Study of Compounds- Hydrogen Chloride-ICSE-Class 10|Biswajit Das

General Information and Introduction

• Formula and Molecular Mass: The chemical formula for hydrogen chloride is HCl. Its molecular mass is approximately 36.5 g/mol.
• Nature: At room temperature, hydrogen chloride is a colorless gas with a pungent, choking smell. It is a covalent compound, but when dissolved in water, it ionizes to form hydrochloric acid (H+ and Cl− ions).
• Physical Properties: It is highly soluble in water, and it is denser than air (approximately 1.28 times heavier). It fumes in moist air due to the formation of tiny droplets of hydrochloric acid.

2. Laboratory Preparation of Hydrogen Chloride Gas

• Reactants: The lab preparation uses sodium chloride (NaCl) and concentrated sulfuric acid (H2​SO4​).
• Reaction and Conditions: The reaction is carried out at a temperature below 200°C. Heating at a higher temperature is avoided to prevent the formation of a hard crust of sodium sulfate (Na2​SO4​), which is difficult to remove from the flask.
  • NaCl+H2​SO4​<200°C​NaHSO4​+HCl
• Drying and Collection: The HCl gas is dried by passing it through concentrated sulfuric acid. Quicklime (CaO) is not used as a drying agent because it’s a basic oxide and would react with the acidic gas. The gas is collected by the upward displacement of air because it’s denser than air and is highly soluble in water, so it cannot be collected over water.

3. The Fountain Experiment

This is a classic experiment used to demonstrate two key properties of HCl gas:

1. High solubility in water: As the gas dissolves rapidly in the flask, it creates a partial vacuum.
2. Acidic nature: The blue litmus solution from the beaker is forced into the flask by atmospheric pressure, where it turns red, confirming the acidic nature of the resulting hydrochloric acid.

4. Preparation of Hydrochloric Acid

Hydrochloric acid is prepared by dissolving HCl gas in water. A special funnel arrangement is used to prevent back-suction (water rushing back into the gas generator). The wide mouth of the inverted funnel increases the surface area for the absorption of the gas and, in case of a pressure drop, the funnel detaches from the water surface, preventing back-suction.

5. Chemical Properties of Hydrochloric Acid

• Reactions with Metals: Dilute HCl reacts with active metals (above hydrogen in the reactivity series) to produce a salt and hydrogen gas.
  • Zn+2HCl→ZnCl2​+H2​
• Reactions with Bases: As an acid, it neutralizes bases and metallic oxides/hydroxides to form salt and water.
  • NaOH+HCl→NaCl+H2​O
  • CuO+2HCl→CuCl2​+H2​O
• Reactions with Carbonates and Bicarbonates: It reacts with carbonates and bicarbonates to produce a salt, water, and carbon dioxide gas.
  • Na2​CO3​+2HCl→2NaCl+H2​O+CO2​
• Reactions with Sulfites and Sulfides: It reacts with sulfites and sulfides to produce sulfur dioxide (SO2​) and hydrogen sulfide (H2​S) gas, respectively.
• Reaction with Silver Nitrate: This is a test for the presence of chloride ions (Cl−). HCl reacts with silver nitrate (AgNO3​) to form a curdy white precipitate of silver chloride (AgCl), which is soluble in ammonium hydroxide (NH4​OH).
  • AgNO3​+HCl→AgCl↓+HNO3​
• Aqua Regia: A mixture of concentrated hydrochloric acid and concentrated nitric acid in a 3:1 ratio by volume is called aqua regia. It can dissolve noble metals like gold and platinum.

6. Uses of Hydrochloric Acid

• Industrial Uses: Used for cleaning steel (“pickling”), in the production of chlorides, and in the tanning of leather.
• Domestic Uses: Used as a cleaning agent for tiles and sanitary ware.
• In the Human Body: It is a vital component of gastric juice in the stomach, helping in the digestion of food