Nitric acid- HNO3

Nitric acid

Nitric acid is the most important oxoacid formed by nitrogen. It is a colourless liquid. In the gaseous state, HNO₃ exists as a planar molecule with the structure as shown below:

Preparation Nitric acid:

Nitric acid is manufactured by the catalytic oxidation of ammonia in Ostwald process.

Laboratorically, nitric acid can be prepared by the heating NaNO₃ or KNO₃ with conc. H₂SO₄.

Properties of nitric acid:

Concentrated HNO₃ is a strong oxidising agent and attacks most metals except noble metals. Cr, Fe and AI do not dissolve in conc. HNO₃ due to the formation of a passive film of oxide on the surface.

The oxidising action of HNO₃ is depends on its concentration and the nature of the reducing agent. The principal product of reduction of HNO₃ is NO when it is dilute but NO₂ when it is concentrated.

For example:

Brown ring test for the detection of nitrates:

Addition of dilute ferrous sulphate solution (Fe²⁺) to an aqueous solution containing nitrate ion, and then carefully adding concentrated sulphuric acid along the sides of the test tube results in the formation of a brown coloured complex [Fe(H₂O)₅ (NO)]SO₄.

Uses of nitric acid:

• HNO₃ is used in the manufacture of fertilisers.
• It is used in the formation of explosives, dynamites, TNT, etc.
• It is also used in the etching of metals.