Question

1. Write a balanced chemical equation for the laboratory preparation of nitric acid.
2. In the preparation of nitric acid from KNO₃, concentrated hydrochloric acid is not used in place of concentrated sulphuric acid. Explain why?
3. Conc. nitric acid prepared in the laboratory is yellow in colour. Why? How is this colour removed?
4. Give reasons for the following:
   In the laboratory preparation of nitric acid, the mixture of concentrated sulphuric acid and sodium nitrate should not be heated very strongly above 200°C.

Answer 1

(a) The chemical equation is:

\[\ce{KNO3 + H2SO4 ->[<200^\circ C] KHSO4 + HNO3}\]

\[\ce{NaNO3 + \underset{(Conc.)}{H2SO4} ->[<200^\circ C] NaHSO4 + HNO3}\]

(b) Concentrated hydrochloric acid cannot replace conc. sulphuric acid for the preparation of nitric acid because hydrochloric acid is a volatile acid, and hence nitric acid vapours will carry HCl vapours.

(c) Conc. Nitric acid prepared in the laboratory is yellow in colour due to the dissolution of reddish-brown-coloured nitrogen dioxide (NO₂) gas in acid. This gas is produced due to the thermal dissociation of a portion of nitric acid.

\[\ce{4HNO3 -> 2H2O + 4NO2 + O2}\]

The yellow colour of the acid is removed:

If dry air or CO₂ is bubbled through the yellow acid, the acid turns colourless because it drives out NO₂ from warm acid, which is further oxidised to nitric acid.

By the addition of an excess of water, nitrogen dioxide gas dissolves in water, and thus the yellow colour of the acid is removed.

(d) The temperature of the mixture of concentrated sulphuric acid and sodium nitrate should not exceed 200°C because sodium sulphate formed at higher temperatures forms a hard crust that sticks to the walls of the retort and is difficult to remove. At higher temperatures, nitric acid may also decompose.

\[\ce{NaNO3 + NaHSO4 ->[>200°C]Na2SO4 + HNO3}\]

Answer 2

(a) Nitric acid is obtained by distilling conc. H₂SO₄ with nitrates of potassium (KNO₃) (nitre) or Sodium nitrate (chile saltpetre) NaNO₃.

Reaction:

\[\ce{KNO3 + H2SO4 ->[<200^\circ C] KHSO4 + HNO3}\]

\[\ce{NaNO3 + \underset{(Conc.)}{H2SO4} ->[<200^\circ C] NaHSO4 + HNO3}\]

(b) To prepare nitric acid from KNO₃, concentrated hydrochloric acid is not utilised instead of concentrated sulphuric acid due to its volatility, which can convey hydrochloric acid vapours.

(c) Yellow colour is due to the disintegration of reddish brown. Coloured NO₂ gas in the acid. To erase the colour, add extra water, dissolve NO₂, and the colour will be gone. Alternatively, bubble dry air or CO₂ through the acid.

(d) Higher temperatures cause sodium sulphate to create a hard crust that adheres to retort walls and is difficult to remove.