Nitric Acid (HNO3)

Nitrogen forms oxoacids such as,

·        H2N2O2 (hyponitrous acid),

·        HNO2 (nitrous acid)

·        HNO3 (nitric acid)

Acids containing oxygen and/or –OH group are called oxoacids.

Oxy Acids

Name of oxy – acid

H2N2O2

Hyponitrous acid

 H2 NO2

Hydro-nitrous acid

HNO2

Nitrous acid

HNO3

Nitric acid

HNO4

Per nitric acid

Preparations of HNO3

A screenshot of a cell phone Description generated with very high confidence

      i.            Lab method (heating of KNO3 or NaNO3 with conc H2SO4)

NaNO3 + H2SO4 (conc) → NaHSO4 + HNO3

   ii.            Ostwald’s process

2NO (g) + O2 (g) ⇔ 2NO2 (g)

3NO2 (g) + H2O (l) → 2HNO3 (g) + NO (g)

NO formed here is recycled.

The aqueous HNO3 can be concentrated by distillation upto ~ 68% by mass. Further concentration to 98% can be achieved by dehydration with concentrated H2SO4.

Physical properties of HNO3

It is a syrupy, colourless, pungent liquid usually available as 68%. 15.7 M aqueous solution is often yellow due to small concentrations of NO2. At concentration the specific gravity is 1.504.

Structure of HNO3

In the gaseous state, HNO3 exists as a planar molecule with the structure as shown.

Vidyarthi Academy

Vidyarthi Academy

Chemical Reactions of HNO3

Acidic character of HNO3

In aqueous solution, nitric acid behaves as a strong acid giving hydronium and nitrate ions.

HNO3 (aq) + H2O (l) → H3O+ (aq) + NO3-  (aq)

Reactions of HNO3 with metals

Concentrated nitric acid is a strong oxidising agent and attacks most metals except noble metals such as gold and platinum. The products of oxidation depend upon the concentration of the acid, temperature and the nature of the material undergoing oxidation.

3Cu + 8HNO3 (dilute) → 3Cu(NO3)2 + 2NO + 4H2O

Cu + 4HNO3 (conc.) → Cu(NO3)2 + 2NO2 + 2H2O

4Zn + 10HNO3 (dilute) → 4Zn(NO3)2 + 5H2O + N2O

Zn + 4HNO3 (conc.) → Zn(NO3)2 + 2H2O + 2NO2

Some metals (e.g., Cr, Al) do not dissolve in concentrated nitric acid because of the formation of a passive film of oxide on the surface.

Reaction of HNO3 with non-metals

Concentrated nitric acid also oxidises non–metals and their compounds. Iodine is oxidised to iodic acid, carbon to carbon dioxide, sulphur to H2SO4, and phosphorus to phosphoric acid.

I2 + 10HNO3 (conc.) → 2HIO3 + 10NO2 + 4H2O

C + 4HNO3 (conc.) → CO2 + 2H2O + 4NO2

S8 + 48HNO3 (conc.) → 8H2SO4 + 48NO2 + 16H2O

P4 + 20HNO3 (conc.) → 4H3PO4 + 20 NO2 + 4H2O

Brown ring test of nitrate

Fe2+ reduces nitrates to nitric oxide, which reacts with Fe2+ to form a brown coloured complex. Dilute ferrous sulphate solution is added to an aqueous solution containing nitrate ion, and then adding concentrated sulphuric acid along the sides of the test tube. A brown ring at the interface between the solution and sulphuric acid layers indicates the presence of nitrate ion in solution.

FeSO4 + 6H2O → [Fe(H2O)6]SO4

2HNO3 + 3H2SO4 + 6FeSO4 → 3Fe2(SO4)3 + 2NO + 4H2O

[Fe(H2O)6]SO4+ NO → [Fe(H2O)5(NO)]SO4 + H2O

or

NO3- + 3Fe2+ + 4H+ → NO + 3Fe3+ + 2H2O

Uses of nitric acid

1.     Used in the manufacture of ammonium nitrate for fertilisers and other nitrates for use in explosives and pyrotechnics.

2.     Used for the preparation of nitroglycerin, trinitrotoluene and other organic nitro compounds.

3.     Used in the pickling of stainless steel, etching of metals and as an oxidiser in rocket fuels.

4.     Used as nitrating reagent.