A student adds aqueous sodium hydroxide to an aqueous solution of iron(II) sulfate. A green precipitate forms, which slowly turns red-brown on its surface when left to stand in air.
Write an ionic equation for the formation of the green precipitate.
Use the standard electrode potential (EθE^\thetaEθ) data below to explain why the green precipitate turns red-brown when left to stand in air, and construct an overall equation for the reaction which occurs.
Redox SystemEquationEθ/V1Fe(OH)3(s)+e−⇌Fe(OH)2(s)+OH−(aq)−0.562O2(g)+2H2O(l)+4e−⇌4OH−(aq)+0.40\begin{array}{c|l|c} \text{Redox System} & \text{Equation} & E^\theta / \text{V} \\ \hline 1 & \text{Fe(OH)}_3(\text{s}) + \text{e}^- \rightleftharpoons \text{Fe(OH)}_2(\text{s}) + \text{OH}^-(\text{aq}) & -0.56 \\ 2 & \text{O}_2(\text{g}) + 2\text{H}_2\text{O}(\text{l}) + 4\text{e}^- \rightleftharpoons 4\text{OH}^-(\text{aq}) & +0.40 \end{array}Redox System12EquationFe(OH)3(s)+e−⇌Fe(OH)2(s)+OH−(aq)O2(g)+2H2O(l)+4e−⇌4OH−(aq)Eθ/V−0.56+0.40