Nitric Acid

Nitric Acid – The p-Block Elements – Class 12

Nitric Acid (HNO₃)

 

The common oxoacids of nitrogen are given below :

 

 

Nitric acid is a very strong oxidising agent.Nitrogen shown an oxidation state of +5 in nitric acid.

Laboratory Preparation of Nitric Acid

In the laboratory, nitric acid can be prepared by heating sodium or potassium nitrate with concentrated sulphuric acid to about 423-475 K.

NaNO₃ + H₂SO₄ ——> NaHSO₄ + HNO₃

Anhydrous nitric acid can be obtained by distillation of concentrated aqueous nitric acid with P₄0₁₀.

 

Manufacture of Nitric Acid

Nitric acid is commonly manufactured by Ostwald process in which it is prepared by the catalytic oxidation of ammonia by atmospheric oxygen. The reaction is carried out at about 500 K and 9 x 10⁵ Pa (9 bar) pressure in the presence of Pt or Rh gauge as catalyst.

4NH₃(g) + 50₂(g)——> 4NO(g) + 6H₂0(g)    ΔH =- 90.2 kJ

Pt/Rh gauge,  500K, 9 bar

Nitric oxide thus formed combines with oxygen to form nitrogen dioxide.

2NO(g) +  O₂ (g) ——> 2 NO₂ (g) 

Nitrogen dioxide so formed, dissolves in water to give nitric acid.

3NO₂ (g) + H₂O(l) —–> 2HNO₃(aq) + NO(g)

Dilute nitric acid is further concentrated by dehydration with concentrated sulphuric acid to get about 98% acid.

Properties of Nitric Acid

Physical Properties

1) Pure nitric acid is a colourless liquid.

2) It has boiling point 355.6 K and freezing point 231.4 K.

3) laboratory grade nitric acid contains about 68% of HNO₃ by mass and has a specific gravity of 1.504.

4) The impure acid is generally yellow due to the presence of nitrogen dioxide as impurity. Nitric acid containing dissolved nitrogen dioxide is known as fuming nitric acid.

5) It has a corrosive action on skin and produces painful blisters.

Chemical Properties

(1) Acidic character: It is one of the strongest acids because it is highly ionised in aqueous solution giving hydronium and nitrate ions.

2HNO₃(aq) +H₂O (l) ——> H₃O⁺ + NO₃¯(aq)

It turns blue litmus red. It forms salts with alkalies, carbonates and bicarbonates.

 

NaOH + HNO₃ —-> NaNO₃ + H₂O

Na₂CO₃ +  HNO₃ —-> 2NaNO₃ + H₂O + CO₂

NaHCO₃ + HNO₃ —-> NaNO₃ + H₂O + CO₂

(2) Action on metals: With the exception of gold and platinum, nitric acid attacks all metals forming a variety of products. The product depends upon the nature of metal, the concentration of acid and temperature.

 

(A) Metals that are more electropositive than hydrogen (Mg, Al, Mn, Zn, Fe, Pb, etc.). In this case nascent hydrogen is liberated which further reduces nitric acid.

M + 2HNO₃ ——> M(NO₃)₂ + 2H

HNO₃ + H —-> Reduction product + H₂O

 

The principal product is NO₂, with conc. HNO₃, N₂O with dil. HNO₃, and ammonium nitrate with very dil. HNO₃.

 

For example: Zn reacts as:

 

(a) Using concentrated nitric acid (forms nitrogen dioxide)

 

Zn + 2HNO₃ —–> Zn(NO₃)₂ + 2H

HNO₃ + H —–> NO₂ + H₂O] x 2

—————————————————-

Zn + 4HNO₃ —-> Zn(NO₃)₂ + 2NO₂ + 2H₂O

 

(b) Using dilute nitric acid (forms nitrous oxide)

 

Zn + 2HNO₃ —–> Zn(NO₃)₂ + 2H ] × 4

2 HNO₃ + 8 H —–> NO₂ + 5 H₂O

—————————————————-

Zn + 10HNO₃ —-> 4Zn(NO₃)₂ + NO₂ + 5H₂O

 

(c) Using very dilute nitric acid (forms ammonium nitrate)

 

Zn + 2HNO₃ —–> Zn(NO₃)₂ + 2H ] × 4 

HNO₃ + 8H —–> NH₃ + 3 H₂O 

NH₃ + HNO₃ ——–> NH₄NO₃ 

———————————————————-

4Zn + 10 HNO₃ —–> 4Zn(NO₃)₂ + NH₄NO₃ + 3 H₂O

 

(B) Metals which are less electropositive than hydrogen (Cu, Bi, Hg, Ag). In this case nascent hydrogen is not liberated.

HNO₃ → Reduction product + H₂O + [0]

Metal + (O) + HNO₃→ Metal nitrate + H₂O

The principal product is NO₂ with conc. HNO₃ and NO with dil. HNO₃

 

For example: Cu reacts as

(a) Using concentrated nitric acid

2HNO₃ ——> 2NO₂ + H₂O + [O]

Cu + O + 2HNO₃ ——–>  Cu(NO₃)₂ + H₂O

—————————————————-

Cu + 4HNO₃  → Cu(NO₃)₂ + 2NO₂ + 2H₂O

 

(b) Using dilute nitric acid

2HNO₃ —–> 2NO + H₂O + 3[O]

Cu + O + 2HNO₃ –> Cu(NO₃)₂ + H₂O] x 3

——————————————————-

3Cu + 8HNO₃ ——-> Cu(NO₃)₂ + 2NO + 4H₂O

 

Hg + 4HNO₃ —–> Hg(NO₃)₂ + 2NO₂ + 2H₂O

6Hg + 8HNO₃ —–> 3Hg₂(NO₃)₂ + 2NO + 4H₂O

Ag + 4HNO₃ —–> AgNO₃ + NO₂ + 2H₂O

3 Ag + 4 HNO₃ —–> 3 AgNO₃ + NO + 2H₂O

 

 

(c) Action on noble metals

Noble metals like gold and platinum are not attacked by nitric acid. However, these metals are attacked by aqua regia (3 parts conc. HCl and 1 part conc. HNO₃) forming their chlorides.

 

NaOH + HNO₃ —–> NaNO₃ + H₂O

HNO₃ + 3 HCl ——> NOCl + 2 H₂O + 2 Cl

 Nitrosyl               chloride

Au + 3 Cl —> AuCl₃

Pt + 4Cl —-> PtCl₄

 

(3) Oxidising nature -Oxidation of non-metals and compounds.

Nitric acid behaves as a strong oxidising agent. It has a tendency to give nascent oxygen as:

2HNO₃ —–> 2 NO₂ + H₂O + O

(conc.)

2HNO₃ —–> 2 NO + H₂O + 3 [O]

Therefore, nitric acid oxidises many non-metals and compounds.

 

(A) Oxidation of non-metals: Dilute nitric acid has no action on non-metals like carbon, sulphur, phosphorus, etc. However, concentrated nitric acid oxidises many non-metals.

 

For example

(1) Nitric acid oxidises sulphur to sulphuric acid

 

2HNO₃ ——> 2NO₂ + H₂O+ O] x 3

1/8 S₈ + H₂O + 3O —–> H₂SO₄

_{——————————————————————}

1/8 S₈ + 6HNO₃ ——–>H₂SO₄ + 6NO₂  + 2 H₂O

———————————————————

S₈ + 48HNO₃ ——–> 8 H₂SO₄ + 48NO₂  + 12H₂O

———————————————————-

 

(ii) Nitric acid oxidises carbon to carbonic acid

 

2HNO₃ —–> 2NO₂ + H₂O + O] x 2

C + H₂O+ 2O —-> H₂CO₃

————————————————–

C+4HNO₃ —–> H₂CO₃ + 4NO₂ + 2H₂O

————————————————–

(iii) Nitric acid oxidises phosphorus to phosphoric acid

 

2HNO₃ —–> 2NO₂ + H₂O + O] x 5

2P + 3H₂O + 5O —-> 2 H₃PO₄

—————————————–

2P+ 10HNO₃ —–> 2 H₃PO₄ + 10 NO₂ + 2 H₂O

p + 5 HNO₃ —> H₃PO₄ + 5 NO₂ + H₂O

P₄ + 20 HNO₃ —-> H₃PO₃ + 20 NO₂ + 4 H₂O

 

 

(iv) It oxidises iodine to iodic acid.

 

2HNO₃ ——> 2NO₂ + H₂O + O]× 5

I₂ + H₂O + 5O —> HIO₃ 

———————————————————

I₂ + 10HNO₃ ——–>  2 HIO₃ + 10 NO₂  + 4 H₂O

———————————————————

 

(v) Nitric acid oxidises arsenic to arsenic acid.

 

2HNO₃ ——> 2NO₂ + H₂O + O] × 5

2As + 3H₂O + 5O ——> 2H₃AsO₄

———————————————————-

2As + 10HNO₃ ——> 10NO₂ +  2H₃AsO₄ + 2H₂O 

As + 5HNO₃ ——> 5NO₂ +  H₃AsO₄ + H₂O 

 

(B) Oxidation of compounds

Dilute as well as concentrated nitric acid oxidises a number of compounds.

 

(1) Nitric acid oxidises hydrogen sulphide to sulphur.

dil HNO₃ :

3H₂S + 2 HNO₃ —–> 2 NO + 4 H₂O + 3S

 

Conc HNO₃

3H₂S + 2 HNO₃ —–> 2 NO₂ + 4 H₂O + S

 

(2) Nitric acid oxidises sulphur dioxide to sulphuric acid

dil HNO₃

3SO₂ + 2HNO₃ + 2H₂O —> 3 H₂SO₄ + 2 NO

 

conc. HNO₃

SO₂ + 2 HNO₃ ——-> H₂SO₄ + 2 NO₂

 

(3) Nitric acid oxidises ferrous sulphate to ferric sulphate

dil HNO₃

6FeSO₄ + 2HNO₃ +3H₂SO₄——> 3Fe₂(SO₄)₃ + 2NO + 4 H₂O

 

conc. HNO₃

2FeSO₄ + 2HNO₃ +3H₂SO₄——> 3Fe₂(SO₄)₃ + 2NO₂ + 4 H₂O

 

(4) Action on organic compounds

 

Nitric acid also reacts with organic compounds. 

For example: sucrose (cane sugar) is oxidised to oxalic acid by nitric acid.

C₁₂H₂₂ O₁₁ + 36 HNO₃ –> 6 (COOH)₂ + 36NO₂ + 23 H₂O

 

In the presence of sulphuric acid, nitric acid reacts with aromatic compounds forming nitro compounds. This process is called nitration.

 

For example: it reacts with benzene to form nitrobenzene.

C₆H₆ + HNO₃ —-> C₆H₅NO₂ + H₂O

Similarly, phenol reacts with nitric acid in the presence of H₂SO₄ to give trinitrophenol (known as picric acid).

 

Nitric acid attacks proteins giving a yellow nitro compound known as xantho protein. Therefore, nitric acid stains skin and renders wool yellow.

 

Structure

Gaseous nitric acid has planar structure. Nitrate ion, NO₃¯ has also planar symmetrical structure 

 

 

Brown Ring test for NO₃¯ ion

Nitrates give brown ring test with Fe²⁺ ions in the presence of conc. H₂SO₄.This is based upon the tendency of Fe²⁺ to reduce nitrates to nitric oxide which reacts with Fe²⁺ to form a brown coloured complex.

 

The test is usually performed by adding dilute FeSO₄ solution to an aqueous solution containing NO₃¯ ion and then adding conc. H₂SO₄ slowly along the sides of the test tube. A brown ring at the interface between the solution and sulphuric acid indicates the presence of NO₃¯ ion.

 

3Fe²⁺ +NO₃¯ + 4 H⁺ —-> NO + 3Fe³⁺ + 2H₂O

Fe³⁺ + NO + 5 H₂O —-> [Fe(H₂O)₅ NO]²⁺

            Pentaaquanitrosyl iron (II) ion

Uses of Nitric Acid

 

(i) It is used in the manufacture of ammonium nitrate for fertilizers.

 

(ii) It is used in the manufacture of sulphuric acid by lead chamber process.

 

(iii) It is used in the manufacture of explosives such as trinitro toluene (TNT), nitroglycerine, picric acid, etc.

 

(iv) It is used in the manufacture of dyes, perfumes and silk.

 

(v) It is used for the manufacture of nitrates for use in explosive and pyrotechnics.

 

(vi) It is used in picking of stainless steel and etching of metals.

 

(vii) It is also used as an oxidiser in rocket fuels.

 

(viii) It is used in the purification of gold and silver as aqua regia.