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Question
Describe the preparation of potassium permanganate. How does the acidified permanganate solution react with iron (II) ions? Write the ionic equation for the reaction.
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Solution
Potassium permanganate is prepared by fusion of MnO2 with an alkali metal hydroxide and an oxidising agent like KNO3. This produces the dark green K2MnO4, which disproportionates in a neutral or acidic solution to give permanganate.
\[\ce{2MnO2 + 4KOH + O2 -> 2K2MnO4 + 2H2O}\]
Potassium manganate is extracted with water and thereafter disproportionates in neutral or acidic solution to yield potassium permanganate.
\[\ce{3MnO^{2-}_4 + 4H^+ -> 2MnO^-_4 + MnO2 + 2H2O}\]
In an acidic medium of dilute sulphuric acid, KMnO4 acts as a strong oxidising agent, and it reacts as:
\[\ce{MnO^-_4 + 8H^+ + 5e- -> Mn^2+ + 4H2O}\]
Ferrous (Fe2+) ion solution to ferric (Fe3+) ion solution
\[\ce{MnO^-_4 + 8H+ + 5e- -> Mn^{2+} + 4H2O}\]
\[\ce{5Fe^{2+} -> 5Fe^{3+} + 5e-}\]
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\[\ce{MnO^-_4 + 5Fe^{2+} + 8H+ -> Mn^{2+} + 5Fe^{3+} + 4H2O}\]
RELATED QUESTIONS
Complete the following equations:
When chromite ore FeCr2O4 is fused with NaOH in presence of air, a yellow-coloured compound (A) is obtained, which on acidification with dilute sulphuric acid gives a compound (B). Compound (B) on reaction with KCl forms an orange coloured crystalline compound (C).
(i) Write the formulae of the compounds (A), (B) and C.
(ii) Write one use of compound (C).
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(ii) \[\ce{Mo(VI)}\] and \[\ce{and W(VI)}\] are more stable than \[\ce{Cr(VI)}\].
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(iv) Lower oxidation states of heavier members of group-6 of transition series are more stable.
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\[\ce{Cr2O^{2-}7 + 3H2S + 8H^+ ->}\]
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Indicate the steps in the preparation of \[\ce{K2Cr2O7}\] from chromite ore.
