Question
How is chlorine prepared in the laboratory and industrially? List the important chemical properties and uses of chlorine.
(NCERT Class 12, Chapter 7 — The p-Block Elements)
Solution — Step by Step
Chlorine is prepared by heating MnO₂ with concentrated HCl:
Here, HCl acts as both an acid (reacting with MnO₂) and a reducing agent (getting oxidised from to ). The gas is collected by upward displacement of air since Cl₂ is denser than air.
Industrially, chlorine is obtained by the electrolysis of brine (concentrated NaCl solution) — the Castner-Kellner process:
Cl₂ is released at the anode (oxidation of Cl⁻ ions).
Reaction with metals: Chlorine reacts with most metals to form metal chlorides.
Reaction with non-metals: (limited Cl₂)
Reaction with water:
HOCl (hypochlorous acid) is the actual bleaching agent — it’s a powerful oxidiser.
Reaction with NaOH:
- Cold dilute NaOH:
- Hot concentrated NaOH:
- Water purification — kills bacteria and pathogens
- Bleaching — of paper pulp, textiles (via HOCl)
- Manufacturing — PVC, pesticides (DDT, BHC), refrigerants (CFCs)
- Preparation of HCl, bleaching powder, chloroform
Why This Works
Chlorine is a strong oxidising agent (it readily gains electrons: ). This explains its reactivity with metals, its bleaching action, and its germicidal properties.
The reaction with NaOH is a disproportionation — chlorine simultaneously gets oxidised and reduced. In cold NaOH, it forms NaOCl (bleach). In hot NaOH, it forms NaClO₃ (chlorate). The temperature determines the product — this is a favourite NEET question.
Alternative Method — Summary of Reactions
| Reaction | Product | Condition |
|---|---|---|
| Cl₂ + H₂O | HOCl + HCl | Room temperature |
| Cl₂ + cold NaOH | NaCl + NaOCl | Cold, dilute |
| Cl₂ + hot NaOH | NaCl + NaClO₃ | Hot, concentrated |
| Cl₂ + Ca(OH)₂ | CaOCl₂ (bleaching powder) | Dry Ca(OH)₂ |
For NEET, the reaction of Cl₂ with NaOH under different conditions is a high-weightage question. Remember: cold → hypochlorite (OCl⁻), hot → chlorate (ClO₃⁻). The mnemonic: “Cold = OCl, Hot = ClO₃” — heat gives the higher oxidation state product.
Common Mistake
Students confuse bleaching powder (CaOCl₂) with calcium chloride (CaCl₂). Bleaching powder is calcium hypochlorite (or calcium oxychloride) — it has the OCl⁻ group responsible for bleaching. CaCl₂ has no bleaching action. Also, bleaching powder is made by passing Cl₂ over dry slaked lime Ca(OH)₂, NOT over quicklime CaO.