Nitric Acid

Manufacture of nitric acid by Ostwald process:

Principle:

  1. Catalytic Oxidation of ammonia: Ammonia is oxidized in the air in presence of platinum as a catalyst at 850°C to give nitric oxide.
catalytic oxidation of ammonia

  1. Oxidation of nitric oxide: Nitric oxide is oxidized into nitrogen dioxide in presence of oxygen.
    2NO + O2 → 2NO2

  1. Absorption of nitrogen dioxide: Nitrogen dioxide is absorbed in water in presence of air to give nitric acid.
    4NO2 + 2H2O + O2 → 4HNO3 (Nitric Acid)

manufacture of nitric acid by Ostwald's process
Fig: Manufacture of nitric acid by Ostwald’s process

Physical properties of nitric acid:
  1. Pure nitric acid is a colorless liquid. In presence of light, it is decomposed into brown color due to dissolved NO2.
  2. Its boiling point is 84°C and the melting point is -42°C.
  3. It is soluble in water.
  4. It is corrosive to the skin and produces blisters.

Chemical properties of nitric acid:
  1. Acidic nature:
    i. It is a strong acid that completely ionizes in an aqueous solution.
    HNO3 + H2O → NO3 + H3O+

    ii. It turns blue litmus paper into red. It neutralizes bases to give salt and water.
    HNO3 + NaOH → NaNO3 + H2O

    iii. It decomposes carbonates to give CO2 gas.
    2HNO3 + Na2CO3 → 2NaNO3 + H2O + CO2

  1. Mg and Mn react with very dilute nitric acid to give hydrogen gas.
    Mg + HNO3 → Mg(NO3)2 + H2
    Mn + HNO3 → Mn(NO3)2 + H2

3. Oxidizing agent:

a. Action with more electropositive metals than hydrogen:

i. With zinc

– With Conc. HNO3
Zn + 2HNO3 → Zn(NO3)2 + 2[H]
[H] + HNO3 → H2O + NO2 ] x 2

Zn + 4HNO3 → Zn(NO3)2 + 2NO2 + 2H2O

– With moderate Conc. HNO3
Zn + 2HNO3 → Zn(NO3)2 + 2[H] ] x 3
3[H] + HNO3 → 2H2O + NO ] x 2

3Zn + 8HNO3 → 3Zn(NO3)2 + 2NO + 4H2O

-With dil. HNO3
Zn + 2HNO3 → Zn(NO3)2 + 2[H] ] x 4
8[H] + 2HNO3 → 5H2O + N2O


4Zn + 10HNO3 → 4Zn(NO3)2 + N2O + 5H2O

-With very dil. HNO3
Zn + 2HNO3 → Zn(NO3)2 + 2[H] ] x 4
8[H] + HNO3 → 3H2O + NH3
NH3 + HNO3 → NH4NO3


4Zn + 10HNO3 → 4Zn(NO3)2 + NH4NO3 + 3H2O


ii. With magnesium:

-With conc. HNO3
Mg + 2HNO3 → Mg(NO3)2 + 2[H]
[H] + HNO3 → H2O + NO2 ] x 2


Mg + 4HNO3 → Mg(NO3)2 + 2NO2 + 2H2O

-With moderate conc. HNO3
Mg + 2HNO3 → Mg(NO3)2 + 2[H] ] x 3
3[H] + HNO3 → 2H2O + NO ] x 2


3Mg + 8HNO3 → 3Mg(NO3)2 + 2NO + 4H2O

-With dil.HNO3
Mg + 2HNO3 → Mg(NO3)2 + 2[H] ] x 4
8[H] + 2HNO3 → 5H2O + N2O


4Mg + 10HNO3 → 4Mg(NO3)2 + N2O + 5H2O

-With very dil. HNO3
Mg + 2HNO3 → Mg(NO3)2 + 2[H] ] x 4
8[H] + HNO3 → 3H2O + NH3
NH3 + HNO3 → NH4NO3


4Mg + 10HNO3 → 4Mg(NO3)2 + NH4NO3 + 3H2O


iii. With Iron:

-With conc. HNO3: Conc. HNO3 reacts with iron to form a layer of ferrosoferric oxide on the surface of the iron. Then the reaction stops. This is called the passivity of iron. Such iron is called passive iron.

2HNO3 → 2NO2 + H2O + [O] ] x 4
4[O] + 3Fe → Fe3O4


3Fe + 8HNO3 → 8NO2 + Fe3O4 + 4H2O

-With moderate conc. HNO3
Fe + 3HNO3 → Fe(NO3)3 + 3[H]
[H] + HNO3 → H2O + NO2 ] x 3


Fe + 6HNO3 → Fe(NO3)3 + 3NO2 + 3H2O

-With very dilute HNO3
Fe + 2HNO3 → Fe(NO3)2 + 2[H] ] x 4
8[H] + HNO3 → 3H2O + NH3
NH3 + HNO3 → NH4NO3


4Fe + 10HNO3 → 4Fe(NO3)2 + NH4NO3 + 3H2O


b. Action with less electropositive metals than hydrogen:

i. Action with copper:

-with conc. HNO3
2HNO3 → 2NO2 + H2O + [O]
[O] + Cu → CuO
CuO + 2HNO3 → Cu(NO3)2 + H2O


Cu + 4HNO3 → Cu(NO3)2 + 2NO2 + 2H2O

-With moderate conc. HNO3
2HNO3 → 2NO + H2O + 3[O]
[O] + Cu → CuO ] x 3
CuO + 2HNO3 → Cu(NO3)2 + H2O


3Cu + 8HNO3 → 3Cu(NO3)2 + 2NO + 4H2O


c. Action with non-metals (only conc.)

i. with carbon
2HNO3 → 2NO2 + H2O + [O] ] x 2
2[O] + C → CO2


C + 4HNO3 → 4NO2 + CO2 + 2H2O

ii. with sulphur
2HNO3 → 2NO2 + H2O + [O] ] x 3
2[O] + S → SO2
SO2 + [O] + H2O → H2SO4


S + 6HNO3 → H2SO4 + 6NO2 + 2H2O


c. Action with reducing agents (only conc.)

i. with H2S
2HNO3 → 2NO2 + H2O + [O]
[O] + H2S → H2O + S


H2S + 2HNO3 → 2NO2 + 2H2O + S

ii. with SO2
2HNO3 → 2NO2 + H2O + [O]
[O] + SO2 + H2O → H2SO4
SO2 + 2HNO3 → H2SO4 + 2NO2


Test of nitrate (NO3) ion (Ring test):

In a test tube, a volume of given salt solution is taken and double the volume of its conc. H2SO4 is added. The solution is cooled and freshly prepared FeSO4 solution is added from the side of the test tube. The formation of a brown ring at the junction indicates the presence of nitrate.

NO3 + H2SO4 → HNO3 + HSO4] x 2
2HNO3 → 2NO + H2O +3[O]
FeSO4 + [O] + H2SO4 → Fe2(SO4)3 + H2O ] x 3


2NO3 + 6FeSO4 + 5H2SO4 → 3Fe2(SO4)3 + 2HSO4 + 4H2O +2NO

or,
FeSO4 + NO → FeSO4.NO (Nitrosyl ferrous sulphate-Brown ring)

Uses of nitric acid:
  • In the manufacture of explosives.
  • To prepare dyes, drugs, etc.
  • To manufacture fertilizer.
  • Purification of gold and silver.
  • Lab reagent.
Some Important Questions
  1. Write the action of conc. HNO3 with H2S, SO2, C, S, Zn, Cu.
  2. Write the name of any two metals that can liberate hydrogen gas from nitric acid?
  3. What is the passivity of iron? Can other metal show such phenomenon.
  4. Why is conc. HNO3 is stored in a dark brown bottle?
  5. What do you mean by the Ring test?

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