A. The formation of this protective oxide layer is precisely why iron exhibits passivation towards concentrated nitric acid.

Short Answer Questions

Question 1

Give the chemical formula and name of a double salt of iron. Why is it preferred over ferrous sulfate for volumetric analysis?

Model Answer:

  • Chemical Formula: (NH4)2Fe(SO4)2·6H2O
  • Name: Ammonium ferrous sulfate hexahydrate, commonly known as Mohr’s Salt.

Reason for Preference: Mohr’s salt is preferred over simple ferrous sulfate (FeSO4·7H2O) for volumetric analysis (e.g., in redox titrations with KMnO4) primarily because of its stability against oxidation.

  1. Resistance to Oxidation: Ferrous sulfate is easily oxidized by atmospheric oxygen to ferric sulfate (Fe2(SO4)3) upon exposure to air, especially in aqueous solution. This changes its concentration over time, making it unsuitable for accurate volumetric measurements.
  2. Stability of Mohr’s Salt: Mohr’s salt is a stable double salt. The presence of the ammonium ion (NH4+) in Mohr’s salt helps to stabilize the Fe2+ ion. In aqueous solution, the ammonium ion causes a slight acidification due to hydrolysis, creating an acidic environment locally. This acidic condition significantly retards the oxidation of Fe2+ to Fe3+ by atmospheric oxygen, thereby maintaining its concentration for a longer period.

Question 2

Describe the main chemical reactions occurring in the different temperature zones of a blast furnace during the extraction of iron from its oxide ore.

Model Answer: The blast furnace is a counter-current reactor where specific chemical reactions occur in different temperature zones:

  1. Combustion Zone (Bosh, 1500-1900 °C): Located at the bottom, this is the hottest zone. Hot air is blown in through tuyeres, reacting with coke (carbon) to generate heat and carbon dioxide.

    • C(s) + O2(g) → CO2(g) (Highly exothermic, provides primary heat)
    • The CO2 produced then reacts with incandescent coke to form carbon monoxide, the main reducing agent: CO2(g) + C(s) → 2CO(g) (Endothermic)

  2. Slag Formation Zone (Middle, 1000-1200 °C): In this zone, limestone (flux) decomposes, and the resulting calcium oxide reacts with acidic impurities like silica (sand) present in the ore.

    • CaCO3(s) → CaO(s) + CO2(g) (Calcination of flux)
    • CaO(s) + SiO2(s) → CaSiO3(l) (Formation of slag, which is lighter than molten iron and floats on top)

  3. Reduction Zone (Top to Middle, 400-900 °C): In the upper cooler regions, iron oxides are progressively reduced by carbon monoxide.

    • Upper zone (400-600 °C): Indirect reduction of higher oxides. 3Fe2O3(s) + CO(g) → 2Fe3O4(s) + CO2(g)
    • Middle zone (600-700 °C): Further reduction. Fe3O4(s) + CO(g) → 3FeO(s) + CO2(g) Fe3O4(s) + 4CO(g) → 3Fe(s) + 4CO2(g) (at higher temperatures)
    • Lower zone (800-900 °C): Ferrous oxide (FeO) is reduced to iron. FeO(s) + CO(g) → Fe(s) + CO2(g)
    • Direct Reduction (at higher temperatures, >900 °C): Some direct reduction by carbon also occurs. FeO(s) + C(s) → Fe(s) + CO(g)

Molten iron, known as pig iron, collects at the bottom of the furnace, while the molten slag floats above it and is tapped off separately.

High-Order Thinking Skills (HOTS) Question

Question 1

Explain why Fe2+ is a strong reducing agent in aqueous solution, while Fe3+ acts as an oxidizing agent. Discuss how the stability of these ions changes with pH, specifically comparing Fe2+ stability in acidic medium versus basic medium.

Detailed Chemical Explanation:

  1. Redox Behavior of Fe2+ and Fe3+:

    • Electronic Configuration: Iron (Fe) has an electronic configuration of [Ar] 3d6 4s2.
      • Fe2+ (ferrous ion): [Ar] 3d6. It has four unpaired electrons.
      • Fe3+ (ferric ion): [Ar] 3d5. This is a half-filled d-subshell, which confers extra stability due to higher exchange energy and symmetrical distribution.
    • Fe2+ as a Reducing Agent: Because of the enhanced stability of the d5 configuration, Fe2+ readily loses one electron to form Fe3+. Fe2+(aq) → Fe3+(aq) + e- The standard electrode potential (E°) for the Fe3+/Fe2+ couple is +0.77 V. However, when written as oxidation, Fe2+ → Fe3+ + e-, the potential is -0.77 V. A negative oxidation potential indicates a strong tendency for oxidation. Thus, Fe2+ acts as a strong reducing agent, meaning it gets oxidized while reducing other species (e.g., reducing KMnO4 to Mn2+ or K2Cr2O7 to Cr3+).
    • Fe3+ as an Oxidizing Agent: Conversely, Fe3+ (d5) can gain an electron to achieve the d6 configuration of Fe2+. Fe3+(aq) + e- → Fe2+(aq) The standard reduction potential of +0.77 V indicates that Fe3+ has a significant tendency to get reduced. Thus, Fe3+ acts as an oxidizing agent, meaning it gets reduced while oxidizing other species (e.g., oxidizing I- to I2).

  2. Stability of Fe(II) and Fe(III) with pH: The stability of the oxidation states of iron is significantly influenced by the pH of the medium, primarily due to the formation of insoluble hydroxides.

    • Fe2+ in Acidic Medium: In acidic solutions, Fe2+ ions exist predominantly as hydrated ions, [Fe(H2O)6]2+. While they are reducing agents, their oxidation to Fe3+ is relatively slow in the absence of strong oxidizing agents or catalysts. The high concentration of H+ ions suppresses the formation of Fe(OH)2, which would otherwise readily oxidize. The standard potential for Fe3+/Fe2+ couple (+0.77 V) refers to acidic conditions.

    • Fe2+ in Basic Medium: In basic (alkaline) solutions, Fe2+ ions rapidly precipitate as ferrous hydroxide, Fe(OH)2(s), a white precipitate. Fe2+(aq) + 2OH-(aq) → Fe(OH)2(s) Fe(OH)2 is much less stable to oxidation than the Fe2+(aq) ion. It is extremely susceptible to oxidation by even atmospheric oxygen. The standard electrode potential for the Fe(OH)3/Fe(OH)2 couple is approximately -0.56 V. This significantly lower potential (more negative) means that Fe(OH)2 is a much stronger reducing agent than Fe2+(aq) and is very easily oxidized to ferric hydroxide, Fe(OH)3, a reddish-brown precipitate (rust). 4Fe(OH)2(s) + O2(g) + 2H2O(l) → 4Fe(OH)3(s) The precipitation of Fe(OH)2 effectively removes Fe2+ ions from the solution, shifting the equilibrium towards further oxidation of any remaining Fe2+. The removal of Fe2+ as a precipitate and the high concentration of OH- ions facilitate the oxidation, making Fe(II) compounds highly unstable in basic media.

Conclusion: While Fe2+ is a reducing agent and Fe3+ is an oxidizing agent in general, their relative stabilities and reactivity are profoundly affected by pH. Fe3+ (d5) is inherently more stable than Fe2+ (d6). This stability difference is exploited in acidic conditions. However, in basic conditions, the formation of insoluble hydroxides drastically enhances the reducing power of Fe(II) species, making Fe(OH)2 far more prone to oxidation compared to Fe2+ in acidic solutions.

Fe

Iron (Fe)

Atomic Number 26

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