Question

(a) Write a balanced chemical equation for the laboratory preparation of nitric acid.
(b) In the preparation of nitric acid from $KN{O}_3$, concentrated hydrochloric acid is not used in place of concentrated sulphuric acid. Explain why.
(c) Conc. nitric acid prepared in laboratory is yellow in colour. Why? How is this colour removed? [4 MARKS]

Answer 1

a) 1 Mark
b) 1 Mark
c) 2 Marks

(a) $\underset{Potassiumnitrate}{KN{O}_3}+\underset{Sulphuricacid}{H_{2}S{O}_4}\stackrel{<{200}^{\circ }C}{\to }\underset{Potassiumbisulphate}{KHS{O}_4}+\underset{Nitricacid}{HN{O}_3}$

(b) In the preparation of nitric acid from $KN{O}_3$, concentrated hydrochloric acid is not used in place of concentrated sulphuric acid because hydrochloric acid is volatile and hence nitric acid vapours will carry hydrochloric acid vapours. Conc.$HN{O}_3$ can also oxidise HCl to $C{l}_2$.

(c) The yellow colour of conc. nitric acid prepared in the laboratory is due to the dissolution of reddish brown coloured nitrogen dioxide gas in it. This gas is produced by the thermal decomposition of a portion of nitric acid.
$4HN{O}_3\to 2H_{2}O+4N{O}_2+O_2$
Removal of colour: If dry air or $C{O}_2$ is bubbled through the yellow acid, the latter turns colourless because the nitrogen dioxide comes out from the warm acid and gets oxidized to form nitric acid. The reaction is the reverse of the decomposition of nitric acid.