Chemistry
(a) Nitric acid cannot be concentrated beyond 68% by the distillation of a dilute solution of HNO3. State the reason.
(b) What is passive iron? How is passivity removed?
Nitric Acid
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Answer
(a) An aqueous solution of nitric acid (68% concentration) forms a constant boiling mixture at 121°C. A constant boiling mixture is one which boils without change in composition.
Hence on boiling further, the mixture evolves out the vapours of both acid and water in the same proportion as in the liquid.
Therefore, dilute HNO3 cannot be concentrated beyond 68% by boiling.
(b) Iron becomes passive or inert when treated with pure concentrated nitric acid due to the formation of the extremely thin layer of insoluble iron oxide (passivity) which stops the reaction.
Fe + 6HNO3 ⟶ Fe(NO3)3 + 3H2O + 3NO2
This inert iron is known as passive iron.
Passivity can be removed by rubbing the surface layer with sandpaper or by treating with strong reducing agent.
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Related Questions
Conc. nitric acid prepared in the laboratory is yellow in colour. Why? How is this colour removed?
Give reasons for the following:
In the laboratory preparation of nitric acid, the mixture of concentrated sulphuric acid and sodium nitrate should not be heated very strongly above 200°C.
Name the products formed when:
(a) carbon and conc. nitric acid is heated,
(b) dilute HNO3 is added to copper.
Give two chemical equations for each of the following:
(a) Reactions of nitric acid with non-metals.
(b) Nitric acid showing as acidic character.
(c) Nitric acid acting as oxidising agent.