CHLORINE (Cl2)

Preparation :

(i) By heating MnO2 with concentrated hydrocloric acid.

MnO2 + 4HCl ® MnCl2 + Cl2 + 2H2O

(ii) By heating chloride with concentrated H2SO4 in presence of MnO2.

4H+  + MnO2 + 2X ® X2 + Mn+2 + 2H2O

(iii) 2KMnO4 + 16 HCl ® 2 KCl + 2 MnCl2 + 5 Cl2 + 8 H2O

(iv) Manufacture of chlorine :

(a) Deacon’s process : By oxidation of hydrogen chloride gas by  atmospheric oxygen in the presence of CuCl2 (catalyst) at 723 K.

4 HCl + O2  2 Cl2 + 2 H2O

(b) Electrolytic process : Chlorine is obtained by the electrolysis of brine (concentrated NaCl solution). Chlorine is liberated at anode. It is obtained as a by–product in many chemical industries e.g.; in mfg. of sodium hydroxide.

NaX (aq) ® Na+ (aq) + X (aq)

Anode : 2X ® X2 + 2e 

Properties :

(i) It is a greenish–yellow gas with pungent and suffocating odour. It is about 2–5 times heavier than air. It can be  liquefied into greenish–yellow liquid which boils at 239 K. It is soluble in water.

(ii) At low temperature it forms a hydrate with water having formula Cl2 . 8H2O which is infact a clathrate compound.

(iii) Reaction with metals :

2Al + 3Cl2 ® 2AlCl3   

2Fe + 3Cl2 ® 2FeCl3

Reaction with non-metals :

P4 + 6Cl2 ® 4PCl3 

S8 + 4Cl2 ® 4S2Cl2

(iv) Affinity for hydrogen : It reacts with compounds containing hydrogen and form HCl.

H2 + Cl2 ® 2HCl  

H2S + Cl2 ® 2HCl + S

(v)  Reaction with NaOH :

(a) 2 NaOH ( cold & dilute) + Cl2 ® NaCl + NaClO + H2O

(b) 6 NaOH (hot & concentrated) + 3 Cl2 ® 5 NaCl + NaClO3 + 3 H2O

(vi) Reaction with dry slaked lime, Ca(OH)2 : To give bleaching powder.

2 Ca(OH)2 + 2 Cl2 ® Ca(OCl)2 + CaCl2 + 2 H2O

The composition of bleaching powder is Ca(OCl)2.CaCl2.Ca(OH)2 .2H2O

(vii)   Oxidising & bleaching properties :

Chlorine dissolves in water (Cl2 water is yellow) giving HCl (colourless) and HOCl (colourless). Hypochlorous acid (HOCl) so formed, gives nascent oxygen which is responsible for oxidising and bleaching properties of chlorine.

(a) It oxidises ferrous to ferric, sulphite to sulphate, sulphur dioxide to sulphuric acid and iodine to iodic acid.

2 FeSO4 + H2SO4 + Cl2 ® Fe2(SO4)3 + 2 HCl

Na2SO3 + Cl2 + H2O ® Na2SO4 +  2 HCl

SO2 + 2 H2O + Cl2 ® H2SO4 + 2 HCl

I2 + 6 H2O + 5 Cl2 ® 2 HIO3 + 10 HCl

(b) It is a powerful bleaching agent ; bleaching action is due to oxidation.

Cl2 + H2O ® 2 HCl + O

Coloured substance + O ® Colourless substance

It bleaches vegetable or organic matter in the presence of moisture. Bleaching effect of chlorine is permanent.

Note :

The bleaching action of SO2 is temporary because it takes place through reduction.

SO2 + 2 H2O ® H2 SO4  + 2 H

SO32– + Coloured material  SO42– + Reduced colourless material.

Reduced Colourless material   Coloured material.

Uses :  Cl2  is used

1. for bleaching wood pulp (required for the manufacture of paper and rayon), bleaching cotton and textiles,

2. in the manufacture of dyes, drugs and organic compounds such as CCl4, CHCl3, DDT, refrigerants, etc.

3. in the extraction of gold and platinum.

4. in sterilising drinking water and

5. preparation of poisonous gases such as phosgene (COCl2), tear gas (CCl3NO2), mustard gas (ClCH2CH2SCH2CH2Cl).