Question

A solution contains magnesium ions (${\mathrm{Mg}}^{2+}$), iron (II) ions (${\mathrm{Fe}}^{2+}$) and copper ions (${\mathrm{Cu}}^{2+}$). On passing an electric current through this solution, which ions will be the first to be discharged at the cathode? Write the equation for the cathode reaction.

Answer 1

• ${\mathrm{Mg}}^{2+}$, ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Cu}}^{2+}$ are ions of the same charge present in a solution.
• Thus, the ions would be selectively discharged.
• According to the electrochemical series, the sequence of the cations in the decreasing order of ease of gaining electrons at cathode would be: ${\mathrm{Cu}}^{2+}$ > ${\mathrm{Fe}}^{2+}$ > ${\mathrm{Mg}}^{2+}$.
• Therefore, ${\mathrm{Cu}}^{2+}$ would be the first to be discharged at cathode, when electric current is passed through the solution.
• Reaction at cathode: ${\mathrm{Cu}}^{2+}\left(\mathrm{aq}\right)+2e^{–}\to \mathrm{Cu}\left(s\right)$