Write the equations for the laboratory preparation of nitric acid mentioning the conditions required.
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 + NaHSO4→Na2SO4 +HNO3
Nitric acid is a very strong and corrosive mineral acid, also called aqua fortis or spirit of niter.
Formula and structure: The chemical formula of nitric acid is HNO3. Its molecular formula is written as NHO3 and its molar mass is 63.01 g/mol. The chemical structure of nitric acid is shown below, with its resonance forms:
The HNO3 molecule is planar with the nitrogen attached to three oxygen atoms, one of which holds the proton. Two of the N-O bonds are equivalent and show resonance with double bond character.
Preparation: Nitric acid is prepared by reaction of nitrogen dioxide (NO2) with water.
3 NO2 + H2O → 2 HNO3 + NO
The nitric oxide (NO) by-product usually gets oxidized again by the oxygen in air to produce additional nitrogen dioxide starting material.
The commercial production of nitric acid is by oxidizing anhydrous ammonia to nitric oxide, in the presence of platinum catalyst at a high temperature (Ostwald process).