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Which of the following is an example of Disproportionation of an oxidation state, in which a single substance is simultaneously reduced and oxidized to form two different products one with a higher oxidation state and one with a lower oxidation state?

$(1)\; 2Mn^{3+} (\text{aq}) + 2H_2O (\text{l}) \rightarrow MnO_2(\text{s}) + Mn^{2+}(\text{aq})$

$(2) \;3MnO_4^{-2} + 4H^{+} \rightarrow 2MnO_4^{-} + MnO_2 + 2H_2O$

$ (3) \;3CrO_4^{3-} + 8H^{+} \rightarrow 2CrO_4^{2-} + Cr^{3+} + 4H_2O$

$\begin {array} {1 1} Only\;(1)\;and \;(2) \\Only\;(2)\;and \;(3) \\ Only\;(1)\;and \;(3)\\ (1),(2),and \;(3) \end {array}$

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Answer: (1), (2) and (3)
$(1)\; 2Mn^{3+} (\text{aq}) + 2H_2O (\text{l}) \rightarrow MnO_2(\text{s}) + Mn^{2+}(\text{aq})$
$\quad Mn^{3+}$ ions undergo disproportionation to produce $Mn(II)$ ions and $MnO_2$ where the oxidation state is +4.
$(2) 3MnO_4^{-2} + 4H^{+} \rightarrow 2MnO_4^{-} + MnO_2 + 2H_2O$
$\quad Mn(VI)$ undergoes disproportionation to produce $MnO_2$ where the oxidation state is +4 and $MnO_4^{-}$ where the oxidation state is +7
$(3) 3CrO_4^{3-} + 8H^{+} \rightarrow 2CrO_4^{2-} + Cr^{3+} + 4H_2O$
$\quad Cr(V)$ undergo disproportionation to produce $Cr(III)$ ions and $CrO_2^{2-}$ where the oxidation states are +6.
answered Jan 29, 2014 by sreemathi.v
edited Aug 4, 2014 by balaji.thirumalai
 

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