Question

(a) Concentrated nitric acid renders iron passive. Give reason.
(b) How is passivity removed?

Answer 1

(a) Concentrated nitric acid $\left({\mathrm{HNO}}_3\right)$ renders iron passive because:

• Conc nitric acid, as a strong oxidizer, converts the metal to ferric oxide, which forms an insoluble protective layer on it.
• The reaction comes to a halt when the metal loses contact with the acid.
• A chemically active metal becomes non-reactive and inactive.

(b) Passivity can be removed by:

• When iron reacts with nitric acid, an extremely thin coating of insoluble metallic oxide forms, effectively stopping the reaction.
• By wiping the surface layer with sand paper or treating with a strong reducing chemical, passivity can be eliminated.