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    I tried these questions on Redox Equilibria; ELECTRODE POTENTIALS and I just wanna know if the answers are correct

    QUESTION:
    “Show whether disproportionation will occur in the following cases under standard conditions”
    a) VO2+ = VO2+ and V3+
    b) MnO2 = MnO4- and Mn2+

    MY ANSWERS: (the = signs represent equilibrium arrows)

    a) VO2+ = VO2+ and V3+

    using the equations:
    VO2+ + 2H+ + e- = V3+ + H2O (E = +0.34V)
    VO2+ + 2H+ + e- = VO2+ + H2O (E = +1.00V)

    Which then makes;
    V3+ + H2O + VO2+ + 2H+ + e- = VO2+ + 2H+ + e- + VO2+ + H2O

    Simplified down to:
    V3+ + VO2+ = 2VO2+

    And the E nought value for this is 1.00 – (-0.34) = +1.34V

    So the answer for a) is that a disproportionation reaction is feasible, but it is the exact opposite reaction than the one thought to occur (in the question);
    Instead of
    VO2+ = VO2+ and V3+
    We have
    V3+ + VO2+ = 2VO2+

    b) MnO2 = MnO4- and Mn2+

    using the equations
    MnO2 + 4H++ 2e- = Mn2+ + 2H2O (E = +1.23V)
    MnO4- + 8H+ + 5e- = Mn2+ + 4H2O (E = +1.51V)
    MnO4- + 4H+ + 3e- = MnO2 + 2H2O (E = +1.70V)

    (I used the first one and the third one, and multiplied them accordingly so the number of electrons were equal)

    Which then makes:
    3Mn2+ + 6H2O + 2MnO4- + 8H+ + 6e- = 3MnO2 + 12H+ + 6e- + 2MnO2 + 4H2O

    Simplified down to:
    3Mn2+ + 2MnO4- + 2H2O = 5MnO2 + 4H+

    And the E nought value for this is 1.70 – 1.23 = +0.47

    So once again the answer for b) is that a disproportionation reaction is feasible, but it is the exact opposite reaction than the one thought to occur (in the question):
    Instead of
    MnO2 = MnO4- and Mn2+
    We have
    3Mn2+ + 2MnO4- = 5MnO2

    Are these right? (thanx for reading this btw)
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    for (b), if the reverse reaction is feasible then the forward reaction will have the negative of the E0 value and not be feasible. I don't think the back reaction is disproportionation either. So for (b), disproportionation is not feasible. I think.. I have only skimmed it
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    (Original post by mik1w)
    for (b), if the reverse reaction is feasible then the forward reaction will have the negative of the E0 value and not be feasible. I don't think the back reaction is disproportionation either. So for (b), disproportionation is not feasible. I think.. I have only skimmed it
    hang on if the backward reaction is +0.47 then it must be feasible (unlike forward reaction)
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    Yes, so the back reaction is feasible, but the question is asking you about the forward reaction isn't it. The E0 value of that is -0.47 and so the reaction they are asking you about will not happen.
 
 
 
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