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Group 15: Nitrogen - Nitric Acid

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Group 15: Nitrogen - Nitric Acid - Lesson Summary

Nitrogen forms a number of oxoacids, such as hypo nitrous acid, nitrous acid, per nitric acid and nitric acid. Of these, nitric acid is an industrially important oxoacids.

Methods to prepare nitric acid:
In the laboratory, nitric acid is prepared by heating potassium nitrate or sodium nitrate with concentrated sulphuric acid.

KNO 3                +     H 2SO 4                 →     KHSO 4 + HNO 3
potassium nitrate   sulphuric acid                                    Nitric acid
NaNO 3                +     H 2SO 4                 →     NaHSO 4 + HNO 3
Sodium nitrate       sulphuric acid                                    Nitric acid

On a commercial scale, it is prepared by Ostwald’s process. The first step in this process is the catalytic oxidation of ammonia into nitric oxide.

                                                           Pt/Rh gauze catalyst
4NHO 3(g)                +     5O 2(g)                          →                4NO(g)               + 6H 2O(g)
potassium nitrate   sulphuric acid                  1155 K , 9 bar         Nitric oxide

The nitric oxide is then oxidised to nitrogen dioxide, from which nitric acid is formed.

2NO(g)        +        O 2(g)     →              2NO 2(g)
Nitric oxide         Oxygen                  Nitrogen dioxide
3NO 2(g)        +        H 2O(g)         →              2HNO 3(aq)   + NO(g)
Nitrogen dioxide         Water                          Nitric acid

98% concentrated nitric acid, called fuming nitric acid.
Properties of nitric acid:
It is a colourless liquid that boils at 84.1 0c.and freezes to a white solid at - 41.55 0c.
Nitric acid undergoes photochemical dissociation to produce brown nitrogen dioxide gas. The brown nitrogen dioxide dissolves in the colourless nitric acid, turning it yellow.
4HNO 3                  →             4NO 2               + 2H 2O
Nitric acid                             Nitrogen dioxide

An aqueous solution of nitric acid undergoes ionisation to produce hydronium ions and nitrate ions.

 HNO 3(aq) + H 2O(l) →     H 3O +(aq)  +       NO 3 -(aq)
Nitric acid                   Hydronium ions     Nitrate ions

It is a strong acid. Hot and concentrated nitric acid acts as a powerful oxidising agent.  Metals such as copper and zinc are insoluble in hydrochloric acid. However, these metals dissolve in nitric acid. Concentrated nitric acid reacts with copper and zinc to give nitrogen dioxide.
   Cu        +   4HNO 3               →           Cu(NO 3) 2        +      2NO 2              +   2H 2O
Copper        Nitric acid(Conc.)            Copper nitrate     Nitrogen dioxide

   Zn         +   4HNO 3               →           Zn(NO 3) 2        +      2NO 2              +   2H 2O
  Zinc        Nitric acid(Conc.)              Zinc nitrate     Nitrogen dioxide
Dilute nitric acid reacts with copper to give nitric oxide, and with zinc to give nitrous oxide.

   3Cu        +   8HNO 3                →           3Cu(NO 3) 2        +      2NO              +   4H 2O
Copper        Nitric acid(Dilute)            Copper nitrate          Nitric oxide

   4Cu        +   10HNO 3                →           4Zn(NO 3) 2        +      2N 2O              +   5H 2O
   Zinc        Nitric acid(Dilute)                  Zinc nitrate          Nitric oxide
Certain metals such as chromium, aluminium and iron are rendered passive towards an attack by nitric acid. This is due to the formation of a thin layer of oxide on the surface of these metals.
Aqua regia:
It is the mixture of 1 part of conc.HNO 3 & 3 parts of conc. HCl. Noble metals like gold and platinum which cannot dissolve in conc. HNO 3Can be dissolved in aqua regia. Concentrated nitric acid oxidises non-metals such as carbon, phosphorus, sulphur and iodine to their corresponding higher oxoacids. The nascent oxygen produced from the dissociation of nitric acid oxidises these non-metals.

 C         +  4HNO 3         → H 2CO 3           +    2H 2O    +     4NO 2
Carbon      Nitric acid        Carbonic acid                      Nitrogen dioxide

P 4         +  20HNO 3       → 4H 3PO 4           +    4H 2O    +     20NO 2
Carbon      Nitric acid        Phosphoric acid                      Nitrogen dioxide

S 8         +  48HNO 3       → 8H 2SO 4           +    16H 2O    +     48NO 2
Sulphur      Nitric acid        Sulphuric acid                      Nitrogen dioxide

To confirms the presence of nitrate ions in a given salt brown ring test conducted.
Uses of nitric acid:
  • Nitric acid is widely used in the manufacture of dyes and drugs, as well as explosives such as trinitrotoluene, or TNT, nitro-glycerine and picric acid.
  • The salts of nitric acid, such as ammonium nitrate and calcium nitrate, are used as important fertilisers.
  • It is used in the purification of silver and gold.
  • It is used in the laboratory as an oxidising and a nitrating reagent.
  • It finds a major use in the pickling of stainless steel, etching of metals and as an oxidiser in rocket fuels. 


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