Nitric acid and nitrous acid are mono basic oxoacids of nitrogen. But they are very different in chemical and physical properties. Nitric acid is widely used in chemical industry for producing other chemicals. Also nitric acid is used as an strong acid and oxidizing acid in laboratories.
In this tutorial, we are leaning about characteristics, preparation and some reactions of HNO3 and HNO2 acids.
HNO2 decomposes to HNO3, NO, H2O when temperature is increased in the solution.
HNO2 does not dissociate completely in the water. Therefore its aqueous solution H+ concentration is very less when it compare with HNO2 concentration.
Nitrous acid can be oxidized because nitrogen atom is at +3 oxidation state. Therefore HNO2 can reduce acidic potassium permanganate ( H+ / KMnO4 ).
Due to unstability of nitrous acid, it is not stored in the laboratory. At requirement, we prepare the nitrous acid as below.
Solid sodium nitrate (NaNO3) is heated to decompose solid sodium nitrite (NaNO2) and oxygen (O2) gas. This reaction is a redox reaction (oxidation - reduction reaction)
Solid sodium nitrite is dissolved in water to prepare aqueous sodium nitrite. No reaction is occurred in this step.
Add cold dilute HCl to aqueous sodium nitrite solution. It gives dilute nitrous acid and NaCl solution.
Add concentrated H2SO4 acid to the solid NaNO3(s) and distill the mixture.
Ostwald process is used in industry to production of nitric acid. Ammonia and oxygen are used as raw materials in the presence of Pt / Rh catalyst. Higher temperatures and pressure are applied in the production to get a successful yield.
When the temperature of the acid solution increases, nitrous acid starts to decomposes to nitrogen dioxide, oxygen and water. Nitrogen dioxide (NO2) is a brown gas.
Nitric acid is a strong acid. But nitrous acid is a weak acid. Therefore acidic strength of nitric acid is much higher than nitrous acid.
When oxidation number increases, acidic characteristics increases. Oxidation number of nitrogen in nitric acid is +5 while it is +3 in nitrous acid. That is a one reason why nitric acid is much acidic than nitrous acid.