Question

Iron becomes passive in conc. HNO3 why?
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Answer 1

Answer:

nitric acid is a strong oxidizing agent. It oxidizes iron to form a layer of Fe2O3. This layer is non-reactive and cannot be oxidized further. Therefore, iron becomes passive to further reaction.

Explanation:

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Answer 2

Explanation:

We know concentrated HNO3 is strong oxidizing acids, so concentrated HNO3 can easily loot Fe’s electron, which leads to a large increasing in valence of Fe, and a decreasing in the valence of N.

(Most of the resultant is Fe3O4, which is very stable in this situation.)

When the iron dipped in concentrated HNO3 (cold or normal temperature), a layer of inert thin oxide film(Fe3O4, and some other oxides) is easily formed on the iron surface and prevents HNO3 from contacting with iron surface.

if you throw the iron into hot concentrated HNO3 the result will be different.

(Fe+6HNO3= Fe(NO3)3+3H2O+3NO2↑

Fe+4HNO3=Fe(NO3)2+2H2O+2NO2↑)

Because the layer of inert thin oxide film is dissolved by the hot HNO3.

HOWEVER, iron won’t becomes passive when dipping in dilute HNO3.

The concentrated HNO3 mainly exists in the form of molecular, which shows oxidizing .

But dilute HNO3 exists in the form of hydrogen ion and nitrate ion, which shows acidity, so it does not passivate.

(Here are the possible equations of iron dipping in dilute HNO3:

Fe+4HNO3=Fe(NO3)3+NO↑+2H2O

3Fe+8HNO3=3Fe(NO3)2+2NO↑+4H2O)

The oxidability of dilute HNO3 is much weaker than concentrated HNO3.