It is found only in the combined state (not free state). Main source is sea water which contains NaCl; other sources are rock salts and other chloride minerals like carmallite, MgCl2.KCl.6H2O; hom silver, AgCl; sylvine, KCl, etc.
Preparation:-
(i) By the action of any oxidising agent (whose oxidation potential is greater than Cl2, e.g. MnO2, KMnO4, K2Cr2O7, PbO2, CaOCl2, O3) on HCl. This also constitutes laboratory method. Chlorine gas is collected by upward displacement of air (Cl2 gas is heavier than air).
(ii) Alternatively, chlorine can be prepared by heating any chloride with concentrated sulphuric acid and manganese dioxide.
Chlorine so obtained is first passed through water for dissolving HCl gas (very soluble) and then through conc. H2SO4 for removing moisture.
Manufacture:-
(i) By the electrolysis of brine (aq. solution of NaCl) in Nelson cell (see also manufacture of NaOH). This is the cheapest method and gives purest chlorine. Chlorine is obtained at the anode and hydrogen at the cathode.
(ii) Deacon's process (old and obsolete method). The temperature should not be below 350°C because at this temperature oxygen would not decompose hydrochloric acid at all.
\(\underset {(4 \ vol.)}{4HCl } + \underset {(1\ vol.)}{O_2 } (\text{from air}) \underset{CuCl_2(Catalyst)}{\xrightarrow {400-500^\circ C}} 2H_2O + 2Cl_2 \uparrow\)
Chlorine gas, thus obtained, contains N2 and O2 and is used for the manufacture of bleaching powder by Hasenclever's process (dilute chlorine process).
Properties:- (A) Physical.
(i) Chlorine is a greenish-yellow gas with a suffocating smell. It is poisonous in nature.
(ii) It is about 2.5 times as heavy as air and hence collected by upward displacement of air.
(iii) It can be easily liquefied by cooling under pressure to a yellow liquid (b.p. -34.6°C and f.p. -101.6°C).
(iv) It is as fairly soluble in water giving yellowish, strongly smelling liquid called chlorine water. A part chlorine reacts chemically with water forming HOCI which explains its oxidation and aching action.
\(Cl_2 + H_2O \rightarrow HCI + HOCI\)
\(2HOCl \rightarrow 2HCl + O_{2}\)
(B) Chemical.
(i) Combination with elements. It combines with hydrogen in light (but not in dark) with explosion to form HCl, However, in presence of charcoal catalyst the reaction is smooth.
It combines directly with all non-metals except N, O and C; and also with many metals e.g Na, Fe, Al, Zn, Hg, Cu, etc.
(ii) Affinity for hydrogen. It decomposes several hydrogen compounds forming HCl.
(a) \(2H_ 2 O+2Cl_ 2\rightarrow 4HCl + O_{2}\)
(b) \(C_6H_{6} + 3Cl_2 \rightarrow 6HCl + 6C\)
(c) \(H_{2}S + Cl_{2} \rightarrow 2HCl + S\)
(d) \(\underset{(\text{turpentine oil})} {C_{10} H_{16} + 8Cl_2} \rightarrow 16HCl + 10C\)
(i) Oxidising and bleaching action. As mentioned earlier, moist chlorine gives oxygen hence it can act as an oxidising and bleaching agent (reactions given above).
Chlorine, being more electronegative than Br2 and I2 displaces them from the solution of their salta.
\(2HBr + Cl_{2} \rightarrow 2HCl + Br_{2} \quad ; \quad 2KI + Cl_2 \rightarrow + I_2\)
Bromine displaces only iodine from iodides.
\(2KI + Br_{2} \rightarrow 2KBr + I_{2}\)
Iodine can't displace any halogen.
Owing to liberation of oxygen from water, moist chlorine bleaches vegetable colouring matter ke litmus, indigo, linen, wood pulp, cotton fabric, etc.
\(Cl_{2} + H_{2}O \rightarrow 2HCl + [O]\)
Coloured matter + [0] Colourless matter
Bleaching done by \(Cl_{2}\) is permanent and the colour is not restored on standing. However, it can't be used for bleaching delicate articles (e.g. straw, silk, wool, etc.) which are damaged by it.
(iv) Action of alkalies.
\( 2NaOH (cold) + Cl_2 \rightarrow NaCl +\underset{\text{Sod. hypochlorite}} { NaClO + H_2O }\)
\(6NaOH (hot) + 3Cl_{2} \rightarrow \underset{\text{Sod. chlorate} }{5NaCl + NaClO3 + 3H2O} \)
Bromine and I2 react in the same way. However, sod, hypoiodite is unstable and decomposes
\((3NaIO \rightarrow 2NaI + NaIO_3).\)
With milk of lime (paste of lime in water), chlorine reacts as above but with slaked lime it forms bleaching powder, CaOCl2.
\(Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O\)
(v) Action of ammonia.
\(8NH 3 (\text{ercess})+ 3Cl_{2} \rightarrow 6NH_{4}Cl + N_{2}\)
\(NH3+3Cl_2 (excess) \rightarrow NCl_3 + 3HCI\)
Bromine reacts in a similar way, but iodine forms an explosive substance, \(NH_{3} .NI_3\)
\((cf. 2NH_{3} + 3I_{2} \rightarrow NH_3 .NI_3 +3HI)\)
(vi) Formation of addition compounds.
\(SO_{2}(dry) +Cl _2 \rightarrow \underset{\text{Sulphuryl chloride} } {SO _2 Cl _2 } \quad ; \quad CO + Cl_2 \rightarrow \underset{\text{Carbonyl chloride} }{ COCl_2}\)
Corresponding \(SO_{2}Br_{2}\) and \(SO_2I_2\) are not stable.
(vii) Action on organic compounds.
\(CH_{4} + Cl_{2} \rightarrow CH_{3}Cl + HCL\) (Substitution reaction)
\(C_{2}H_{4} + Cl_{2} \rightarrow \underset{\text{ Ethylene chloride }}{C_{2}H_{4}Cl_{2}}\) (Addition reaction)
Bromine and iodine also react in a similar way.
Uses. Chlorine is used.
(i) as a bleaching agent,
(ii) in the sterilisation of drinking water,
(iii) in swimming pools as germicide and disinfectant,
(iv) in the preparation of domestic antiseptic solutions (NaOCl),
(v) in the manufacture of bleaching powder, chlorates, bromine, HCl, CHCl3, CCl4., CH2Cl2, DDT, synthetic plastics, rubbers, etc.
(vi) in the extraction of gold and platinum,
(vii) in warfare for the manufacture of poisonous gases like phosgene (COCl2) , tear gas ( CCl3 .NO2 ) and mustard gas (ClC2H4 - S - C2H4Cl).
