ICSE Class 10 Chemistry Chapter 08 Hydrogen Chloride

Hydrogen Chloride

Important Points To Remember

1. Hydrogen chloride gas was first prepared by Glauber. Humphry Davy proved the gas to be compound of hydrogen and chlorine.

2. It is a polar covalent compound.

3. It is synthesized from its elements in the presence of diffused sunlight.

\[H_2 + Cl_2 \xrightarrow{\text{diffused sunlight}} 2HCl\]

4. Hydrogen chloride is prepared in laboratory by the reaction of metallic chloride with concentrated Sulphuric acid (Fig. 1)

Reactants used for the preparation of Hydrogen chloride gas are Sodium chloride and concentrated Sulphuric acid. Sodium chloride is preferred over other metallic chlorides as it is easily and cheaply available.

\[NaCl + H_2SO_4(conc.) \xrightarrow{\text{below 200}°C} NaHSO_4 + HCl\]

Rock salt - Sodium bisulfate - Hydrogen chloride gas

\[2NaCl + H_2SO_4(conc.) \xrightarrow{\text{above 200}°C} Na_2SO_4 + 2HCl\]

Rock salt - Sodium sulphate - Hydrogen chloride gas

5. Precautions observed during the laboratory preparation of Hydrogen chloride gas.

(i) The reaction mixture should not be heated beyond 200°C. As above 200°C a sticky mass of Sodium sulphate is formed which sticks to glass apparatus and is difficult to remove.

(ii) Heat energy is wasted if heated above 200°C.

6. Hydrogen chloride gas is dried by passing through concentrated Sulphuric acid. It is not dried by passing through Phosphorous pentaoxide and Calcium oxide as both of these drying agents undergo chemical reaction with Hydrogen chloride gas.

\[2P_2O_5 + 3HCl \longrightarrow POCl_3 + 3HPO_3\]

\[CaO + 2HCl \longrightarrow CaCl_2 + H_2O\]

Teacher's Note

Hydrogen chloride is commonly used in laboratory settings and industrial processes. Understanding its preparation and properties helps us appreciate how many everyday cleaning products and food processing chemicals are produced safely.

7. Hydrogen chloride gas is collected by upward displacement of air as the gas is heavier than air and it is not collected over water as it is highly or extremely soluble in water. Therefore it forms Hydrochloric acid.

8. When the gas jar is completely filled with the gas then dense white fumes appear at the mouth of the jar.

9. In order to know whether the gas jar is full of Hydrogen chloride gas then bring a glass rod dipped in Ammonium hydroxide near the mouth of the gas jar. If dense white fumes appear immediately, then it shows that the gas jar is full of Hydrogen chloride gas.

10. Hydrogen chloride gas is dissolved in water with the help of special arrangement called funnel arrangement to prevent the back suction of water and gives greater surface area for the absorption of gas.

11. The physical properties of Hydrogen chloride gas are

(a) It is a colorless gas having pungent suffocating smell which gives dense fumes in moist air.

(b) It is highly or extremely soluble in water.

The extreme solubility of Hydrogen chloride gas is demonstrated by fountain experiment. (Fig. 3)

Teacher's Note

The fountain experiment dramatically shows how some gases dissolve extremely well in water, which is why HCl gas immediately forms hydrochloric acid when exposed to moisture - important to remember when handling these chemicals safely in labs or industrial settings.

(c) Hydrogen chloride gas is heavier than air. Set up the apparatus as shown in fig. 4

It is seen that the dense white fumes appear more closer to the Hydrogen chloride gas suggesting that the gas is heavier than air as it has travelled less distance as compared to air (Fig. 4).

12. Hydrogen chloride gas is neither combustible nor it is a supporter of combustion. It extinguishes the burning splinter.

13. Hydrogen chloride gas is thermally decomposed to give Chlorine.

\[2HCl \xrightarrow{500°C} H_2 + Cl_2\]

-14. Metals which are lying above Hydrogen in the metal activity series displace Hydrogen when heated with hydrogen chloride gas.

\[Mg + 2HCl \xrightarrow{\triangle} MgCl_2 + H_2\]

\[Zn + 2HCl \xrightarrow{\triangle} ZnCl_2 + H_2\]

\[Fe + 2HCl \xrightarrow{\triangle} FeCl_2 + H_2\]

\[Ca + 2HCl \xrightarrow{\triangle} CaCl_2 + H_2\]

15. Hydrogen chloride gas reacts with ammonia to form dense white fumes of Ammonium chloride.

\[NH_3 + HCl \longrightarrow NH_4Cl\]

Dense white fumes

16. Hydrogen chloride gas is a polar covalent compound which on dissolving in water produces ions.

17. Hydrogen chloride gas on dissolving in water produces strong, monobasic acid (Hydrochloric acid).

\[HCl + H_2O \longrightarrow H_3O^+ + Cl^-\]

18. Dry Hydrogen chloride gas and liquified Hydrogen chloride gas has no effect on litmus, showing the neutral (non-acidic) character of the dry gas.

19. Liquified Hydrogen chloride gas does not allow the electric current to pass through it, showing the covalent nature of Hydrogen chloride gas.

20. In laboratory Hydrochloric acid is prepared by dissolving Hydrogen chloride gas in water. The gas is dissolved in Water with the help of funnel arrangement in order to prevent the back suction of Water and provides maximum area for the absorption of Hydrogen chloride gas.

21. Dilute Hydrochloric acid is a typical acid. So, it undergoes the following reactions:

(a) Reaction with Indicator: Dilute solution of Hydrochloric acid is strongly acidic and it shows characteristic colours with indicators.

Teacher's Note

Indicators are used in chemistry labs and everyday applications like swimming pool testing to determine acidity - understanding how acids change indicator colors helps us monitor pH in water treatment and food preparation.

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