Indicate the steps in the preparation of: (i) K2Cr2O7 from chromite ore. (ii) KMnO4 from pyrolusite ore.
Preparation of potassium dichromate K2Cr2O7 from chromite ore FeO⋅Cr2O3(FeCr2O4): (i) Chromite ore is fused with NaOH or sodium carbonate in air to form sodium chromate. 4FeCr2O4+16NaOH+7O2→8Na2CrO4+2Fe2O3+8H2O (ii) Sodium chromate solution is filtered and acidified to form sodium dichromate. 2Na2CrO4+H2SO4→Na2Cr2O7+Na2SO4+H2O The solution is coled to remove crystallized sodium sulphate. Sodium dichromate remains soluble. (iii) Equimolar proportions of hot solution of sodium dichromate and KCl are mixed to form potassium dichromate. (iii) Equimolar proportions of hot solution of sodium dichromate and KCl are mixed to form potassium dichromate. Na2Cr2O7+2KCl→K2Cr2O7+2NaCl Sodium chloride precipitates out in hot solution and is removed by filtration. On cooling, potassium dichromate separates out from mother liquor. Preparation of potassium permanganate KMnO4 from pyrolusite ore MnO2 . (i) Pyrolusite ore is fused with KOH (or K2CO3) in presence of air (or oxidising agents such as KNO3 or KClO3) to give potassium manganate K2MnO4 (green mass) 2MNO2+4KOH+O2Delta2K2MnO4+2H2O (ii) Extraction of green mass with water followed by oxidation with chlorine (or ozone or CO2) gives potassium permanganate. 2K2MnO4+Cl2→2KCl+2KMnO4 On concentrating the solution, dark purple crystals of potassium permanganate separates out. Note: In neutral or acidic medium, manganate ion will disproportionate to permanganate ion. 3MnO42−+4H+→2MnO4−+MnO2+2H2O Convertion of manganate to permanganate can also be carried by electrolytic oxidation
