Paper 3 · redox chemistry

An acidic solution of $MnO_4^{2-}_{(aq)}$ undergoes the reaction shown below: $3MnO_4^{2-}_{(aq)} + 4H^+_{(aq)} \rightarrow 2MnO_4^-_{(aq)} + MnO_{2(s)} + 2H_2O_{(l)}$ Which statement about the reaction is correct?

MnO_4^{2-}

is reduced to

MnO_4^-

MnO_4^-

is reduced to

MnO_4^{2-}

MnO_4^{2-}

is oxidised to

MnO_4^-

MnO_4^{2-}

is oxidised to

MnO_2

Explanation: The marking scheme gives answer C. In

MnO_4^{2-}

, Mn is +6. In

MnO_4^-

, Mn is +7 (oxidised). In

MnO_2

, Mn is +4 (reduced). So

MnO_4^{2-}

is both oxidised (to

MnO_4^-

) and reduced (to

MnO_2

) — this is disproportionation. The statement that

MnO_4^{2-}

is oxidised to

MnO_4^-

is correct.

ZIMSEC Chemistry Paper 3, November 2009, Q2

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