Nitrous Acid

Nitrous Acid

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Nitrous Acid [HNO₂]

Method of preparation :

a) By the reaction of NaNO₂ with cold and dilute H₂SO₄ : nitrous  acid is formed.

2 NaNO₂ (Sodium nitrite) + H₂SO₄ (dilute)  ——-> 2HNO₂ (Nitrous acid) +Na₂SO₄

b) By the reaction of Barium nitrite [ Ba(NO₂)_2] with cold and dilute H₂SO₄  :  At 00c nitrous  acid is formed.

Ba(NO₂)₂ +H₂SO₄ ——–> 2HNO₂ +BaSO₄

Properties :

1) Acidic nature :

It is a weak  monobasic acid. It forms salt with base.

HNO₂ + NaOH ——-> NaNO₂ (sodium nitrite) +H₂O

2) Oxidizing properties :

a) SO₂ is oxidized into H₂SO₄ :

2HNO₂ ——-> H₂O + 2NO + O

SO₂ + O + H₂O —–>H₂SO₄

Equation as a whole,

2HNO₂ + SO₂  ——> 2NO + H₂SO₄

b) It oxidizes H₂S into sulphur :

2HNO₂ ——-> H₂O + 2NO + O

H₂S+ O ——> H₂O + S

Equation as a whole,

2HNO₂ +H₂S ——> 2H₂O + 2NO + S

c) It oxidizes Na₂SO₃ into Na₂SO₄ :

2HNO₂ ——-> H₂O + 2NO + O

Na₂SO₃ + O ——> Na₂SO₄

Equation as a whole,

2HNO₂ +Na₂SO₃ —-> H₂O + 2NO + Na₂SO₄

d) It oxidizes acidic KI into K₂SO₄ :

2HNO₂   —-> H₂O + 2NO + O

2KI +H₂SO₄ +O —–>K₂SO₄ +I₂ +H₂O

Equation as a whole,

2HNO₂ + 2KI +  H₂SO₄ ——> 2H₂O + 2NO +K₂SO₄ +I₂

e) It oxidizes SnCl₂  into SnCl₄ :

2HNO₂ ——> H₂O + 2NO + O

SnCl₂ + 2HCl +O ——-> SnCl₄ + H₂O

Equation as a whole,

2HNO₂ + SnCl₂ + 2HCl —–>  SnCl₄  + 2NO +  2H₂O

3) Reducing properties :

HNO₂ itself is oxidized into HNO₃ therefore HNO₂ acts as reducing agent.

a) Cl₂ is reduced to HCl :

Cl₂+ H₂O —–> 2HCl + [O]

HNO₂ + [O]  ——> HNO₃

Equation as a whole,

Cl₂ + H₂O + HNO₂ ——> HNO₃ + 2HCl

b) H₂O₂ is reduced into H₂O :

H₂O₂ ——> H₂O + [O]

HNO₂ + [O]  —–> HNO₃

Equation as a whole,

H₂O₂ + HNO₂ —–>H₂O + HNO₃

c) Acidic KMnO₄ is reduced to MnSO₄ and pink color of KMnO₄ disappears :

2KMnO₄ + 3H₂SO₄ ——->K₂SO₄ +   2MnSO₄ + 3H₂O + 5[O]

5HNO₂ + 5[O]    ————-> 5HNO₃

Equation as a whole,

2KMnO₄ +3H₂SO4 + 5HNO₂ ——-> K₂SO₄ + 2MnSO₄ + 3H₂O + 5HNO₃

d) Acidic K₂Cr₂O₇ is reduced to Cr₂(SO₄)₃ :

K₂Cr₂O₇ + 4H₂SO₄ ——->K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O + 3[O]

3HNO₂ + 3[O] —–> 3HNO₃

Equation as a whole,

K₂Cr₂O₇ + 4H₂SO₄ + 3HNO₂ ———-> K₂SO₄ + Cr₂(SO₄)₃ +4H₂O +3HNO₃

HNO₂ acts as oxidizing as well as reducing agent:

4) Reaction with Urea :

Nitrogen , carbon dioxide & water is formed.

NH₂CONH₂ + 2HNO₂ ——> 2N₂ + CO₂ + 3H₂O

5) Reaction with ethylamine : Ethyl alcohol is formed.

C₂H₅NH₂ + HNO₂ ——> C₂H₅OH + N₂ + H₂O

Question: What is the effect of heat on NH₄NO₂ and NH₄NO₃ ?

Solution :

NH₄NO_{3( ammonium nitrate)} —–> N₂O (Nitrous oxide) + 2H₂O

NH₄NO_{2 (ammonium nitrite)} ——>  N_{2 (Nitrogen)} + H₂O

Test : 

1) It decolourises the pink colour of acidic KMnO₄

2) It gives  green  colour  with  acidic K₂Cr₂O₇.

3) It makes Starch iodide solution violet.