Manganese may play an important role in chemical cycles in the oceans. Two react

Question

Manganese may play an important role in chemical cycles in the oceans. Two reactions involving manganese (in acid solution) are the reduce nitrate ions (to NO) with Mn^2+ ions and the oxidation of ammonium ions (to N_2) with MnO_2. Calculate E degree cell for the reactions. You may use the standard reduction potential found in the table below. MnO_2(s) + 4 H^+ + 2 e^- doubleheadarrow Mn^2+ + 2 H_2O, E degree = 1.23 V N_2 + 8H^+ +6e^- doubleheadarrow 2NH_4^+, degree = -0.272 V NO_3^- + 4 H^+ + 3 e^+ doubleheadarrow NO(g) + 2 H_2O, E degree = 0.96 V Reduction: V Oxidation: V

Explanation / Answer

MnO2 +4 H+ +2e Mn2+ + 2H2O                         E0 = 1.23 V

N2 + 8 H+ + 6e 2 NH4 +                                      E0= -0.272 V

NO3- + 4 H+ +3e NO +H2O                               E0 = 0.96 V

REDUCTION of nitrate ions E0 = 0.96 V

E0anode = 1.23 V

E0 cell = E0cathode - E0 anode = 0.96 -1.23=- 0.27 V

FOR Oxidation E0anode= -0.272 V

E0 cell = 1.23- (-.272 ) = 1.5 V

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