Question

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

1. Name the acid used. Why is this particular acid preferred to other acids?
2. Give the balanced equation for the reaction.
3. Name the drying agent used in drying hydrogen chloride gas.
4. Phosphorous pentoxide and calcium oxide are good drying agents, but they cannot be used to dry hydrogen chloride gas. Why?
5. Why is the direct absorption of HCl gas in water not feasible?
6. What arrangement is done to dissolve HCl gas in the water?

Answer 1

a. Hydrogen chloride gas when dissolved in water forms hydrochloric acid. The gas is very soluble in water and so cannot be transferred through water via a delivery tube because it causes back suction, which means that water can move back into the hot flask via the delivery tube. The delivery tube is attached to a funnel, and gas is dissolved in water. This avoids back suction of water.

b. \[\ce{NaCl + H2SO4 ->[<200^\circ C]NaHSO4 + HCl ^}\]

\[\ce{2NaCl + \underset{(Conc.)}{H2SO4} ->[above 200^\circ C] Na2SO4 + 2HCl}\]

c. The drying agent used in drying hydrogen chloride gas is conc. Sulphuric acid.

d. Phosphorous pentoxide (P₂O₅) and calcium oxide (CaO) are good drying agents, but they cannot be used to dry hydrogen chloride gas because they react with hydrogen chloride.

\[\ce{2P2O5 + 3HCl -> POCl3 + 3HPO3}\]

\[\ce{CaO + 2HCl -> CaCl2 + H2O}\]

e. Direct absorption of hydrogen chloride gas in water is not feasible as it leads to back suction. The reaction is very exothermic.

f. The ‘inverted funnel arrangement’ is used to dissolve hydrogen chloride gas in water. It provides a large surface area and prevents back suction.