Nitrous acid is a weak and unstable acid. It has the ability to either donate a proton or form a covalent bond with an electron pair; therefore, it is also called monoprotic acid. It is treated with amines to produce diazonium salts which are used in azo coupling reactions of azo dyes. It is formed under cold, acidic conditions, and it decomposes to form nitric oxide (NO) and nitric acid (HNO3).  

Nitrous Acid (HNO2)

• Structure- The structure of nitrous acid contains two oxygen atoms to a nitrogen atom, one bonded with a single bond and the other with a double bond. In the lower atmosphere, when nitric oxide reacts with water, nitrous acid is formed naturally. It is found in two forms called ‘syn’ and ‘anti’ in the gas phase. At room temperature, the ‘anti form’ is found to be more prevailing than syn. The ‘anti-form is more stable as per infrared spectroscopy measurements.
• Physical properties- The molecular formula of nitrous acid is HNO2, and its molar mass is 47.013 g/mol. It appears to be pale blue in colour, and the acid dissociation constant (pKa) is 3.15. The density of nitrous acid is 1g/ml. Melting point is only known in solution or in gas.
• Chemical properties- It is a weak acid and cannot completely dissociate in water. Nitrous acid needs to be prepared freshly as it is unstable in its free form and found in a liquid state most of the time. It can donate a proton (H+) to a base in an acid-base reaction, thus called monobasic acid. It forms salts when reacted with bases.

Reduction

Reduction is a reaction where the chemical compound gains electrons, and this leads to a decrease in the oxidation number. The compound gaining electrons will get reduced. The compound undergoing reduction reaction attracts the hydrogen or electropositive element or loses oxygen or electronegative elements.

For example- 1) The removal of oxygen from mercuric oxide.          

              2HgO(s)  →    2Hg(l)  +  O2(g) 

 2) The removal of an electronegative element that is chlorine, from ferric chloride.

             2FeCl3(aq) + H2(g)  →  2FeCl2(aq) + 2HCl(aq)

Reduction of Nitrous Acid

• In alkali solution, it reacts with Zn to give NH3 (ammonia)

5H2O + KNO2 + 3Zn   →   NH3 + KOH + 3Zn (OH)2

• When nitrous acid reacts with Sn2+ ions in the solution, the product formed is N2O (nitrous oxide)

    2HNO2 + 6HCl + 2SnCl2 → 2SnCl4 + N2O + 3H2O + 2KCl

• Nitrous acid reacts with SO2 gas to produce (hydroxylamine) NH2OH

2HNO2 + 6H2O + 4SO2→  3H2SO4 + K2SO4 + 2NH2OH

• When reacted with I– and Fe2+ ions, NO (nitric oxide)

     2HNO2 + 2KI + 2H2SO4 →  I2 + 2NO + 2H2O + 2K2SO4

    2HNO2 + 2FeSO4 + 2H2SO4 → Fe2(SO4)3 + 2NO + 2H2O + K2SO4

• Nitrous acid reacts with [N2H5]+ (hydrazinium) and HN3 (hydrazoic acid) to produce N2 gas.

                   HNO2 + [N2H5]+→  HN3 + H2O + H3O+

                         HNO2 + HN3 → N2O + N2 + H2O

Conclusion

Nitrous acid is a weak acid and has been used in many industries to prepare other chemicals; it acts as a solvent/ reagent for these reactions. It plays an important role in the identification of alkaloids called spot-tests. It is present in the Liebermann reagent (A mixture of potassium nitrate and concentrated sulphuric acid) as an agent responsible for colour production (chromophore) for the detection of alkaloids.

Nitrous acid is found to be the faster oxidising agent as compared to dilute nitric acid. Nitrous acid has the ability to destroy or eliminate the toxic and potentially hazardous explosive sodium azide. In the organic synthesis of the Sandmeyer reaction that synthesises aryl halides and aryl diazonium salts, these salts produced with the help of nitrous acid are widely used. The reactions of nitrous acid-like reduction are used in the production of ammonia, nitrous oxide, nitric oxide, etc. Nitrous acid reacts with aliphatic alcohols to produce potent vasodilators. It acts as an oxidising agent/oxidiser in liquid fuel rockets. It is also present in the atmosphere of the earth in the ozone budget, which is the troposphere.