Question

Study the reaction given below and answer the questions that follow:
NaNO₃ + H₂SO₄ → NaHSO₄ + HNO₃ (below 200°C)
(a) Give one reason for maintaining the temperature below 200°C.
(b) Why is concentrated Sulphuric acid used in the above reaction?

Hint:

Nitric acid prepared in the lab often appears yellow. This is due to the presence of dissolved nitrogen dioxide (NO_2) produced by the slight decomposition of the acid!

Answer 1

(a) Reason for temperature < 200°C:
Maintaining the temperature below 200°C is important for several reasons:
1. It prevents the formation of a hard crust of sodium sulphate (Na2SO4), which is difficult to remove from the glass retort.
2. It prevents the thermal decomposition of the product, nitric acid (HNO3).
3. It saves fuel and prevents the glass apparatus from cracking.

(b) Use of concentrated sulphuric acid:
Concentrated sulphuric acid is used because it is a non-volatile acid. It can displace the more volatile nitric acid from its salt (sodium nitrate) when heated.