Write the equations for the laboratory preparation of nitric acid mentioning the conditions required.

Dear Students,

Nitric Acid (HNO3)
The most important and useful oxoacid of nitrogen is nitric acid. Its molecular formula is HNO3 and molar mass 63.01 g mol-1. It is also known as aqua fortis and spirit of niter.It is highly corrosive mineral acid.


Laboratory preparation of Nitric Acid
(1). Nitric acid is usually prepared by heating (below 200°C) sodium nitrate with concentrated sulphuric acid. This heating is done in a glass retort and vapors of nitric acid are condensed in a receiver, which is cooled by water. The reaction is:
NaNO3 + H2SO4→  NaHSO4 + HNO3
Since HNO3 attacks rubber and cork ,only glass apparatus is used.
(2) Concentrated hydrochloric acid (HCl) cannot replace Conc. Sulphuric acid (H2SO4)  for the preparation of nitric acid (HNO3) because HCl  is volatile acid and hence nitric acid vapours will carry HCl vapours.
(3) Concentrate nitric acid prepared in laboratory is yellow in colour due to the presence of dissolved nitrogen dioxide (NO2). NO2 gas is produced due to the thermal dissociation of a portion of nitric acid.
4HNO3 2H2O +4 NO2 + O2
This yellow colour of the nitric acid can be removed by:
          (a) If CO2 or dry air is bubbled through the yellow nitric acid,the acid turns colourless because it drives out nitrogen dioxide gas from warm acid which is further oxidized to nitric acid.
          (b) Addition of excess of water (H2O),NO2 gas dissolves in water and thus the yellow colour of the nitric acid is removed.
 (4) The temperature of the mixture of concentrated Sulphuric acid (H2SO4) and sodium nitrate(NaNO3) should be below 200°C because sodium sulphate (Na2SO4) formed at high temperature forms a hard crust which stick to the walls of the retort and is difficult to remove and glass apparatus may damage.At higher temperature HNO3 may also decompose.
NaNO3 + NaHSO4Na2SO4 +HNO3

Regards.

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