Chlorine ($Cl_2$)
Preparation, Permanent Bleaching, and The Great Alkali/Ammonia Traps.
Discovered by Carl Wilhelm Scheele in 1774, Chlorine is a dense, highly toxic, greenish-yellow gas with a pungent, suffocating odor. Its name is derived from the Greek word 'chloros', meaning greenish-yellow. Due to its intense reactivity and oxidizing power, the chemical reactions of chlorine are absolute favorites for JEE and NEET examiners.
1. Preparation of Chlorine Gas
A. Laboratory Preparation
In the lab, chlorine is prepared by the oxidation of Hydrochloric acid ($HCl$) using strong oxidizing agents like Manganese dioxide ($MnO_2$) or Potassium permanganate ($KMnO_4$).
Note: The $KMnO_4$ reaction occurs readily at room temperature without heating.
B. Industrial: Deacon's Process & Electrolysis
Chlorine is manufactured by the oxidation of hydrogen chloride gas by atmospheric oxygen. This reaction requires a specific catalyst: Copper(II) chloride ($CuCl_2$) at 723 K.
Chlorine is obtained on a massive scale by the electrolysis of brine (concentrated aqueous $NaCl$ solution). Chlorine gas is liberated at the Anode, while $H_2$ gas and aqueous $NaOH$ are obtained at the cathode.
2. The Grand Exam Trap: Reaction with Alkalies ($NaOH$)
Chlorine reacts with sodium hydroxide ($NaOH$) to undergo a disproportionation reaction (where $Cl_2$ is simultaneously oxidized and reduced). However, the products formed depend entirely on the temperature and concentration of the alkali. Memorize these two reactions perfectly.
Reacting $Cl_2$ with cold and dilute $NaOH$ yields a mixture of chloride and hypochlorite.
Products: Sodium chloride (-1) & Sodium hypochlorite (+1)
Reacting $Cl_2$ with hot and concentrated $NaOH$ yields a mixture of chloride and chlorate.
Products: Sodium chloride (-1) & Sodium chlorate (+5)
When chlorine gas is passed over dry slaked lime ($Ca(OH)_2$), it produces bleaching powder.
3. The Second Grand Trap: Reaction with Ammonia ($NH_3$)
Similar to the alkali reaction, the products here change entirely depending on which reactant is in excess!
When ammonia is in excess, chlorine oxidizes it to Nitrogen gas ($N_2$), and the remaining ammonia forms ammonium chloride.
When chlorine is in excess, it strips away all the hydrogens from nitrogen, replacing them to form highly explosive Nitrogen trichloride ($NCl_3$).
4. Powerful Oxidizing and Bleaching Action
Chlorine is a powerful oxidizing agent. It oxidizes:
- Ferrous ($Fe^{2+}$) to Ferric ($Fe^{3+}$): $2FeSO_4 + H_2SO_4 + Cl_2 \rightarrow \mathbf{Fe_2(SO_4)_3} + 2HCl$
- Sulfur dioxide to Sulfuric Acid: $SO_2 + 2H_2O + Cl_2 \rightarrow \mathbf{H_2SO_4} + 2HCl$
- Iodine to Iodic Acid: $I_2 + 6H_2O + 5Cl_2 \rightarrow \mathbf{2HIO_3} + 10HCl$
The Mechanism of Bleaching
Chlorine's bleaching action requires the presence of moisture. Dry chlorine gas will not bleach a dry flower!
It dissolves in water to form $HCl$ and $HOCl$ (Hypochlorous acid). The highly unstable $HOCl$ rapidly breaks down, releasing nascent oxygen $[O]$.
$HOCl \rightarrow HCl + \mathbf{[O]}$
Because this bleaching happens via oxidation (adding oxygen to the pigment), the color loss is permanent (unlike the temporary reduction-based bleaching of $SO_2$).
Chlorine's extreme reactivity made it infamous in WWI. It is a precursor for the synthesis of several highly poisonous gases you must memorize:
- Phosgene: $COCl_2$
- Tear Gas: $CCl_3NO_2$
- Mustard Gas: $ClCH_2CH_2-S-CH_2CH_2Cl$
Mastery Check: Chlorine Chemistry
15 High-Yield Questions to test your JEE/NEET Preparation
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