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Chlorine
Chlorine is one of the most essential non-metallic elements in the periodic table, represented by the symbol Cl and atomic number 17. Belonging to Group 17 (Halogens) of the periodic table, it is a highly reactive element commonly used in water purification, bleaching, and various chemical industries.
It exists as a greenish-yellow gas at room temperature, with a strong and pungent odor. Chlorine was discovered by Carl Wilhelm Scheele in 1774, and later identified as an element by Sir Humphry Davy in 1810.
1.0Position of Chlorine in the Periodic Table
Chlorine lies between fluorine and bromine in the halogen family, making it less reactive than fluorine but more reactive than bromine.
2.0Occurrence of Chlorine
Chlorine does not occur freely in nature due to its high reactivity. It is found in combined form as chlorides in minerals and sea water.
Major Sources of Chlorine
- Sodium chloride (NaCl): Common salt, found abundantly in seawater and salt lakes.
- Sylvite (KCl): Mineral source of potassium and chlorine.
- Carnallite (KCl·MgCl₂·6H₂O): Double salt containing magnesium and chlorine.
Sea water contains about 2% sodium chloride, making it the major source of chlorine production.
3.0Physical Properties of Chlorine
Explanation
When dissolved in water, chlorine forms a weak solution known as chlorine water, which exhibits both oxidizing and bleaching properties.
4.0Chemical Properties of Chlorine
Chlorine is a highly reactive element, readily forming compounds with metals and nonmetals. Its reactivity is mainly due to its strong oxidizing nature.
1. Reaction with Hydrogen
Chlorine reacts with hydrogen explosively in the presence of sunlight:
H2 + Cl2 → 2HCl
This reaction forms hydrogen chloride gas.
2. Reaction with Metals
It combines with many metals to form metal chlorides.
2Na + Cl2 → 2NaCl
Fe + Cl2 → FeCl3
3. Reaction with Non-metals
Chlorine also reacts with non-metals like phosphorus and sulfur:
2P + 3Cl2 → 2PCl3
S + Cl2 → SCl2
4. Reaction with Water
When dissolved in water, chlorine partially decomposes:
Cl2 + H2O ⇌ HCl + HOCl
Here, hypochlorous acid (HOCl) is formed, responsible for the bleaching and disinfecting action of chlorine.
5.0Preparation of Chlorine
Chlorine can be prepared both in the laboratory and industrially.
A. Laboratory Preparation of Chlorine
1. From Hydrochloric Acid and Manganese Dioxide
When concentrated hydrochloric acid reacts with manganese dioxide (MnO₂), chlorine gas is liberated.
MnO2 + 4HCl → MnCl2 + 2H2O + Cl2↑
Apparatus setup:
A flask fitted with a delivery tube and a gas jar for collecting chlorine by the upward displacement of air (since chlorine is heavier than air).
2. By Oxidation of HCl
Concentrated hydrochloric acid can be oxidized by potassium permanganate (KMnO₄) or lead dioxide (PbO₂) to release chlorine.
B. Industrial Preparation of Chlorine
1. By Electrolysis of Brine
Electrolysis of aqueous sodium chloride (brine) is the most common method:
2NaCl + 2H2O → 2NaOH + H2 + Cl2↑
At the anode, chlorine gas is liberated. This process also produces caustic soda (NaOH) and hydrogen gas, collectively known as the Chlor-Alkali process.
6.0Bleaching Action of Chlorine
Chlorine acts as a strong bleaching agent due to the oxidizing power of nascent oxygen (O) released during its reaction with water:
Cl2 + H2O → HCl + [O]
The nascent oxygen oxidizes coloring matter to colorless substances.
Applications of bleaching:
- Whitening of cotton fabrics.
- Bleaching of paper pulp.
- Removal of stains from textiles.
7.0Disinfecting Action of Chlorine
Chlorine destroys harmful microorganisms present in water and air. It reacts with water to produce hypochlorous acid, which acts as a powerful germicide.
Applications:
- Sterilization of drinking water.
- Disinfection of swimming pools.
- Used in sewage treatment.
8.0Industrial Production: The Chlor-Alkali Process
Chlorine is rarely found free in nature due to its reactivity. It is produced almost exclusively through the electrolysis of brine (saturated sodium chloride solution). This industrial method is known as the Chlor-Alkali Process.
Electrolysis Mechanism
When electricity is passed through brine, the ions dissociate and migrate to their respective electrodes. The overall chemical equation for the process is:
Three valuable products are generated:
- Chlorine Gas: Produced at the Anode (positive electrode).
Anode Reaction:
- Hydrogen Gas: Produced at the Cathode (negative electrode).
Cathode Reaction:
- Sodium Hydroxide (NaOH): Commonly known as caustic soda, remains in the solution.
Membrane Cell Technology
Modern production utilizes Membrane Cells rather than the older Mercury or Diaphragm cells. An ion-selective membrane separates the anode and cathode compartments. This membrane allows positive sodium ions (Na^+) to pass through to the cathode side but blocks chloride ions (Cl^−) and hydroxide ions (OH^−). This results in a much purer Sodium Hydroxide product and uses significantly less energy, making it the most environmentally standard method today.
9.0Uses of Chlorine
Chlorine has numerous industrial, domestic, and scientific uses.
1. In Water Purification
Chlorine is widely used to disinfect water supplies, making it safe for human consumption.
2. In the Chemical Industry
It is used in the manufacture of:
- Hydrochloric acid (HCl)
- PVC (Polyvinyl Chloride)
- Chlorofluorocarbons (CFCs)
- DDT and other pesticides
3. In Bleaching
Used for bleaching wood pulp, textiles, and paper.
4. In Medicines
Chlorine derivatives are used in the preparation of antiseptics and disinfectants.
10.0Environmental and Health Effects of Chlorine
Although chlorine is highly useful, excessive exposure can be harmful.
1. Health Hazards
- Inhalation of chlorine gas can irritate the respiratory system.
- High concentration may cause lung damage or suffocation.
2. Environmental Impact
- Chlorine compounds like CFCs deplete the ozone layer.
- Overuse in water treatment can produce toxic by-products like trihalomethanes.
Safety Precautions
- Use proper ventilation when handling chlorine.
- Avoid direct inhalation and wear protective equipment.