Question

In the electrolysis of aqueous Copper sulphate between Copper electrodes, the sulphate and hydroxyl ions remain as spectator ions. why?

Answer 1

Electrolysis of Copper sulphate$\left({\mathrm{CuSO}}_4\right)$:

1. In this electrolysis, the electrolyte is aqueous Copper sulphate $\left({\mathrm{CuSO}}_4\right)$ and both the electrodes are Copper.
2. Ionization of electrolytes:
3. $$\begin{gathered} {\mathrm{CuSO}}_4\rightleftharpoons {\mathrm{Cu}}^{2+}+{\mathrm{SO}}_4^{2-} \\ \mathrm{Copper}\mathrm{Sulphate} \\ {\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{H}}^{+}+{\mathrm{OH}}^{-} \\ \mathrm{Water} \end{gathered}$$
4. The ions present in the electrolyte are: ${\mathrm{Cu}}^{+2},{\mathrm{H}}^{+},{\mathrm{SO}}_4^{2-},{\mathrm{OH}}^{-}$
5. Since Cathode is connected to the negative terminal so positively charged ion ${\mathrm{Cu}}^{+2}\mathrm{and}{\mathrm{H}}^{+}$ will migrate to the cathode but ${\mathrm{Cu}}^{+2}$ gets discharge preference because ${\mathrm{Cu}}^{+2}$ lies lower in electrochemical series than ${\mathrm{H}}^{+}$
6. The anode is connected to positive terminal so negatively charged ions ${\mathrm{SO}}_4^{2-}\mathrm{and}{\mathrm{OH}}^{-}$ will migrate toward the anode but Copper being an active electrode ionizes much easier and loses electrons more easily than ${\mathrm{SO}}_4^{2-}\mathrm{and}{\mathrm{OH}}^{-}$.
7. At anode: $\mathrm{Cu}\to {\mathrm{Cu}}^{+2}+2{\mathrm{e}}^{-}$.

So, Sulphate$\left({\mathrm{SO}}_4^{2-}\right)$ and hydroxyl $\left({\mathrm{OH}}^{-}\right)$ions remain as spectator ions because of better ionization of Copper $\left(\mathrm{Cu}\right)$.