April 07, 2012

d - Block Elements Five Marks

1. Explain how potassium dichromate is extracted from chromite ore. Or How is potassium dichromate prepared from chrome iron ore?
1. Conversion of chrome iron ore to sodium chromate
The powdered chrome iron ore is mixed with Na2CO3 and quick lime, then roasted in a reverberatory furnace with free expose to air.
4FeO.Cr2O3 + 8Na2CO3 + 7O2  8Na2CrO4 + 2Fe2O3 + 8CO2
2. Conversion of Na2CrO4 to Na2Cr2O7
Sodium chromate solution so obtained is filtered and treated with con.H2SO4, when sodium chromate is converted to sodium dichromate.
2Na2CrO4 + H2SO4  Na2Cr2O7 + Na2SO4 + H2O
3. Conversion of sodium dichromate into potassium dichromate
Hot concentrated solution of sodium dichromate is treated with KCl, when potassium dichromate, being much less soluble than sodium salt, crystallizes out on cooling.
Na2Cr2O7 + 2KCl K2Cr2O7 + 2NaCl
2.  Explain the extraction of silver from its chief ore. Or How is silver extracted from its ore?
Extraction of silver from the Argentite ore
Silver is extracted from the argentite ore by the Mac-Arthur and Forrest’s cyanide process.
1. Concentration: Froth-floatation process.
2. Treatment of the ore with NaCN
The concentrated ore is treated with 0.4-0.6% solution of sodium cyanide for several hours. The mixture is continuously agitated by a current of air, so that Ag present in the ore is converted into soluble sodium argento complex.
Ag2S + 4NaCN 2Na [Ag(CN)2] + Na2S
3. Precipitation of silver
The solution containing sodium argento cyanide is filtered to remove insoluble impurities and filtrate is treated with zinc dust, silver gets precipitated.
2Na [Ag(CN)2] + Zn Na2[Zn(CN)4] + 2Ag
4. Electrolytic refining
Anode - Impure Silver
Cathode - Pure Silver
Electrolyte - Silver nitrate acidified with 1% Nitric acid. On passing electricity pure silver gets deposited at the cathode.
3.  Explain the extraction of Zinc from its chief ore. Or How is zinc extracted from its chief ore?
Extraction of Zinc from Zinc blende
1. Concentration: Froth-floatation process.
2. Roasting
The concentrated ore is then roasted in the presence of excess of air at about 1200 K.
2 ZnS + 3O2  2ZnO + 2SO2
3. Reduction
Zinc oxide is mixed with powdered coke and heated to 1673 K in a fire clay retort, in which ZnO is reduced to zinc metal.
ZnO + C Zn + CO
4. Electrolytic refining
Anode - Impure Zinc
Cathode - Pure Zinc
Electrolyte - ZnSO4 solution containing a little of dil.H2SO4. On passing electric current, pure zinc get deposited at the cathode.
4. How is Cr2O3 reduced to chromium by aluminothermic process? Or Explain aluminothermic process involved in the extraction of chromium.
Aluminothermic process
Chromic oxide is mixed with powdered Aluminium in the ratio 3:1 and is placed in a large fire clay crucible. A mixture of barium peroxide and Mg powder is placed over this. The crucible is surrounded by sand which prevents loss of heat by radiation. The mixture is ignited by a piece of Mg ribbon. During this process a large amount of heat is liberated, in which Cr2O3 is reduced to chromium.
The molten chromium is collected in the crucible and aluminium oxide is removed as slag.
Cr2O3 + 2Al 2Cr + Al2O3 + 468.6 kJ
5.  How is gold extracted from its chief ore? Or How is gold extracted?
Mac Arthur Forrest Cyanide Process
The extraction of gold from the sulphide or telluride ore involves the following steps:
1. Crushing, grinding and concentration
The gold ore is crushed and powdered, and then concentrated by froth floatation process.
2. Roasting
The concentrated ore is roasted in excess of air or oxygen when impurities of sulphur, arsenic and tellurium are oxidised and escape as their volatile oxides.
3. Treatment with KCN
The finely powdered roasted ore is taken in large vats made up of cement or iron and having false wooden bottoms with holes. It is then treated with a dilute (0.5%) solution of KCN in presence of excess of air for 24 hours. As a result, gold dissolves in KCN to form a soluble complex called potassium aurocyanide.
4Au + 8KCN + 2H2O + O2  4K[Au(CN)2] + 4KOH
4. Precipitation of gold
The above solution containing potassium aurocyanide is filtered to remove insoluble impurities and then treated with zinc dust. Gold being less electropositive than zinc, it gets precipitated.
2K[Au(CN)2] + Zn K2[Zn(CN)4] + 2 Au
The precipitated gold is recovered by filtration. It is further purified by electrorefining.
5. Electrolytic refining
Anode - Impure Gold
Cathode - Pure Gold
Electrolyte - Auric chloride (AuCl3) acidified with 10-12% HCl. On passing current pure gold gets deposited on the cathode.

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