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((1) Occurrence of zinc: Zinc does not occur in the native form since it is a reactive metal. The chief ores of zinc are (i) Zinc blende (ZnS) (ii) Calamine or zinc spar (ZnCO3) and (iii) Zincite (ZnO)
(2) Extraction of zinc : Zinc blende, after concentration by Froth floatation process, is roasted in air to convert it into ZnO. In case of calamine, ore is calcined to get ZnO. The oxide thus obtained is mixed with crushed coke and heated at 1673 K in fire clay retorts (Belgian Process) when ZnO gets reduced to metallic zinc. Being volatile at this temperature, the metal distils over and is condensed leaving behind Cd, Pb and Fe as impurities. The crude metal is called spelter. The metal may be refined either by electrolysis or by fractional distillation.
Properties of Zn : Zinc is more reactive than mercury. It is a good conductor of heat and electricity. Zinc readily combines with oxygen to form ZnO. Pure zinc does not react with non-oxidising acids (HCl or
H2SO4) but the impure metal reacts forming
Zn2+ ions and evolving
H2 gas.
Zn + 2HCl → ZnCl2 + H2↑
Hot and conc. H2SO4 attacks zinc liberating SO2 gas
Zn + 2H2SO4 → ZnSO4 + SO2 + 2H2O
Zinc also reacts with both dilute (hot and cold) HNO3 and conc. HNO3 liberating nitrous oxide (N2O), ammonium nitrate (NH4NO3) and nitrogen dioxide (NO2) respectively.
4Zn + 10HNO3 (warm, dilute) → 4Zn(NO3)2 + N2O + 5H2O
4Zn + 10HNO3 (cold very dilute) → 4Zn(NO3)2 + 2NO2 + 2H2O
Zn + 4HNO3 (hot and conc.) → Zn(NO3)2 + 2N2O + 2H2O
Zinc dissolves in hot concentrated NaOH forming the soluble sod. Zincate
Zn + 2NaOH + 2H2O → Na2 [Zn(OH)4] + H2
Or Zn + 2NaOH → Na2ZnO2 + H2
(3) Special varieties of zinc. (i) Zinc dust : It is prepared by melting zinc and then atomising it with a blast of air.
(ii) Granulated zinc : It is prepared by pouring molten zinc into cold water.
Both these varieties of zinc are used as reducing agents in laboratory.
Compounds of zinc
(1) Zinc oxide (Zinc white or Chinese white), ZnO : It is obtained by burning zinc in air or by heating zinc carbonate or zinc nitrate.
2Zn + O2 → 2ZnO
ZnCO3 → ZnO + CO2
2Zn(NO3)2 → 2ZnO + 4NO2 + O2
It is a white powder but becomes yellow and heating and again white on cooling.
It is insoluble in water and is vey light and hence commonly known as philosopher's wool.
It is amphoteric in nature.
ZnO + 2HCl → ZnCl + H2O
(Black)
ZnO + 2NaOH → Na2ZnO2 + H2O
(Acidic) Sod. sincate
Or ZnO + 2NaOH + H2O → Na2[Zn(OH)4]
Sod. tetrahydrozincate(II)
It is reduced both by carbon adn H2 and is used as a white paint
ZnO + C → Zn + CO; ZnO + H2 → Zn + H2O
(2) Zinc chloride, ZnCl2 : It is obtained when Zn metal, ZnO or ZnCO3
is treated with dil. HCl. It crystallizes as ZnCl2.2H2O
and becomes anhydrous on heating.
ZnCl2 is highly deliquescent and is highly soluble in H2O
and also readily dissolves in organic solvents like acetone, alcohol, ether etc. its aqueous solution is acidic due to hydrolysis.
ZnCl2 + H2O → Zn(OH)Cl + HCl
Anhydrous
ZnCl2 is used as a Lewis acid catalyst in organic reactions. Mixed with moist zinc oxide, it is used for filling teeth and its solution is used for preserving timber. Anhydrous ZnCl2
used as a Lucas reagent with conc. HCl.
(3) Zinc sulphide, ZnS : It is a white solid. It is soluble in dil. HCl and thus does not get precipitated by H2S
in the acidic medium.
ZnS + 2HCl → ZnCl2 + H2S
It is a constituent of lithopone (ZnS + BaSO4)
(4) Zinc sulphate, ZnSO4.7H2O : It is commonly known as white vitriol and is obtained by the action of dil.
H2SO4 on zinc metal, ZnO or ZnCO3
. On heating, it first loses six molecules of water of crystallization at 373 K. At 723 K, it becomes anhydrous and on further heating, it decomposes.
373K 723K
ZnSO4.7H2O —————→ ZnSO4.H2O ————→
1073K
2ZNSO4 —————→ 2ZnO + 2SO2 + O2
It is used to prepare lithopone (BaSO4 + ZnS), a white paint and also in galvanishing iron.
ZnSO4 + BaS → ZnS + BaSO4
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