Question

Give a reason for the following: Iron is rendered passive with fuming nitric acid.

Answer 1

• Concentrated nitric acid $\left({\mathrm{HNO}}_3\right)$ is a strong oxidizing agent and being a strong oxidizing agent, it oxidizes the iron metal into its respective oxide ${\mathrm{Fe}}_2{\mathrm{O}}_3$$\mathrm{Fe}\left(\mathrm{s}\right)\stackrel{{\mathrm{HNO}}_3}{\to }{\mathrm{Fe}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)(\mathrm{protective}\mathrm{layer})$
• An insoluble reddish-brown layer of iron oxide over the Iron metal stop the further reaction with bulk.

So, the prevent iron from corrosion, it is rendered passive fuming with nitric acid $\left({\mathrm{HNO}}_3\right)$.