Question

State the factors that influence the preferential discharge of ions at the electrodes.

Answer 1

If the presence of cations and anions are more than one in the electrolytic cell, then the ions of same charge move towards the similar electrode. But the ion that has more preference alone gets discharged at the electrode. There are some factors which influence the preferential discharge of ions at the electrodes. They are:

1. The placement of metallic ion in electrochemical series: The ion located lower in the electrochemical series gets discharged first at the respective electrodes in preference with other ions placed higher in the series.
For example, in the electrolysis reaction, if the electrolyte is aqueous copper sulphate solution, the dissociation of electrolyte takes place as:
$$\begin{gathered} {\mathrm{CuSO}}_4\stackrel{}{\to }{\mathrm{Cu}}^{2+}+{\mathrm{SO}}_4^{2-} \\ {\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{OH}}^{-} \end{gathered}$$

The ions migrating to cathode would be ${\mathrm{Cu}}^{2+},{\mathrm{H}}_3{\mathrm{O}}^{+}$ ions. But the ion that is preferentially discharged at cathode is ${\mathrm{Cu}}^{\mathrm{2+}}$ ions as it is located lower than ${\mathrm{H}}^{+}$ ion in the electrochemical series. Similarly ions migrating to anode would be ${\mathrm{SO}}_4^{2-},{\mathrm{OH}}^{-}$ ions. But the ion that is preferentially discharged at anode is ${\mathrm{OH}}^{-}$ as it is located lower than ${\mathrm{SO}}_4^{2-}$ in the electrochemical series.

2. Ionic concentration in the electrolyte: If the concentration of an anion present in the electrolyte is high, then this anion is preferentially discharged at anode than other anions. For example, in the electrolysis of concentrated NaCl, chloride ions get preferentially discharged at anode, even though ${\mathrm{OH}}^{-}$ ions are located lower than ${\mathrm{Cl}}^{-}$ ions in the electrochemical series.

3. Type of electrode: If the electrode involved in the electrolysis is inert like graphite or platinum, then it does not decide the preferential ion discharged at it. But if the electrode involved in the electrolysis is reactive like Cu, Ni, etc. then it gets involved in the electrode reaction and decides the preferential ion discharged at it. In these cases, anions migrate to anode but they do not discharge, instead reactive anode itself loses electrons and forms ions.
For example, in the electrolysis of aqueous copper sulphate between copper electrodes, ${\mathrm{OH}}^{-}$ ions do not get discharged but the copper anode itself loses electrons to form ${\mathrm{Cu}}^{\mathrm{2+}}$ ions. The electrolytic reactions that take place are:
$$\begin{gathered} {\mathrm{CuSO}}_4\to {\mathrm{Cu}}^{2+}+{\mathrm{SO}}_4^{2-} \\ {\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}^{+}+{\mathrm{OH}}^{-} \end{gathered}$$

Reaction at cathode: ${\mathrm{Cu}}^{2+}+2{\mathrm{e}}^{-}\to \mathrm{Cu}(\mathrm{is}\mathrm{preferentially}\mathrm{discharged}\mathrm{to}{\mathrm{H}}^{+}\mathrm{ions})$

Reaction at anode: $\mathrm{Cu}\stackrel{}{\to }{\mathrm{Cu}}^{2+}+2{\mathrm{e}}^{-}$ (SO₄^2-, OH^- are not discharged at anode while Cu anode oxidises itself).