Nitrogen forms the oxo acids such as H₂N₂O₂ (hyponitrous acid), HNO₂ (Nitrous acid) and HNO₃ (Nitric Acid) with their structures:

Preparation of Nitric acid:-

(1) Laboratory Preparation:- In laboratory, nitric acid is prepared by heating KNO₃ or NaNO₃ and conc H₂SO₄ in a glass retort.

KNO₃ + H₂SO₄ → KHSO₄ + HNO₃

(2) Commercial Preparation:- On commercial scale, HNO₃ is prepared by Ostwald’s process. This method is based upon the catalytic oxidation of ammonia by atmospheric oxygen.

Nitric oxide thus formed combines with oxygen to give NO₂.

2NO + O₂ → 2NO₂

Nitrogen di – oxide so formed, dissolves in water to give HNO₃.

3NO₂ + H₂O → 2HNO₃ + NO

The NO gas is then recycled. The aq. HNO₃ cannot be concentrated by distillation above 68% by mass due to formation of azeotropic mixture. Further conc. upto 98% can be achieved by dehydration with conc. H₂SO₄.

Physical Properties of Nitric Acid:-

Pure Nitric acid is colourless, fuming liquid with pungent smell and contains some dissolved oxides of nitrogen, hence is called Fuming Nitric Acid.

(2) It freezes at 231.4K and boils at 355.6K.

(3) Laboratory grade Nitric acid is a constant boiling mixture (394K) i.e. Azeotropic mixture containing 68% HNO₃ and 32% H₂O by mass. It has a specific gravity 1.504.

Chemical Properties:-

(1) Acidic nature:- Nitric Acid is a strong mono basic acid. In aqueous solution, it ionizes as:

HNO₃ + H₂O → H₃o⁺ + NO₃^-

Hence it reacts with bases and oxide of metals to give nitrates and turns Blue litmus Red e.g. 

CaO + 2HNO₃ → Ca(NO₃)₂ + H₂O

Na₂CO₃ + 2HNO₃ → 2NaNO₃ + CO₂ + H₂O

(2) Oxidation nature:- Nitric acid is a strong oxidizing agent and oxidizes most of the metals (except Gold & Pt.) and non metals. The products of oxidation depend upon the conc. of acid, temperature and nature of material undergoing oxidation. The process of oxidation can be explained as:

(a) Metals which are more electro positive than hydrogen (i.e. have –ve standard electrode Potential e.g. Na, K, Mg, Zn, Fe etc.), liberate nascent hydrogen which reduces nitric acid. Let us take the example of Zn

(i) Zn with dilute HNO₃

Zn + 2HNO₃ → Zn(NO₃)₂ + 2H] * 4

HNO₃ + 8H → NH₃ + 3H₂O

NH₃ + HNO₃ → NH₄NO

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4Zn + 10HNO₃ → 4Zn(NO₃)₂ + 3H₂O + NH₄NO₃

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However, on heating NH₄NO₃ decomposes as

NH₄NO₃ → N₂O + 2H₂O

Hence the overall reaction with Hot Dilute HNO₃ becomes:-

4Zn + 10HNO₃ → 4Zn(NO₃)₂ + 5H₂O + N₂O

(ii) Zn with conc. HNO₃:

Zn + 2HNO₃ → Zn(NO₃)₂ + 2H

HNO₃ + H → H₂O + NO₂]*2

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Zn + 4HNO₃ → Zn(NO₃)₂ + 2H₂O +2NO₂

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(b) Metals which are less electropositive than hydrogen (i.e. have +ve standard reduction potential e.g. Cu, Hg, Ag, etc.) do not liberate hydrogen, instead nitric acid librates nascent oxygen. e.g. Cu reacts with HNO₃ as:

(i) Cu with dilute HNO₃:

2HNO₃ → H₂O + 2NO + 3[O]

Cu + [O] → CuO]*3

CuO + 2HNO₃ → Cu(NO₃)₂ + H₂O]*3

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3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 4H₂O + 2NO

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(ii) Cu with conc. HNO₃:

2HNO₃ → H₂O + 2NO₂ + [O]

Cu + [O] → CuO

CuO + 2HNO₃ → Cu(NO₃)₂ + H₂O

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Cu + 4HNO₃ → Cu(NO₃)₂ + 2H₂O + 2NO₂

(c) The Metals only magnesium and manganese react with very dilute HNO₃ to give H₂ gas. The metals iron chromium, Nickel and Aluminum when dipped in conc. HNO₃ there develops a thin protective layer of the metal oxide on their surfaces. Hence they lose their normal activity and become passive. This phenomenon is known as Passivity.

(d) Non-Metals are not attacked by dilute nitric acid. But conc. Nitric and oxidizes many non-metals such as carbon, sulphur, iodine, phosphorous to their respective

oxo- acids, while nitric acid itself is reduced to NO₂. e.g.

(i) Carbon is oxidized to carbonic acid (H₂CO₃):-

2HNO₃ → H₂O + 2NO₂ + [O]*2

C + 2[O] + H₂O → H₂CO₃

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C + 4HNO₃ → H₂CO₃ + H₂O + 4NO₂

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(ii) Sulphur is oxidized to sulphuric acid (H₂SO₄):-

2HNO₃ → H₂O + 2NO₂ + [O]*24

S₈ + 24[O] + 8H₂O → 8H₂SO₄

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S₈ + 48HNO₃ → 8H₂SO₄ + 16H₂O + 48NO₂

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(iii) Iodine is oxidized to Iodic acid (HIO₃):-

2HNO₃ → H₂O + 2NO₂ + [O]*5

I₂ + 5[O] + H₂O → 2HIO₃

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I₂ + 10HNO₃ → 2HIO₃ + 4H₂O + 10NO₂

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(iv) Phosphorus is oxidized to Phosphoric acid (H₃PO₄):-

2HNO₃ → H₂O + 2NO₂ + [O]…… ] * 10

P₄ + 10[O] + 6H₂O → 4H₃PO₄

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P₄ + 20HNO₃ → 4H₃PO₄ + 4H­₂O + 2NO₂

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Ring Test for Nitrate ion:- In qualitative analysis, the presence of nitrate ion is detected by ring test. In this test a freshly prepared solution of ferrous sulphate is added to the aqueous solution of a nitrate (salt). Pure conc. H₂SO₄ is then added carefully drop wise along the walls of test tube. The appearance of dark brown ring at the junction of the two layers indicates the presence of a nitrate ion.

NO^-₃ + 3Fe⁺² + 4H⁺ → NO + 3Fe⁺³ + 2H₂O

Fe⁺³ + NO + 5H₂O → [Fe(H₂O)₅ NO⁺]⁺²

                                      Penta-aqua-nitrosonium iron (I) ion (Brown Complex)

Uses of Nitric acid:- Nitric acid is used in

(i) Preparation of ammonium nitrate and basic calcium nitrate [CaO.Ca(NO₃)₂] which are used as fertilizers.

(ii) Manufacture of explosives and Pyrotechnics such as gun cotton, nitroglycerine, TNT, Picric acid etc.

(iii) Pickeling (cleaning) of stainless steel and etching of metals.

(iv) Rocket fuels as an oxidizer.