Doc: Electrolytic Refining Class 12 Notes | EduRev

Chemistry Class 12

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Class 12 : Doc: Electrolytic Refining Class 12 Notes | EduRev

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(D) Electrolytic refining :

Anode : Impure Zn

Cathode : Aluminium Electrode

Electrolyte : ZnSO4(aq) H2SO4

Extraction of Cr From FeCr2O4(FeO.Cr2O 3) Chromite ORE :

Conc. Step : Chromite ore is concentrated by gravity sep. Method to remove light impurites followed by magnetic sep. method to remove non-magnetic impurities.

Chemical Method to Separate Cr2O3 :

4FeO.Cr2O3  7O2 (air) 8 Na2CO3/16NaOH Doc: Electrolytic Refining Class 12 Notes | EduRev 2Fe2O3  8Na2CrO4  8CO2/8H2O

Doc: Electrolytic Refining Class 12 Notes | EduRev Doc: Electrolytic Refining Class 12 Notes | EduRev

2Na  (aq) Cr2O72-(aq) SO42- (aq) Doc: Electrolytic Refining Class 12 Notes | EduRev 2Na (aq) CrO42- (aq)

(Orange Solution) yellow solution

Doc: Electrolytic Refining Class 12 Notes | EduRev

Na2Cr2O7.2H2Doc: Electrolytic Refining Class 12 Notes | EduRev NaCrO2 Doc: Electrolytic Refining Class 12 Notes | EduRev Cr(OH)3¯

(orange crystal) (green ppt)

Doc: Electrolytic Refining Class 12 Notes | EduRev

Cr2O3 (green solid)

Al-reduction method : (Goldsmith alumino thermite process)

Doc: Electrolytic Refining Class 12 Notes | EduRev

Impure Cr is purified by electrolytic refining method.

Anode : Impure chromium

Cathode : Hg-electrode

Electrolyte : CrCl3  HCl

Doc: Electrolytic Refining Class 12 Notes | EduRev

 

Extraction of Mn from MnO2 (Pyrolusite) :

Pyrolusite is concentrated by gravity separation method followed by magnetic separation method to remove non-magnetic impurity.

Calcination :

MnO2 gives explosive rxn. with Al therefore in this step it is converted into mixed oxide Mn3O4.

3MnO2 Doc: Electrolytic Refining Class 12 Notes | EduRev Mn3O4(MnO Mn2O3) + O2­

Al-reduction method : (Goldsmith alumino thermite process)

Doc: Electrolytic Refining Class 12 Notes | EduRev

Impure Mn is purified by electrolytic refining method.

Anode : Impure Mn

Cathode : Hg-electrode

Electrolyte : MnSO4  H2SO4

Doc: Electrolytic Refining Class 12 Notes | EduRev

8. Aluminium :

Ores :

(i) Bauxite Al2O3.2H2O (main ORE) ; Diaspore Al2O3.H2O ; Corundum Al2O3

(ii) Mica K2O.3Al2O3.6SiO 2.2H2O ; Kaolin Al2O3.2SiO2.2H 2O

(iii) Cryolite Na3AIF6

Extraction of Al from Bauxite :

Purification of Bauxite : 

Doc: Electrolytic Refining Class 12 Notes | EduRev

 Electrolysis of pure fused Al2O3 (Hall - Heroult method)

Cathode - iron tank lined with graphite

Anode - Carbon rods dipped in molten electrolyte

Electrolyte - molten (20% Al2O3  60% Cryolite 20% CaF2) Temp ≈ 1100°C

Cryolite lowers the melting point of mixture and makes the molten mix conducting.

Na3AlF6 Doc: Electrolytic Refining Class 12 Notes | EduRev 3NaF+ AlF3

AlF3 → Al 3  + 3F-

At cathode Al 3 + 3e- → Al

At anode 3F- → 3F+ 3e-

Al2O3 + 6F → 2AIF3  +  Doc: Electrolytic Refining Class 12 Notes | EduRevO2­

C + Doc: Electrolytic Refining Class 12 Notes | EduRev O2 → CO

CO+  Doc: Electrolytic Refining Class 12 Notes | EduRev O2 → CO2

Doc: Electrolytic Refining Class 12 Notes | EduRev

Anodes are periodically changed as they are consumed by oxygen liberated at anode.

Electrolytic refining (Hoppe's Method)

Iron box lined with carbon, contains the three molten layers.

Bottom layer : Impure Al as anode

Middle layer : (Na3AlF6  BaF2) Molten electrolyte

Tope layer : molten pure Al as cathode.

On passing the current aluminium is deposited at cathode from the middle layer and an equivalent amount is taken from andoe (bottom layer) levaing behind the impurites. In this way aluminium obtained is 99.98% pure.

9. Magnesium : (Dow's process)

Ores :

Carnallite MgCl2.KCl 6H2O (main ore) ; Epsom MgSo4.7H2O magnesite MgCO3 ; Kiesserite MgSO4. H2O ; Dolomite MgCO3. CaCO3 ; Kainite K2SO4. MgSO4. MgCl2.6H2O (Mg is also found in Talc, sabestos and chlorophyll)

Sea water contains 0.13% magnesium as chloride and sulphate. It involves following steps.

(a) Precipitation of magnesium as magnesium hydroxide by slaked lime :

MgCl2 + Ca(OH)2 Mg(OH)2 + CaCl2 (ppt. reaction)

(b) Preparation of hexahydrated magnesium chloride :

Mg(OH)2  + 2HCl(aq) → MgCl2  + 2H2O (Neutralisation reaction)

The solution on concentration and crystallisation gives the crystals of MgCl2.6H2O

(c) Preparation of anhydrous magnesium chloride :

MgCl2.6H2Doc: Electrolytic Refining Class 12 Notes | EduRev MgCl2 + 6H2O

* It is not made anhydrous by simple heating because it gets hydrolysed.

MgCl2.6H2Doc: Electrolytic Refining Class 12 Notes | EduRev MgO+ 5H2O +2HCl

(d) Electrolysis of fused anhydrouns MgCl2 :

Magnesium chloride obtained by any of the above methods is fused and mixed with sodium chloride and calcium chloride in the temperature range of 972 - 1023K. The molten mixture is electrolysed. Magnesium is liberated at the carhode (iron pot) and chlorine is evolved at graphite anode.

MgCl2 Doc: Electrolytic Refining Class 12 Notes | EduRev Mg  2Cl-

At cathode : Mg + 2e- → Mg(99% pure) ;

At anode : 2Cl- → Cl2 + 2e-

Doc: Electrolytic Refining Class 12 Notes | EduRev

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