Question

Pure conc. HNO\(_3\) make 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 prevents further reaction of metal with acid.

Answer 1

The Correct Option is C

To understand why pure concentrated HNO\(_3\) makes iron passive, we need to delve into the concept of passivation and the chemistry behind the reaction of iron with nitric acid.

Passivation is a process where a material becomes less affected or corroded by the environment due to the formation of a protective coating. In the case of iron, when it reacts with concentrated nitric acid, it forms a protective layer on its surface. This protective layer prevents further reaction and hence, the term "passive" is used.

When iron (Fe) is exposed to concentrated nitric acid (HNO\(_3\)), it forms a layer of iron oxide. The specific compound formed is magnetite, which has the chemical formula Fe\(_3\)O\(_4\). This oxide layer is very adherent and non-porous, thus effectively stopping further corrosion of the underlying metal by acting as a barrier.

Let's consider why the other options are incorrect:

• \(Fe_2O_3\): This is hematite, another form of iron oxide, but it is not the compound responsible for passivating iron in nitric acid.
• \(FeO\): This form of iron oxide is not stable in the environment provided by concentrated nitric acid and does not form the protective layer.
• \(Fe(NO_3)_3\): This is iron(III) nitrate, which is a salt and is soluble in water. It does not form a protective, passive layer on iron.

Thus, the correct answer is: Fe\(_3\)O\(_4\).

This highlights that the distinctive reaction between iron and concentrated nitric acid leads to the formation of a passive film of Fe\(_3\)O\(_4\), protecting iron from further chemical reaction.

The reaction can be simplified and illustrated as:

It is important to note that the passivation by nitric acid provides an excellent induced passivity that is useful in various applications.