Nitrous acid and nitric acid are two main oxoacids of Nitrogen. Nitrous acid (HNO2) is unstable and occurs only in aqueous solution. It acts as oxidizing and reducing agent and possesses complex forming ability, Nitric acid (HNO3) forms a part of aqua regia and acts as a strong oxidizing agent. It is manufactured by Ostwald’s process.
Nitrous acid HNO2
• H − O − N = O
• Prepared by the action of any nitrite with dil. H2 SO4 or dil HCl
NaNO2 + H2 SO4 → HNO2 + NaHSO4
• HNO2 → Not stable
On standing it disproportionates
• Test of NO2− (reducing agent)
Nitric acid (HNO3)
• Prepared by the action of hot & conc. H2 SO4 on a nitrate salt.
Commercial method.
Ostwald’s method
• Specific reactions of HNO3
I) Aqua regia mix of [conc. HCl] & [Conc. HNO3] in the ratio 3 : l
It dissolves Au, Pt, etc.
II) HNO3 is a good oxidising agent
III) On metals
i) Lighter metal + v.dil. HNO3 → N2 O
4Sn + 10HNO3 → 4Sn(NO3)2 + 5H2 O + N2 O
ii) Lighter metal + conc. HNO3 (hot) → NO2
Zn + 4HNO3 → Zn(NO3)2 + 2H2 O + 2NO2
iii) Heavier metal + Conc. HNO3 (hot) → NO2
Cu + 4HNO3 → Cu(NO3)2 + 2H2 O + 2NO2
iv) Heavier metal + dil HNO3 → NO
3Cu + 8HNO3 → 3Cu(NO3)2 + 4H2 O + 2NO
Test for NO\(^-_3\) ion
Brown ring test
Salt + freshly prepared FeSO4 + Conc. H2 SO4 → Reddish brown
From ‘NO’ 1e− undergoes a transfer to Fe2+ & this electronic transition causes the brown colour.
