Question

What happens when iron is dipped in conc. ${\mathrm{HNO}}_3$?

Answer 1

Nitric acid

• Nitric acid $\left({\mathrm{HNO}}_3\right)$ is a mineral acid that is highly corrosive.
• When dilute Nitric acid reacts with a metal forms Metal nitrate $\left({\mathrm{MNO}}_3\right)$ and releases Hydrogen $\left({\mathrm{H}}_2\right)$ gas.
• The type of products generated in reactions involving nitric acid varies a lot depending on the concentration.
• Concentrated nitric acid acts as a strong oxidizing agent.

Iron dipped in concentrated nitric acid

• It forms a layer of oxide on a metal surface and hence stops any further reaction of metal.
• When iron $\left(\mathrm{Fe}\right)$ is dipped in concentrated nitric acid, a layer of iron oxide $\left({\mathrm{Fe}}_3{\mathrm{O}}_4\right)$ forms on its surface and it becomes chemically unreactive or passive.
• The reaction is as follows:

$3\mathrm{Fe}\left(\mathrm{s}\right)+8{\mathrm{HNO}}_3\left(\mathrm{aq}\right)\to {\mathrm{Fe}}_3{\mathrm{O}}_4\left(\mathrm{aq}\right)+4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+8{\mathrm{NO}}_2\left(\mathrm{g}\right)$