Aqueous sodium hydroxide is added to an aqueous solution of cobalt(II) chloride. A blue precipitate forms which turns brown when left to stand in air.
Write an ionic equation for the formation of the blue precipitate.
Use the standard electrode potential (EθE^\thetaEθ) data below to explain why the blue precipitate turns brown when left to stand in air, and construct an overall equation for the reaction which occurs.
Redox SystemEquationEθ/V1Co(OH)3(s)+e−⇌Co(OH)2(s)+OH−(aq)+0.172O2(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{Co(OH)}_3(\text{s}) + \text{e}^- \rightleftharpoons \text{Co(OH)}_2(\text{s}) + \text{OH}^-(\text{aq}) & +0.17 \\ 2 & \text{O}_2(\text{g}) + \text{2H}_2\text{O}(\text{l}) + 4\text{e}^- \rightleftharpoons 4\text{OH}^-(\text{aq}) & +0.40 \end{array} Redox System12EquationCo(OH)3(s)+e−⇌Co(OH)2(s)+OH−(aq)O2(g)+2H2O(l)+4e−⇌4OH−(aq)Eθ/V+0.17+0.40
