Question

Pure cold conc \(HNO_3\) makes iron passive, as the surface is covered with protective layer of

• A. Fe\(_2\)O\(_3\)
• B. FeO
• C. Fe\(_3\)O\(_4\)
• D. Fe(NO\(_3\))\(_3\)

Hint:

Passivation often forms mixed oxide layer like \(Fe_3O_4\), not just \(Fe_2O_3\).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Passivity refers to the state where a chemically active metal becomes inert under certain conditions. This is usually due to the formation of a thin, non-porous protective oxide layer on the metal surface.
Step 2: Detailed Explanation:
Concentrated Nitric Acid (\( \text{HNO}_{3} \)) is a strong oxidizing agent.
When Iron (\( \text{Fe} \)) is dipped into cold concentrated \( \text{HNO}_{3} \), the acid immediately oxidizes the surface of the iron.
This leads to the formation of a very thin, invisible, and insoluble layer of magnetic iron oxide (\( \text{Fe}_{3}\text{O}_{4} \)) on the surface.
This oxide layer acts as a barrier, preventing further reaction between the underlying iron and the acid. As a result, the iron becomes "passive" and ceases to dissolve.
Step 3: Final Answer:
The protective layer consists of \( \text{Fe}_{3}\text{O}_{4} \).