Question

____ metal cannot be extracted from their aqueous solutions by electrolysis.

Answer 1

Electrolysis of $\mathrm{Ca}$ from its aqueous salt solution$\left({\mathrm{CaCl}}_2\right)$.

• Dissociation of the compounds into the ions takes place as:

$$\begin{gathered} {\mathrm{CaCl}}_2\rightleftharpoons {\mathrm{Ca}}^{+2}+2{\mathrm{Cl}}^{-} \\ {\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{H}}^{+}+{\mathrm{OH}}^{-} \end{gathered}$$

• The cations present are ${\mathrm{Ca}}^{+2}\mathrm{and}{\mathrm{H}}^{+}$, and anions are ${\mathrm{Cl}}^{-}\mathrm{and}{\mathrm{OH}}^{-}$
• At the cathode, both the cations ${\mathrm{Ca}}^{+2}\mathrm{and}{\mathrm{H}}^{+}$ can migrate but because of discharge preference ${\mathrm{H}}^{+}$ will get discharged because it lies lower than $\mathrm{Ca}$ in the electrochemical series.
• So instead of $\mathrm{Ca}$, ${\mathrm{H}}^{+}$ will get reduced and form ${\mathrm{H}}_2$gas.

$2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2$

• At the anode, the anions ${\mathrm{OH}}^{-}\mathrm{and}{\mathrm{Cl}}^{-}$ will migrate but none will get discharged instead $\mathrm{Ca}$ will self ionise to form ${\mathrm{Ca}}^{+2}$ ion.

$\mathrm{Ca}\to {\mathrm{Ca}}^{+2}+2{\mathrm{e}}^{-}$

So, Calcium$\mathbf{\left(}\mathbf{Ca}\mathbf{\right)}$ metal cannot be extracted from their aqueous solutions by electrolysis.