Draw a labelled diagram for the laboratory preparation of hydrogen chloride gas and answer the following.

(a) Name the acid used. Why is this particular acid preferred to other acids?

(b) Give the balanced equation for the reaction.

(c) Name the drying agent used in drying hydrogen chloride gas.

(d) Phosphorus pentoxide and calcium oxide are good drying agents, but they cannot be used to dry hydrogen chloride gas. Why?

(e) Why is direct absorption of HCl gas in water not feasible?

(f) What arrangement is done to dissolve HCl gas in water.

The labelled diagram for the laboratory preparation of hydrogen chloride gas is shown below:

(a) Conc. H₂SO₄ is used for the preparation of hydrogen chloride gas in the laboratory.

As conc. H₂SO₄ is non-volatile and has a high boiling point, therefore, it displaces the volatile hydrogen chloride from the salt sodium chloride. Hence, conc. H₂SO₄ is used as a reactant in the laboratory preparation of HCl from sodium chloride.

(b) $\text{NaCl} + \text{H}$2\text{SO}4 \xrightarrow{\lt 200 \degree\text{C}} \text{NaHSO}_4 + \text{HCl [g.]}

(c) Conc. H₂SO₄

Reason — Drying agent used for drying should only remove the moisture and not react with it, hence, conc. sulphuric acid is used as the drying agent.

(d) Phosphorus pentoxide and calcium oxide are good drying agents, but they cannot be used to dry hydrogen chloride gas since they react with it.

2P₂O₅ + 3HCl ⟶ POCl₃ + 3HPO₃

CaO + 2HCl ⟶ CaCl₂ + H₂O

(e) Direct absorption of hydrogen chloride gas in water is not feasible because it causes back suction in the delivery tube.

1. Hydrogen chloride gas is extremely soluble in water.
2. If a delivery tube through which HCl gas is passed is directly immersed in water, the rate of absorption of HCl gas is high and hence a partial vacuum is created in the tube.
3. The pressure outside being higher causes the water to be pushed up into the delivery tube and damages the apparatus. This is called back-suction.

(f) A special funnel arrangement is used to dissolve HCl gas in water as it provides a large surface area for absorption while preventing back-suction. The rim of the funnel is positioned so that it just touches the water in the trough. If back-suction occurs, the water rises up the funnel and the air gap between the rim of the funnel and the water's surface equalizes the pressure inside and outside.