Test Index

NCERT Class XII Chemistry
Chapter - The d- and f-Block Elements
Questions with Solutions

© examsnet.com
Question : 16 of 49
Marks: +1, -0
Describe the preparation of potassium permanganate. How does the acidified permanganate solution react with (i) iron (ii) ions (ii) SO2\mathrm{SO}_2 and (iii) oxalic acid ? Write the ionic equations for the reactions.
Solution:  
Potassium permanganate (KMnO4)(\mathrm{KMnO}_4) is prepared by the fusion of a mixture of pyrolusite (MnO2)(\mathrm{MnO}_2) , potassium hydroxide and oxygen, first green coloured potassium manganate is formed.
2MnO2+4KOH+O2→2K2MnO4+2H2O2\mathrm{MnO}_2 + 4\mathrm{KOH} + \mathrm{O}_2 \rightarrow 2\mathrm{K}_2\mathrm{MnO}_4 + 2\mathrm{H}_2\mathrm{O}
The potassium manganate is extracted bywater, which then undergoes disproportionation in neutral or acidic solution to give potassium permanganate.
Electrolytically :
3MnO42−+4H+→2MnO4−+MnO2+2H2O3\mathrm{MnO}_4^{2-} + 4\mathrm{H}^+ \rightarrow 2\mathrm{MnO}_4^- + \mathrm{MnO}_2 + 2\mathrm{H}_2\mathrm{O}
In acidic medium of dilute sulphuric acid, KMnO4 acts as strong oxidising agent andit reacts as:
MnO4−+8H++5e−→Mn2++4H2O\mathrm{MnO}_4^- + 8\mathrm{H}^+ + 5e^- \rightarrow \mathrm{Mn}^{2+} + 4\mathrm{H}_2\mathrm{O}
(i) Iron(II) solution : Ferrous (Fe2+)(\mathrm{Fe}^{2+}) ion solution to ferric (Fe3+)(\mathrm{Fe}^{3+}) ion solution.
MnO4−+8H++5e−→Mn2++4H2O\mathrm{MnO}_4^- + 8\mathrm{H}^+ + 5e^- \rightarrow \mathrm{Mn}^{2+} + 4\mathrm{H}_2\mathrm{O}
                                        5Fe2+→5Fe3++5e−\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;5\mathrm{Fe}^{2+} \rightarrow5\mathrm{Fe}^{3+} + 5e^-
———————————————-\text{----------------------------------------------}
MnO4−+5Fe2++8H+→Mn2++5Fe3++4H2O\mathrm{MnO}_4^- + 5\mathrm{Fe}^{2+} + 8\mathrm{H}^+ \rightarrow \mathrm{Mn}^{2+} + 5\mathrm{Fe}^{3+} + 4\mathrm{H}_2\mathrm{O}
————————————————\text{------------------------------------------------}
(ii) Sulphur dioxide (SO2)(\mathrm{SO}_2)
MnO4−+8H++5e−→Mn2++4H2O]×2\mathrm{MnO}_4^- + 8\mathrm{H}^+ + 5e^- \rightarrow \mathrm{Mn}^{2+} + 4\mathrm{H}_2\mathrm{O}] \times 2
10H2O+5SO2→5SO42−+20H++10e−10\mathrm{H}_2\mathrm{O} + 5\mathrm{SO}_2 \rightarrow 5\mathrm{SO}_4^{2-} + 20\mathrm{H}^+ + 10e^-
————————————————\text{------------------------------------------------}
2MnO4−+5SO2+2H2O→5SO42−+2Mn2++4H+2\mathrm{MnO}_4^- + 5\mathrm{SO}_2 + 2\mathrm{H}_2\mathrm{O} \rightarrow 5\mathrm{SO}_4^{2-} + 2\mathrm{Mn}^{2+} + 4\mathrm{H}^+
————————————————\text{------------------------------------------------}
(iii)Oxalic acid
MnO4−+8H++5e−→Mn2++4H2O]×2\mathrm{MnO}_4^- + 8\mathrm{H}^+ + 5e^- \rightarrow \mathrm{Mn}^{2+} + 4\mathrm{H}_2\mathrm{O}] \times 2
10H2O+5SO2→5SO42−+20H++10e−10\mathrm{H}_2\mathrm{O} + 5\mathrm{SO}_2 \rightarrow 5\mathrm{SO}_4^{2-} + 20\mathrm{H}^+ + 10e^-
————————————————\text{------------------------------------------------}
2MnO4−+5SO2+2H2O→5SO42−+2Mn2++4H+2\mathrm{MnO}_4^- + 5\mathrm{SO}_2 + 2\mathrm{H}_2\mathrm{O} \rightarrow 5\mathrm{SO}_4^{2-} + 2\mathrm{Mn}^{2+} + 4\mathrm{H}^+
————————————————\text{------------------------------------------------}
© examsnet.com
Go to Question:
Prev Question
Next Question