Chlorine

Chlorine, represented by the symbol Cl, is a chemical element found in Group 17 (the Halogen Group) and Period 3 of the Periodic Table. Carl Wilhelm Scheele discovered chlorine in 1774 by reacting hydrochloric acid (HCl) with manganese dioxide (MnO₂). In 1810, Sir Humphry Davy confirmed chlorine as an element and named it "chlorine" due to its greenish-yellow colour (from the Greek word "chloros," meaning greenish-yellow).

1.0Introduction 

Chlorine is the second lightest halogen, positioned between fluorine and bromine in the periodic table. It has an atomic number of 17 and an atomic mass of 35.45. At room temperature, chlorine is a yellow-green gas with a pungent odour. It is commonly used as a disinfectant, especially for cleaning swimming pools. Despite its effectiveness as a disinfectant, chlorine is a toxic and corrosive gas that can irritate the eyes and respiratory system upon direct exposure.

2.0Structure of Chlorine

Chlorine has an atomic number of 17, which means its atomic structure includes 17 protons in its nucleus and 17 electrons surrounding the nucleus. These electrons are distributed across three electron shells: K, L, and M. The first electron shell (K) contains 2 electrons, the second shell (L) holds 8 electrons, and the outermost shell (M), also known as the valence shell, has 7 electrons.

Since chlorine has 7 electrons in its valence shell, and chlorine needs just one more electron to complete its octet, it exhibits a valency of 1, as it tends to gain one electron during chemical reactions.

The electronic configuration of chlorine can be expressed as 1s² 2s² 2p⁶ 3s² 3p⁵

Alternatively, using noble gas notation, it can be represented as

[Ne] 3s² 3p⁵, indicating that Chlorine has 17 electrons in total, with 7 of these electrons in its valence shell.

Chlorine gas (Cl_2​) consists of diatomic molecules, meaning each molecule comprises two chlorine atoms. These two atoms are joined by a single covalent bond, with each chlorine atom sharing one electron to form the bond.

The bond length between the two chlorine atoms is approximately 199 pm (picometers).

The chlorine molecule is linear, with the two atoms directly bonded to each other.

3.0Isotopes of Chlorine

Chlorine (Cl) has 25 known isotopes, ranging from ²⁸Cl to ⁵²Cl. Among these, only two isotopes are stable: ³⁵Cl and ³⁷Cl.

1. Stable Isotopes:

• ³⁵Cl: This isotope makes up 75.77% of naturally occurring chlorine.
• ³⁷Cl: This isotope accounts for 24.23% of natural chlorine.

The relative abundances of these two stable isotopes result in chlorine having a standard atomic weight of 35.45.

2. Radioactive Isotope:

• The longest-lived radioactive isotope of chlorine is ³⁶Cl^, which has a half-life of 3.01×10^53.01 years.

4.0Preparation of Chlorine

Chlorine can be prepared using the following methods:

• By Heating Manganese Dioxide with Concentrated Hydrochloric Acid:

MnO_{2 ​}+ 4HCl → MnCl₂ ​+ Cl_{2 ​}+ 2H₂​O

In this method, manganese dioxide (MnO₂) reacts with concentrated hydrochloric acid (HCl) to produce chlorine gas (Cl₂), manganese chloride (MnCl₂), and water (H₂O). Alternatively, a mixture of common salt (NaCl) and concentrated sulfuric acid (H₂SO₄) can be used instead of HCl:

4NaCl + MnO₂​ + 4H₂​SO_4​ → MnCl_2​ + 4NaHSO₄ ​+ 2H_2​O + Cl₂​

• By the Action of Hydrochloric Acid on Potassium Permanganate:

2KMnO₄ ​+ 16HCl → 2KCl + 2MnCl₂​ + 8H_2​O +5Cl_2​

In this method, potassium permanganate (KMnO₄) reacts with hydrochloric acid (HCl) to produce chlorine gas (Cl₂), potassium chloride (KCl), manganese chloride (MnCl₂), and water (H₂O).

5.0Manufacture of Chlorine

Chlorine can be manufactured using the following methods:

• Deacon’s Process: In this method, chlorine is produced by oxidising hydrogen chloride (HCl) gas with atmospheric oxygen (O₂) in the presence of copper(II) chloride (CuCl₂) as a catalyst at 723 K:

4HCl + O₂​   → 2Cl_2​ + 2H₂​O

This process efficiently converts HCl, a by-product in various industrial processes, into chlorine gas.

• Electrolytic Process: In this method, chlorine is obtained by brine electrolysis, a concentrated sodium chloride (NaCl) solution. During electrolysis, chlorine is liberated at the anode:

2NaCl + 2H₂​O  electrolysis​  →  Cl₂​+ 2NaOH + H_2​

Chlorine is also produced as a by-product in many chemical industries, especially in the production of sodium hydroxide (NaOH) through this process.

6.0Physical and Chemical Properties of Chlorine

Physical Properties:

• Chlorine gas is greenish-yellow in appearance with a pungent and suffocating odour.
• It is approximately 2-5 times heavier than air.
• Chlorine can be easily liquefied into a greenish-yellow liquid with a boiling point of 239 K.
• It is soluble in water.

Chemical Properties:

1. Reaction with Metals and Non-Metals: Chlorine reacts with various metals and non-metals to form chlorides: 

2Al+3Cl_2​→2AlCl_3​ 

P₄​+6Cl₂​→4PCl₃​ 

2Na+Cl₂​→2NaCl 

S₈​+4Cl_2​→4S₂​Cl₂​ 

2Fe+3Cl₂​→2FeCl₃​

2. Affinity for Hydrogen: Chlorine has a strong affinity for hydrogen and reacts with hydrogen-containing compounds to form hydrogen chloride (HCl): H_2​+Cl₂​→2HCl 

H₂​S+Cl₂​→2HCl+S 

C₁₀​H₁₆​+8Cl₂​→16HCl +10C

3. Reaction with Ammonia: With excess ammonia, chlorine produces nitrogen and ammonium chloride: 

8NH₃​+3Cl_2​→6NH₄​Cl+N_2​

With excess chlorine, nitrogen trichloride (an explosive compound) is formed: NH₃​+3Cl₂​→NCl_3​+3HCl

4. Reaction with Alkali: With cold and dilute alkali, chlorine forms a mixture of chloride and hypochlorite: 

2NaOH+Cl₂​ → NaCl + NaOCl + H₂​O

With hot and concentrated alkali, chlorine forms chloride and chlorate:

6NaOH+3Cl₂​→5NaCl+NaClO_3​+3H_2​O

5. Reaction with Slaked Lime: When chlorine reacts with dry slaked lime, it produces bleaching powder: 

2Ca(OH)₂​+2Cl_2​→Ca(OCl)_2​+CaCl₂​+2H_2​O

The composition of bleaching powder is Ca(OCl)₂​⋅CaCl₂​⋅Ca(OH)₂​⋅2H₂​O.

Chlorine reacts with hydrocarbons in the following ways:

6. Reaction with hydrocarbons

Chlorine undergoes substitution reactions. For example:CH4​+Cl₂​UV​CH3​Cl+HCl(Methane to Methyl Chloride)

• Reaction with Unsaturated Hydrocarbons: Chlorine undergoes addition reactions. For example:

C2​H4​+Cl2​Room temp.​C2​H4​Cl2​            Ethene to 1,2-Dichloroethane)

Note: When chlorine water is left standing, it loses its yellow colour due to hydrochloric acid (HCl) and hypochlorous acid (HOCl) formation. The hypochlorous acid formed releases nascent oxygen, which is responsible for chlorine's oxidising and bleaching properties.

7. Oxidising Reactions:

• Oxidation of Ferrous to Ferric:2FeSO₄​+H_2​SO_4​+Cl₂​→Fe_2​(SO₄​)_3​+2HCl
• Oxidation of Sulphite to Sulphate:Na_2​SO₃​+Cl_2​+H₂​O→Na₂​SO₄​+2HCl
• Oxidation of Sulphur Dioxide to Sulphur Trioxide:SO_2​+2H₂​O+Cl₂​→H₂​SO_4​+2HCl
• Oxidation of Iodine to Iodate:I2​+6H₂​O+5Cl_2​→2HIO₃​+10HCl

8. Bleaching Action:

Chlorine is a powerful bleaching agent due to its ability to oxidise. The reaction responsible for bleaching is:

Cl₂+H₂O→2HCl+[O]

Here, the nascent oxygen [O] oxidises the coloured substance, turning it colourless.

7.0Uses of Chlorine

Chlorine has several key applications:

• Bleaching: Used for wood pulp, cotton, and textiles.
• Metal Extraction: Involved in extracting gold and platinum.
• Chemical Manufacturing: Essential for dyes, pharmaceuticals, and organic compounds like CCl₄ and CHCl₃.
• Water Treatment: Disinfects and sterilises drinking water.
• Poison Preparation: Used to make toxic gases such as phosgene and chlorine trifluoride.