Question

In the laboratory preparation of hydrochloric acid, HCl gas is dissolved in water.
(i) Draw a diagram to show the arrangement used for the absorption of HCl in water.
(ii) Why is such an arrangement necessary? Give two reasons.
(iii) Write the chemical equation for the laboratory preparation of HCl gas when the reactants are:
(a) below 200°C
(b) above 200°C

Answer 1

(i) An inverted funnel arrangement is used for the absorption of HCl in water as shown below:

(ii) Inverted funnel arrangement is necessary because of the following reasons:
(a) When HCl gas is dissolved directly in water, it leads to back suction. HCl gas reacts rapidly with water. This creates a partial vacuum inside the delivery tube, resulting in low pressure. The air from outside exerts pressure on the water in the beaker, so as to compensate for the low pressure. The water is therefore, forced upwards through the delivery tube in the flask. This reverse rise of water in the delivery tube is known as back suction.
To avoid back suction, hydrochloric acid is prepared using the ‘Inverted Funnel Arrangement’.
(b) Funnel used in arrangement provide a large surface for dissolution of hydrochloric acid gas.

(iii) Sodium chloride and sulphuric acid are used as the main reactants for the production of hydrogen chloride gas.
At temperature less than 200⁰C, the reaction takes place as follows:

NaCl (s) + H₂SO₄ (aq) → NaHSO₄ (s) + HCl (g)
Sodium chloride Sulphuric acid Sodium hydrogen Hydrogen chloride
sulphate gas

At temperatures above 200^o C, sodium sulphate is formed as shown below:

NaCl (s) + H₂SO₄​ (aq) → NaHSO₄​ (s) + HCl (g)
Sodium chloride Sulphuric acid Sodium hydrogen Hydrogen chloride
sulphate gas

NaCl (s) + NaHSO₄ (s) → Na₂SO₄ (s) + HCl (g)
Sodium
Sulphate