Indicate the steps in the preparation of:
- K2Cr2O7 from chromite ore.
- KMnO4 from pyrolusite ore.
Indicate the steps in the preparation of:
- K2Cr2O7 from chromite ore.
- KMnO4 from pyrolusite ore.
Solution and Explanation
(i)
Potassium dichromate (K2Cr2O7) is prepared from chromite ore (FeCr2O4) in the following steps.
Step (1):Preparation of sodium chromate
4FeCr2O4+16NaOH+7O2 \(\rightarrow\) 8Na2CrO4+2Fe2O3+8H2O
Step (2):Conversion of sodium chromate into sodium dichromate
2Na2CrO4+conc.H2SO4 \(\rightarrow\) Na2Cr2O7+Na2SO4+H2O
Step(3):Conversion of sodium dichromate to potassium dichromate
Na2Cr2O7+2KCl \(\rightarrow\) K2Cr2O7+2NaCl
Potassium chloride being less soluble than sodium chloride is obtained in the form of orange coloured crystals and can be removed by filtration
The dichromate ion (Cr2\(O^{2-}_7\)) exists in equilibrium with chromate (\(CrO^{2-}_4\)) ion at pH4. However, by changing the pH, they can be interconverted.
\(2Cr^{2-}_4 \underleftrightarrow{Acid \,Alkali}\; 2HCrO^-_4 \underleftrightarrow{Acid \,Alkali}\;Cr_2O^{2-}_7\)
Chromate Hydrogen Dichromate
(Yellow) Chromate (Orange)
(ii)
Potassium permanganate (KMnO4) can be prepared from pyrolusite (MnO2). The ore is fused with KOH in the
presence of either atmospheric oxygen or an oxidising agent, such as KNO3 or KClO4, to give K2MnO4.
2MnO2+4KOH+O2 \(\underrightarrow{heat}\) 2K2MnO4+2H2O
(green)
The green mass can be extracted with water and then oxidized either electrolytically or by passing chlorine/ozone into the solution.
Electrolytic oxidation
K2MnO4 \(\leftrightarrow\) 2K++\(MnO^{2-}_4\)
H2O \(\leftrightarrow\) H++OH-
At anode, manganate ions are oxidized to permanganate ions
\(MnO^{2-}_4\)\(\leftrightarrow\) \(MnO^{-}_4\)+e-
Green Purple
Oxidation by chlorine
2K2MnO4+Cl2 \(\rightarrow\) 2KMnO4+2KCl
2\(MnO^{2-}_4\)+Cl2 \(\rightarrow\) 2\(MnO^{-}_4\)+2Cl
Oxidation by ozone
2K2MnO4+O3+H2O \(\rightarrow\) 2KMnO4+2KOH+O2
2\(MnO^{2-}_4\)+O3+H2O \(\rightarrow\) 2\(MnO^{2-}_4\)+2OH-+O2
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Concepts Used:
D and F Block Elements
The d-block elements are placed in groups 3-12 and F-block elements with 4f and 5f orbital filled progressively. The general electronic configuration of d block elements and f- block elements are (n-1) d 1-10 ns 1-2 and (n-2) f 1-14 (n-1) d1 ns2 respectively. They are commonly known as transition elements because they exhibit multiple oxidation states because of the d-d transition which is possible by the availability of vacant d orbitals in these elements.
They have variable Oxidation States as well as are good catalysts because they provide a large surface area for the absorption of reaction. They show variable oxidation states to form intermediate with reactants easily. They are mostly lanthanoids and show lanthanoid contraction. Since differentiating electrons enter in an anti-penultimate f subshell. Therefore, these elements are also called inner transition elements.
Read More: The d and f block elements