Question 20.13: Predicting the Half-Reactions in an Aqueous Electrolysis Wha...
Predicting the Half-Reactions in an Aqueous Electrolysis
What do you expect to be the half-reactions in the electrolysis of aqueous copper(II) sulfate?
The species you should consider for half-reactions are \mathrm{Cu}^{2+}(a q), \mathrm{SO}_{4}{ }^{2-}(a q), and \mathrm{H}_{2} \mathrm{O}.
Possible cathode half-reactions are
\begin{gathered} \mathrm{Cu}^{2+}(a q)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Cu}(s) ; E^{\circ}=0.34 \mathrm{~V} \\ 2 \mathrm{H}_{2} \mathrm{O}(l)+2 \mathrm{e}^{-} \longrightarrow \mathrm{H}_{2}(g)+2 \mathrm{OH}^{-}(a q) ; E^{\circ}=-0.83 \mathrm{~V} \end{gathered}
Because the copper electrode potential is much larger than the reduction potential of water, you expect \mathrm{Cu}^{2+} to be reduced.
Possible anode half-reactions are
\begin{gathered} 2 \mathrm{SO}_{4}{ }^{2-}(a q) \longrightarrow \mathrm{S}_{2} \mathrm{O}_{8}{ }^{2-}(a q)+2 \mathrm{e}^{-} ;-E^{\circ}=-2.01 \mathrm{~V} \\ 2 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{O}_{2}(g)+4 \mathrm{H}^{+}(a q)+4 \mathrm{e}^{-} ;-E^{\circ}=-1.23 \mathrm{~V} \end{gathered}
You expect \mathrm{H}_{2} \mathrm{O} to be oxidized.
The expected half-reactions are
\begin{gathered} \mathrm{Cu}^{2+}(a q)+2 \mathrm{e}^{-} \longrightarrow \mathrm{Cu}(s) \\ 2 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{O}_{2}(g)+4 \mathrm{H}^{+}(a q)+4 \mathrm{e}^{-} \end{gathered}