Question

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

Answer 1

Hint: Think about the reaction between iron and fuming nitric acid. The surface iron atoms and bulk iron atoms behave differently when iron metal rod is dipped into fuming nitric acid.

Complete Step by step answer: When we say that iron becomes passive, you say that it becomes inert. In other words, iron metal does not react further with the reagent. This is due to the formation of a thin layer of iron oxide on the surface of the ion.
When an iron metal rod is dipped into a beaker containing fuming nitric acid, the iron atoms present at the surface of the iron metal rod react with fuming nitric acid to form iron oxide. This iron oxide forms a coating on the surface of iron metal rod. Thus, the surface of iron metal rod is covered with a coating of iron oxide. This iron oxide coating is insoluble in nitric acid.
Now no iron atom is present at the surface of iron metal rod. Iron oxide does not react with fuming nitric acid. The iron atoms present in the bulk of iron metal rod, having a coating of iron oxide, do not come in contact with fuming nitric acid, and they do not react with fuming nitric acid. Thus, you can say that the iron is made passive or inert by treatment with fuming nitric acid.
This happens with several other metals such as aluminium, cobalt and nickel.

Note: Fuming nitric oxide is a strong oxidizing agent. It oxidizes iron to iron oxide. Addition of oxygen is oxidation. When iron gains oxygen to form iron oxide, it is called oxidation. In the process, the fuming nitric acid is itself reduced.