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Home » Class 12 » Chemistry » The p-Block Elements » Nitric Acid

Nitric Acid

Last Updated on February 16, 2023 By Mrs Shilpi Nagpal

Nitric Acid – The p-Block Elements – Class 12

Contents

  • 1 Nitric Acid (HNO3)
  • 2 Laboratory Preparation of Nitric Acid
  • 3 Manufacture of Nitric Acid
  • 4 Properties of Nitric Acid
    • 4.1 Physical Properties
    • 4.2 Chemical Properties
  • 5 Brown Ring test for NO3¯ ion
  • 6 Uses of Nitric Acid

Nitric Acid (HNO3)

 

The common oxoacids of nitrogen are given below :

 

Oxides of nirogen

 

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 

 

Structure of nitric acid and nitrate ion

 

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.

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Filed Under: Chemistry, Class 12, The p-Block Elements

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