Chlorine (Cl) - Properties, Reactions & Importance

Introduction

Chlorine (Cl) is a highly reactive non-metallic element belonging to Group 17 (Halogens) of the periodic table. It is crucial in various industrial processes and biological systems. As a greenish-yellow gas (Cl₂), its strong oxidizing properties make it an indispensable agent in water treatment, sanitation, and the synthesis of numerous organic and inorganic compounds. Its widespread applications underscore its significance in modern chemistry and daily life.

CBSE/JEE Quick Revision Notes

• Atomic Number (Z): 17
• Atomic Mass: 35.45 u
• Electronic Configuration: [Ne] 3s² 3p⁵
• Period: 3
• Group: 17 (Halogens)
• Block: p-block
• Valency: Typically -1; exhibits +1, +3, +5, +7 in oxoacids and oxides.
• Electronegativity (Pauling): 3.16 (High)
• Physical State (Room Temperature): Greenish-yellow gas (Cl₂)
• Nature: Highly reactive non-metal, strong oxidizing agent.
• Isotopes: Two main stable isotopes, ³⁵Cl (75%) and ³⁷Cl (25%).

Electron Configuration & Bonding Behavior

Chlorine has the electron configuration [Ne] 3s² 3p⁵. With seven valence electrons, it readily accepts one electron to achieve a stable octet, forming the chloride ion (Cl⁻).

Oxidation States

Chlorine exhibits a variety of oxidation states:

• -1: Most common, as in metal chlorides (e.g., NaCl, HCl).
• +1: In hypochlorous acid (HOCl) and hypochlorites (e.g., NaOCl), and chlorine monoxide (Cl₂O).
• +3: In chlorous acid (HClO₂) and chlorites (e.g., NaClO₂), and chlorine trifluoride (ClF₃).
• +5: In chloric acid (HClO₃) and chlorates (e.g., NaClO₃).
• +7: In perchloric acid (HClO₄) and perchlorates (e.g., NaClO₄), and dichlorine heptoxide (Cl₂O₇).

Bonding Types

1. Ionic Bonding: With highly electropositive metals (e.g., NaCl).
2. Covalent Bonding:
   • Diatomic Molecule (Cl₂): Forms a single covalent bond with another chlorine atom.
   • With Non-metals: Forms covalent compounds (e.g., HCl, CCl₄, PCl₅).
   • Interhalogen Compounds: Forms compounds like ClF, ClF₃, ClF₅ where chlorine can be the central atom with positive oxidation states.

Crucial Chemical Reactions

1. Preparation of Chlorine Gas (Cl₂)

• Deacon’s Process:

  4HCl(g) + O₂(g) --(CuCl₂, 723 K)--> 2Cl₂(g) + 2H₂O(g)

• Electrolytic Process (Castner-Kellner Cell for Brine Electrolysis):

  2NaCl(aq) + 2H₂O(l) --(electrolysis)--> 2NaOH(aq) + Cl₂(g) + H₂(g)

• From HCl and Oxidizing Agents:

  MnO₂(s) + 4HCl(aq) → MnCl₂(aq) + Cl₂(g) + 2H₂O(l)
  2KMnO₄(s) + 16HCl(aq) → 2KCl(aq) + 2MnCl₂(aq) + 5Cl₂(g) + 8H₂O(l)

2. Reactions with Metals

Chlorine reacts with metals to form their respective chlorides.

• With Sodium:

  2Na(s) + Cl₂(g) → 2NaCl(s)

• With Iron:

  2Fe(s) + 3Cl₂(g) → 2FeCl₃(s) (Anhydrous ferric chloride)

3. Reactions with Non-metals

• With Hydrogen:

  H₂(g) + Cl₂(g) --(diffused sunlight)--> 2HCl(g)

• With Phosphorus:

  P₄(s) + 6Cl₂(g) → 4PCl₃(l) (Limited Cl₂)
  P₄(s) + 10Cl₂(g) → 4PCl₅(s) (Excess Cl₂)

• With Sulfur:

  S₈(s) + 4Cl₂(g) → 4S₂Cl₂(l)

4. Reactions with Compounds

• With Water (Hydrolysis):

  Cl₂(g) + H₂O(l) ⇌ HCl(aq) + HOCl(aq) (Hypochlorous acid)

  This reaction is responsible for chlorine’s bleaching and disinfecting action (due to HOCl).
• With Cold, Dilute NaOH:

  Cl₂(g) + 2NaOH(aq) → NaCl(aq) + NaOCl(aq) + H₂O(l) (Sodium hypochlorite - bleaching agent)

• With Hot, Concentrated NaOH:

  3Cl₂(g) + 6NaOH(aq) → 5NaCl(aq) + NaClO₃(aq) + 3H₂O(l) (Sodium chlorate)

• With Slaked Lime (Calcium Hydroxide):

  Ca(OH)₂(s) + Cl₂(g) → CaOCl₂(s) + H₂O(l) (Bleaching powder)

• With Hydrogen Sulfide:

  H₂S(g) + Cl₂(g) → 2HCl(g) + S(s)

• Substitution Reaction with Methane (Photochemical):

  CH₄(g) + Cl₂(g) --(hv)--> CH₃Cl(g) + HCl(g)

  Further substitution can occur to form CH₂Cl₂, CHCl₃, and CCl₄.
• Addition Reaction with Ethene:

  C₂H₄(g) + Cl₂(g) → C₂H₄Cl₂(l) (1,2-dichloroethane)

Industrial and Biological Importance

Industrial Importance

• Water Purification: Chlorine and its compounds (e.g., bleaching powder, sodium hypochlorite) are widely used as disinfectants for drinking water and swimming pools due to their ability to kill bacteria and viruses.
• Bleaching Agent: Employed in the paper and textile industries for bleaching pulp and fabrics.
• Manufacturing of Organic Compounds: Essential in the synthesis of polymers (e.g., PVC - polyvinyl chloride), refrigerants, solvents (e.g., CCl₄, chloroform), pesticides (e.g., DDT, though largely phased out), and pharmaceuticals.
• Production of Hydrochloric Acid (HCl): A major industrial acid.
• Extraction of Metals: Used in the extraction of gold and platinum.
• Synthesis of Bleaching Powder (CaOCl₂): A solid bleaching and disinfecting agent.

Biological Importance

• Electrolyte Balance: Chloride ions (Cl⁻) are the most abundant anions in the extracellular fluid of the human body, playing a crucial role in maintaining osmotic pressure and acid-base balance.
• Gastric Acid: Chloride ions are essential for the production of hydrochloric acid (HCl) in the stomach, which is vital for digestion and activating digestive enzymes.
• Nerve Impulse Transmission: Chloride channels in cell membranes are involved in regulating nerve impulse transmission and muscle contraction.
• Toxicity: Gaseous chlorine (Cl₂) is highly toxic and corrosive to respiratory tissues, acting as a potent pulmonary irritant.