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(2) Occurrence : Alkali metals are very reactive and thus found in combined state some important ores of alkali metals are given ahead.
(i) Lithium :Triphylite, Petalite, lepidolite, Spodumene [LiAl(SiO3)3], Amblygonite [Li(Al F)PO4]
(ii) Sodium :Chile salt petre (NaNO3), Sodium chloride (NaCl), Sodium sulphate (Na2SO4), Borax (Na2B4O710H2O), Glauber salt (Na2 SO4.10H2O)
(iii) Potassium :Sylime (KCl), carnallite (KCl.MgCl2.6H2O) and Felspar (K2O.Al2O3.6SiO2)
(iv) Rubidium :Lithium ores Lepidolite, triphylite contains 0.7 to 3% Rb2O
(v) Caesium :Lepidolite, Pollucite contains 0.2 to 7% Cs2O
(3) Extraction of alkali metals : Alkali metals cannot be extracted by the usual methods for the extraction of metals due to following reasons.
(i) Alkali metals are strong reducing agents, hence cannot be extracted by reduction of their oxides or other compounds.
(ii) Being highly electropositive in nature, it is not possible to apply the method of displacing them from their salt solutions by any other element.
(iii) The aqueous solutions of their salts cannot be used for extraction by electrolytic method because hydrogen ion is discharged at cathode instead of an alkali metal ions as the discharge potentials of alkali metals are high. However, by using Hg as cathode, alkali metal can be deposited. The alkali metal readily combines with Hg to form an amalgam from which its recovery difficult. The only successful method, therefore, is the electrolysis of their fused salts, usually chlorides. Generally, another metal chloride is added to lower their fusion temperature.
fusion
Fused NaCl :NaCl ————> Na+ + Cl–
Electrolysis: Anode: 2Cl– —> Cl2 + 2e–
of fused salt: Cathode: 2Na+ + 2e– —> 2 Na
(4) Alloys Formation
(i) The alkali metals form alloys among themselves as well as with other metals.
(ii) Alkali metals also get dissolved in mercury to form amalgam with evolution of heat and the amalgamation is highly exothermic . |
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