Iron(II) ions, Fe2+\text{Fe}^{2+}Fe2+, act as a homogeneous catalyst in the reaction between peroxodisulfate ions (S2O82−\text{S}_2\text{O}_8^{2-}S2O82−) and iodide ions (I−\text{I}^-I−). Which of the following equations represents the step in this catalytic mechanism in which iron is reduced?
S2O82−+2Fe2+→2SO42−+2Fe3+\text{S}_2\text{O}_8^{2-} + 2\text{Fe}^{2+} \rightarrow 2\text{SO}_4^{2-} + 2\text{Fe}^{3+}S2O82−+2Fe2+→2SO42−+2Fe3+
2Fe2++I2→2Fe3++2I−2\text{Fe}^{2+} + \text{I}_2 \rightarrow 2\text{Fe}^{3+} + 2\text{I}^-2Fe2++I2→2Fe3++2I−
2Fe3++2I−→2Fe2++I22\text{Fe}^{3+} + 2\text{I}^- \rightarrow 2\text{Fe}^{2+} + \text{I}_22Fe3++2I−→2Fe2++I2
2SO42−+2Fe3+→S2O82−+2Fe2+2\text{SO}_4^{2-} + 2\text{Fe}^{3+} \rightarrow \text{S}_2\text{O}_8^{2-} + 2\text{Fe}^{2+}2SO42−+2Fe3+→S2O82−+2Fe2+