The reaction between peroxodisulfate ions (S2O82−\text{S}_2\text{O}_8^{2-}S2O82−) and bromide ions (Br−\text{Br}^-Br−) in aqueous solution is very slow:
S2O82−(aq)+2Br−(aq)→2SO42−(aq)+Br2(aq)\text{S}_2\text{O}_8^{2-}(\text{aq}) + 2\text{Br}^-(\text{aq}) \rightarrow 2\text{SO}_4^{2-}(\text{aq}) + \text{Br}_2(\text{aq})S2O82−(aq)+2Br−(aq)→2SO42−(aq)+Br2(aq)
This reaction can be catalysed by cobalt(II) ions, Co2+\text{Co}^{2+}Co2+.
Table 1 gives standard electrode potentials for some related half-equations.
Electrode half-equationE∘ / VS2O82−(aq)+2e−→2SO42−(aq)+2.01Co3+(aq)+e−→Co2+(aq)+1.82Br2(aq)+2e−→2Br−(aq)+1.09\begin{array}{|c|c|} \hline \text{Electrode half-equation} & E^\circ \text{ / V} \\ \hline \text{S}_2\text{O}_8^{2-}(\text{aq}) + 2\text{e}^- \rightarrow 2\text{SO}_4^{2-}(\text{aq}) & +2.01 \\ \hline \text{Co}^{3+}(\text{aq}) + \text{e}^- \rightarrow \text{Co}^{2+}(\text{aq}) & +1.82 \\ \hline \text{Br}_2(\text{aq}) + 2\text{e}^- \rightarrow 2\text{Br}^-(\text{aq}) & +1.09 \\ \hline \end{array}Electrode half-equationS2O82−(aq)+2e−→2SO42−(aq)Co3+(aq)+e−→Co2+(aq)Br2(aq)+2e−→2Br−(aq)E∘ / V+2.01+1.82+1.09
Define the term homogeneous catalyst in the context of this reaction.
Explain why the un-catalysed reaction is slow at first, despite being thermodynamically highly favorable.
Use the electrode potential data in Table 1 to explain how Co2+\text{Co}^{2+}Co2+ acts as a catalyst for this reaction. Write balanced equations for the two steps, explaining their feasibility.
