CBSE Class 12 Chemistry Notes: The p Block Elements – Nitrogen Family (Oxyacids of Nitrogen)

Oxides of N, P and As when dissolves in water to form oxy acids. Oxy acids of Sb and Bi are not stable.

Some important oxy acids:

Element	Formula of the oxyacid	Name of the oxyacid	Oxidation state of element in oxyacid
Nitrogen	H₂N₂O₂	Hyponitrous acid	+1
	HNO₂	Nitrous acid	+3
	HNO₃	Nitric acid	+5
Phosphorus	H₃PO₂	Hypo phosphorus acid	+1
	H₃PO₃	Phosphorus acid	+3
	H₃PO₄	Phosphoric acid	+5
	(HPO₄)_n	Metaphosphoric acid	+5
	H₄P₂O₇	Pyrophosphoric acid	+5
Arsenic	H₃AsO₃	Arsenious acid	+3
	H₃AsO₄	Arsenic acid	+5
Sb & Bi	No stable acid

Oxyacids of Nitrogen , HNO₃ , Nitric acid
Nitric acid is also known as aqua fortis (meaning strong water) which was given by alchemists.

Methods of preparation
(a) Laboratory preparation
KNO₃ + H₂SO₄ → KHSO₄ + HNO₃

(b) Birkeland Eyde process

N₂ + O₂ ⇌ 2NO – 43,200 calories

The formation of nitric oxide is favoured by high temperature thus it is maintained at 3000°C.
The nitric oxide is cooled to 1000oC as to prevent its decomposition. Nitric oxide further combines with O₂ .
2NO + O₂ → 2NO₂
The vapours then passed through water when nitric acid is produced.
2NO₂ + H₂O → HNO₃ + HNO₂
3HNO₂ → HNO₃ + 2NO + H₂O

(c) Ostwald’s process (Modern process)
In this process the nitric acid is formed first when NH₃ and air passed over platinum gauze at 750 – 900°C

The nitric oxide is then oxidised to NO₂ by air which is cooled to 50°C and absorbed in water.
2NO + O₂ → 2NO₂
3NO₂ + H₂O → 2HNO₃ + NO

Fuming nitric acid
When NO₂ is dissolved in conc. nitric acid it forms fuming nitric acid. It is brown in colour. It is obtained by distilling concentrated HNO₃ with a little starch.

Physical properties
HNO₃ is colourless fuming with pungent smell soluble in water. Nitric acid usually acquires yellow
colour due to its decomposition by sunlight into NO₂ .

Chemical properties