ü  Nitric Acid (HNO3):

Ø  Preparation: In the laboratory, nitric acid is prepared by heating KNO3 or NaNO3 and concentrated H2SO4 in a glass retort.

NaNO3 + H2SO4 → NaHSO4 + HNO3

On a large scale it is prepared by Ostwald’s process. It involves following steps:

·       In this process NH3 is catalytically oxidized to NO over a Pt-Rh catalyst at 1200K.

4NH3 + 5O2 ¾® 4NO + 6H2O: DH = -904 kJ

·       About 96 to 98 % of NH3 is converted into NO. The mixture is then diluted with air. NO combines with O2 to give NO2

2NO + O2 ¾® 2NO2

·       NO2 is absorbed in water to give HNO3 and NO, which is then recycled.

3NO2 + H2O ¾® 2HNO3 + NO

Nitric acid can be concentrated to 68 % by distillation, when a constant boiling mixture is formed. More concentrated acid can be made by distilling the mixture with concentrated sulphuric acid. Further concentration to 98% can be achieved by dehydration with concentrated H2SO4. 98% HNO3 is known as fuming nitric acid.

 

Ø  Properties:

·       It is a colourless liquid. In the gaseous state.

                    ·    HNO3 exists as a planar molecule with the structure as shown below:


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

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

 

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

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

 

·       Zinc reacts with dilute nitric acid to give N2O and with concentrated acid to give NO2.

4Zn + 10HNO3(dilute) → 4 Zn (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.

 

·       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 → 2HIO3 + 10 NO2 + 4H2O

C + 4HNO3 → CO2 + 2H2O + 4NO2

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

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

 

·       Brown Ring Test: It is a test used for the detection of nitrates. The test is carried out by adding dilute ferrous sulphate solution to an aqueous solution containing nitrate ion, and then carefully adding concentrated sulphuric acid along the sides of the test tube. A brown ring at the interface between the solution and sulphuric acid layers indicate the presence of nitrate ion in solution.

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

[Fe (H2O)6 ]2+ + NO → [Fe (H2O)5 (NO)]2+ + H2O

                                                                    (brown ring)

ü  Uses of Nitric Acid: It is used

i)                In the manufacture of ammonium nitrate for fertilizers and other nitrates for use in explosives and pyrotechnics.

ii)              For the preparation of nitroglycerin, trinitrotoluene and other organic nitro compounds.

iii)             In the pickling of stainless steel (chemical treatments applied to the surface of stainless steel to remove contaminants and assist the formation of a continuous chromium-oxide, passive film), etching of metals and as an oxidiser in rocket fuels.