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EMF of a disproportionation reaction

  1. Oct 13, 2013 #1
    1. The problem statement, all variables and given/known data
    Consider the following sequence of reactions occuring in the acid medium. All emf (standard) are measured at ##25^{\circ} C## and expressed in volt unit:
    $$MnO_4^-\stackrel{0.564}{\rightarrow}MnO_4^{2-}\stackrel{2.26}{\rightarrow}MnO_2\stackrel{0.95}{\rightarrow}Mn^{3+} \stackrel{1.51}{\rightarrow}Mn^{2+}\stackrel{-1.18}{\rightarrow}Mn$$
    Determine the standard emf of the following reaction:
    $$3MnO_4^{2-}+4H^+ \rightarrow MnO_2(s)+2MnO_4^-+2H_2O$$

    2. Relevant equations



    3. The attempt at a solution
    $$MnO_4^{2-}+4H^++2e^- \rightarrow MnO_2+2H_2O \,\,\,\, \Delta G^{\circ}=-4.52F$$
    $$MnO_4^{2-}\rightarrow MnO_4^{-}+e^- \,\,\,\, \Delta G^{\circ}=+0.564F$$
    Multiplying the second equation by two and adding with the first, I get the equation presented in the problem and ##\Delta G^{\circ}=-3.392F##. Let E be the emf of the resulting reaction, then ##\Delta G^{\circ}## is also equal to ##-2FE##. Equating and solving for E, I get ##E=1.696 V## which is incorrect.

    Any help is appreciated. Thanks!
     
  2. jcsd
  3. Oct 14, 2013 #2
  4. Oct 17, 2013 #3
    Bump.
     
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