Question

Why is iron rendered passive in Nitric acid?

Answer 1

The reaction of Nitric acid with Iron:

• On reaction with concentrated nitric acid $\left({\mathrm{HNO}}_3\right)$ Iron transforms into a protective layer of ferric oxide $\left({\mathrm{Fe}}_2{\mathrm{O}}_3\right)$.
• This happens because Nitric acid is a very good oxidizing agent.
• When the layer of oxide forms on the surface the reaction then further stops as the acid lacks contact with metal.
• A reactive metal is now behaving passive non-reactive.

Formation of rust:

• Due to the formation of a reddish-brown layer (Rust ${\mathrm{Fe}}_2{\mathrm{O}}_3.{\mathrm{xH}}_2\mathrm{O}$)of iron oxide which prevents its further reaction with the acid, iron is made passive with fuming nitric acid.
• $\mathrm{Fe}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)+\mathrm{x}.{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to \underset{(\mathrm{Reddish}\mathrm{brown}\mathrm{rust})}{\underbrace{{\mathrm{Fe}}_2{\mathrm{O}}_3.{\mathrm{xH}}_2\mathrm{O}\left(\mathrm{s}\right)}}$
• On the surface of iron, ${\mathrm{HNO}}_3$ forms a protective coating of oxides.