Chlorine and Its Compounds


Chlorine and Its Compounds

Chlorine is a chemical element with the symbol Cl and atomic number 17. It belongs to Group 17 of the Periodic Table, known as the halogens. It has an atomic mass of 35.45 and an electronic configuration of 2, 8, 7. Chlorine is a greenish-yellow gas with a pungent, irritating smell. It is denser than air and poisonous, making it important to handle it with care in laboratories and industries.

Chlorine does not occur freely in nature due to its high reactivity. It is found mainly in the form of salts such as sodium chloride (NaCl) in seawater and rock salt (halite), and in minerals like sylvite (KCl) and carnallite (KCl·MgCl₂·6H₂O).



Laboratory Preparation of Chlorine


Chlorine can be prepared in the laboratory by reacting manganese(IV) oxide (MnO₂) with concentrated hydrochloric acid (HCl). The reaction is as follows: MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O


This is a redox reaction where Mn is reduced and chloride ions are oxidized. Chlorine gas produced is collected by downward delivery since it is denser than air.


Industrial Preparation of Chlorine


Industrially, chlorine is obtained by the electrolysis of brine (concentrated sodium chloride solution). During electrolysis:


At the anode: 2Cl⁻ → Cl₂ + 2e⁻


At the cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻

This process also produces hydrogen gas and sodium hydroxide (NaOH) as byproducts.


Physical Properties of Chlorine


Chlorine is a greenish-yellow gas with a pungent smell. It is poisonous, heavier than air, and soluble in water to form chlorine water. It is stored in steel cylinders under pressure due to its toxicity and reactivity.


Chemical Properties of Chlorine


  • Chlorine reacts with water to form hydrochloric acid and hypochlorous acid: Cl₂ + H₂O ⇌ HCl + HOCl


  • It reacts with metals to form metal chlorides. For example: 2Na + Cl₂ → 2NaCl Fe + 1.5Cl₂ → FeCl₃


  • It also reacts with non-metals. For instance: H₂ + Cl₂ (in the presence of light) → 2HCl


  • When chlorine reacts with cold, dilute sodium hydroxide, it undergoes disproportionation: Cl₂ + 2NaOH → NaCl + NaOCl + H₂O


  • With hot, concentrated NaOH: 3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O


Uses of Chlorine


Chlorine has a wide range of uses. It is used to purify drinking water and swimming pools due to its ability to kill bacteria. It is an essential component in the manufacture of hydrochloric acid, PVC (polyvinyl chloride), pesticides, disinfectants, and bleaching agents. It is also used in the paper and textile industries for bleaching.


Important Compounds of Chlorine


1. Hydrogen Chloride (HCl): Prepared by reacting sodium chloride with concentrated sulfuric acid:

NaCl + H₂SO₄ → NaHSO₄ + HCl

Hydrogen chloride is a colorless gas that forms hydrochloric acid when dissolved in water.


2. Hydrochloric Acid (HCl(aq)): A strong acid that ionizes completely in water. It reacts with metals to produce hydrogen gas, and with carbonates to release carbon dioxide.


3. Bleaching Powder (Ca(OCl)Cl): Made by passing chlorine over dry slaked lime:

Ca(OH)₂ + Cl₂ → Ca(OCl)Cl + H₂O

It is used for bleaching and disinfection.


4. Sodium Hypochlorite (NaOCl): The active ingredient in liquid bleach, used for household cleaning and water treatment.


5. Polyvinyl Chloride (PVC): A plastic polymer used to make pipes, cables, and packaging materials.


Health and Environmental Effects


Chlorine is toxic when inhaled and can cause severe irritation of the eyes, nose, throat, and lungs. It must be handled carefully in controlled environments. It can also contribute to environmental pollution, especially when used in the manufacture of CFCs, which deplete the ozone layer.


Summary of Common Chlorine Compounds


Hydrogen chloride (HCl): Used to make hydrochloric acid and in industrial processes.


Hydrochloric acid (HCl(aq)): Used in cleaning, digestion, and pH control.


Bleaching powder (Ca(OCl)Cl): Used in bleaching and disinfecting.


Sodium hypochlorite (NaOCl): Used in household bleach and water purification.


PVC (polyvinyl chloride): Used in making plastic materials.


Sample Questions for Practice


1. Write a balanced chemical equation for the reaction between chlorine and cold dilute sodium hydroxide.

2. State two physical properties of chlorine.

3. Explain why chlorine is stored under pressure in steel cylinders.

4. Describe a test to identify the presence of chlorine gas (e.g., moist litmus paper turns white due to bleaching action)


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