Chlorine

Chlorine is highly reactive hence it doesn’t occur free in nature. It is usually distributed as various metal chlorides. The most important chloride is sodium chloride which occurs in sea water.

Preparation:

Chlorine is prepared by the action of conc. sulphuric acid on chlorides in presence of manganese dioxide.

4NaCl + MnO₂ + 4H₂SO₄ → Cl₂ + MnCl₂ + 4NaHSO₄ + 2H₂O

It can also be prepared by oxidising hydrochloric acid using various oxidising agents such as manganese dioxide, lead dioxide, potassium permanganate or dichromate.

PbO₂ + 4HCl → PbCl₂ + 2H₂O + Cl₂

MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂

2KMnO₄ + 16HCl → 2KCl + 2MnCl₂ + 8H₂O + 5Cl₂

K₂Cr₂O₇ + 14HCl → 2KCl + 2CrCl₃ + 7H₂O + 3Cl₂

When bleaching powder is treated with mineral acids chlorine is liberated

CaOCl₂ + 2HCl → CaCl₂ + H₂O + Cl₂

CaOCl₂ + H₂SO₄ → CaSO₄ + H₂O + Cl₂

Manufacture of chlorine:

Chlorine is manufactured by the electrolysis of brine in electrolytic process or by oxidation of HCl by air in Deacon’s process.

Electrolytic process: When a solution of brine (NaCl) is electrolysed, Na+ and Cl ions are formed. Na+ ion reacts with OH^- ions of water and forms sodium hydroxide. Hydrogen and chlorine are liberated as gases.

Deacon’s process: In this process a mixture of air and hydrochloric acid is passed up a chamber containing a number of shelves, pumice stones soaked in cuprous chloride are placed. Hot gases at about 723 K are passed through a jacket that surrounds the chamber.

The chlorine obtained by this method is dilute and is employed for the manufacture of bleaching powder. The catalysed reaction is given below,

2Cu₂Cl₂ + O₂ → 2Cu₂OCl₂ Cuprous oxy chloride

Cu₂OCl₂ + 2HCl → 2CuCl₂ Cupric chloride + H₂O

2CuCl₂ → Cu₂Cl₂ Cuprous chl + Cl₂

Physical properties:

Chlorine is a greenish yellow gas with a pungent irritating odour. It produces headache when inhaled even in small quantities whereas inhalation of large quantities could be fatal. It is 2.5 times heavier than air.

Chlorine is soluble in water and its solution is referred as chlorine water. It deposits greenish yellow crystals of chlorine hydrate (Cl₂.8H₂O). It can be converted into liquid (Boiling point – 34.6° C) and yellow crystalline solid (Melting point -102° C)

Chemical properties:

Action with metals and non-metals: It reacts with metals and non metals to give the corresponding chlorides.

2Na + Cl₂ → 2NaCl

2Fe + 3Cl₂ → 2FeCl₃

2Al + 3Cl₂ → 2AlCl₃

Cu + Cl₂ → CuCl₂

H₂ + Cl₂ → 2HCl ; H = − 44kCal

2B + 3Cl₂ → 2BCl₃

2S + Cl₂ → S₂Cl₂ disulphur dichloride

P₄ + 6Cl₂ → 4PCl₃

2As + 3Cl₂ → 2AsCl₃

2Sb + 3Cl₂ → 2SbCl₃

Affinity for hydrogen : When burnt with turpentine it forms carbon and hydrochloric acid.

C₁₀H₁₆ + 8Cl₂ → 10C + 16HCl

It forms dioxygen when reacting with water in presence of sunlight. When chlorine in water is exposed to sunlight it loses its colour and smell as the chlorine is converted into hydrochloric acid.

2Cl₂ + 2H₂O → O₂ + 4HCl

Chlorine reacts with ammonia to give ammonium chloride and other products as shown below:

With excess ammonia,

2NH₃ + 3Cl₂ → N₂ + 6HCl

6HCl + 6 NH₃ → 6 NH₄Cl

overall reaction

8NH₃ + 3Cl₂ → N₂ + 6 NH₄Cl

With excess chlorine,

NH₃ + 3Cl₂ → NCl₃ + 3HCl

3HCl + 3NH₃ → 3NH₄Cl

overall reaction

4NH₃ + 3Cl₂ → NCl₃ + 3NH₄Cl

Chlorine oxidises hydrogen sulphide to sulphur and liberates bromine and iodine from iodides and bromides. However, it doesn't oxidise fluorides

H₂S + Cl₂ → 2HCl + S

Cl₂ + 2KBr → 2KCl + Br₂

Cl₂ + 2KI → 2KCl + I₂

Reaction with alkali: Chlorine reacts with cold dilute alkali to give chloride and hypochlorite while with hot concentrated alkali chlorides and chlorates are formed.

Cl₂ + H₂O → HCl + HOCl

HCl + NaOH → NaCl + H₂O

HOCl + NaOH → NaOCl + H₂O

overall reaction

Cl₂ + 2NaOH → NaOCl sodium hypo chlorite + NaCl + H₂O

( Cl₂ + H₂O → HCl + HOCl) × 3

( HCl + NaOH → NaCl + H₂O) × 3

( HOCl + NaOH → NaOCl + H₂O) × 3

3NaOCl → NaClO₃ + 2NaCl

overall reaction

3Cl₂ + 6NaOH → NaClO₃ + 5NaCl + 3H₂O

Oxidising and bleaching action: Chlorine is a strong oxidising and bleaching agent because of the nascent oxygen.

H₂O + Cl₂ → HCl + HOCl Hypo chlorous acid

HOCl → HCl + (O)

Colouring matter + Nascent oxygen → Colourless oxidation product

The bleaching of chlorine is permanent. It oxidises ferrous salts to ferric, sulphites to sulphates and hydrogen sulphide to sulphur.

2FeCl₂ + Cl₂ → 2FeCl₃

Cl₂ + H₂O →HCl + HOCl

2FeSO₄ + H₂SO₄ + HOCl →Fe₂ (SO₄ )₃ + HCl + H₂O

overall reaction

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

Cl₂ + H₂O →HCl + HOCl

Na₂SO₃ + HOCl →Na₂SO₄ + HCl

overall reaction

Na₂SO₃ + H₂O + Cl₂ →Na₂SO₄ + 2HCl

Cl₂ + H₂S →2HCl + S

Preparation of bleaching powder: Bleaching powder is produced by passing chlorine gas through dry slaked lime (calcium hydroxide).

Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Displacement redox reactions: Chlorine displaces bromine from bromides and iodine from iodide salts.

Cl₂ + 2KBr → 2KCl + Br₂

Cl₂ + 2KI →2KCl + I₂

Formation of addition compounds: Chlorine forms addition products with sulphur dioxide, carbon monoixde and ethylene. It forms substituted products with alkanes/arenes.

SO₂ + Cl₂ → SO₂Cl₂Sulphuryl chloride

CO + Cl₂ → COCl₂Carbonyl chloride

C ₂ H₄ + Cl₂ → C₂H₄Cl₂ethylene dichloride

CH₄ + Cl₂ →CH₃Cl + HCl

C₆H₆ + Cl₂ →^FeCl3→C₆H₅Cl + HCl

Uses of chlorine:

·           It is used in

·           Purification of drinking water

·           Bleaching of cotton textiles, paper and rayon

·           Extraction of gold and platinum