The skeletal equation is : $\mathrm{Mn}^{3+}_{(aq)}$ → $\mathrm{Mn}^{2+}_{(aq)}+\mathrm{MnO}_{2(s)}+\mathrm{H}^{+}_{(aq)}$ Oxidation half equation : $\mathrm{Mn}^{3+}_{(aq)}$ → $\mathrm{MnO}_{2(s)}^{+4}$ or, $\mathrm{Mn}^{+3}_{(aq)}+2\mathrm{H}_2\mathrm{O}_{(l)}$ → $\mathrm{MnO}_{2(s)} + 4\mathrm{H}^{+}_{(aq)} + e^{-}$ ... (i) Reduction half equation : $\mathrm{Mn}^{3+}_{(aq)}$ → $\mathrm{Mn}^{2+}_{(aq)}$ or, $\mathrm{Mn}^{3+}_{(aq)}+e^{-}$ → $\mathrm{Mn}^{2+}_{(aq)}$ ... (ii) Adding Eq. (i) and Eq. (ii), the balanced equation for the disproportionation reaction is $2\mathrm{Mn}^{3+}_{(aq)} + 2\mathrm{H}_2\mathrm{O}_{(l)}$ → $\mathrm{MnO}_{2(s)}+\mathrm{Mn}^{2+}_{(aq)} + 4\mathrm{H}^{+}_{(aq)}$