Question

Rusting of iron occurs due to formation of

• A. \( \text{Fe}_2\text{O}_3 \cdot n\text{H}_2\text{O} \)
• B. FeO
• C. \( \text{Fe}_3\text{O}_4 \)
• D. \( \text{Fe}(\text{OH})_4 \)

Hint:

Unlike copper or aluminum oxide scales, iron rust is highly porous and flaky, offering no protection to the underlying steel substrate.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks for the chemical formula of the primary compound formed during the rusting of iron under atmospheric conditions.

Step 2: Key Formula or Approach:
Rusting is an electrochemical process where iron reacts with oxygen and water:
Anode: \( \text{Fe} \rightarrow \text{Fe}^{2+} + 2e^- \)
Cathode: \( \text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^- \)
The resulting iron(II) hydroxide is further oxidized by dissolved oxygen to produce hydrated iron(III) oxide:
\[ 4\text{Fe(OH)}_2 + \text{O}_2 \rightarrow 2(\text{Fe}_2\text{O}_3 \cdot n\text{H}_2\text{O}) + 2\text{H}_2\text{O} \]

Step 3: Detailed Explanation:

• Hydrated Oxide Formation: Rust is chemically described as hydrated iron(III) oxide, formulated as \( \text{Fe}_2\text{O}_3 \cdot n\text{H}_2\text{O} \), where \( n \) represents varying amounts of water of hydration.
It is a reddish-brown, porous compound that does not form a protective barrier, allowing corrosion to continue into the underlying metal.

• Analysis of Other Options:
-

Wüstite (FeO, Option B) and

Magnetite (\( \text{Fe}_3\text{O}_4 \), Option C) are dry, high-temperature oxide scales that form in the absence of liquid water.
- \( \text{Fe}(\text{OH})_4 \) (Option D) is not a stable rust constituent.


Step 4: Final Answer:
Hence, rusting of iron occurs due to the formation of hydrated iron(III) oxide (\( \text{Fe}_2\text{O}_3 \cdot n\text{H}_2\text{O} \)), corresponding to Option (A).