Question

State the observation at the anode and at the cathode during the electrolysis of:

copper sulfate solution using copper electrode

Answer 1

Electrolysis of aqueous Copper sulfate using copper electrodes

• Saturated solution of copper (II) sulfate is prepared in distilled water and concentrated sulfuric acid is added in it so that the conductivity of the electrolyte gets increased.
• Copper rods or plates act as cathode and anode.
• When the current passes through the electrolyte, the cations ${\mathrm{Cu}}^{2+}$and ${\mathrm{H}}^{+}$ ions migrate towards the cathode, while ${{\mathrm{SO}}_4}^{2-}$and ${\mathrm{OH}}^{-}$ ions migrate towards the anode.

Chemical reactions:

$$\begin{gathered} {\mathrm{CuSO}}_4\left(\mathrm{aq}\right)\to {\mathrm{Cu}}^{2+}\left(\mathrm{aq}\right)+{{\mathrm{SO}}_4}^{2-}\left(\mathrm{aq}\right) \\ {\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to 2{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{{\mathrm{SO}}_4}^{2-}\left(\mathrm{aq}\right) \\ {\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right) \end{gathered}$$

Observations at the cathode:

• Copper ions $\left({\mathrm{Cu}}^{2+}\right)$ are placed lower in the activity series when compared to Hydrogen. Thus, they are discharged in preference to ${\mathrm{H}}^{+}$ ions to produce neutral Copper atoms.
• The copper atoms get deposit at the cathode due to which the size of cathode increases.
• The reaction at the cathode is:

${\mathrm{Cu}}^{2+}\left(\mathrm{aq}\right)+2{\mathrm{e}}^{-}\to \mathrm{Cu}\left(\mathrm{s}\right)$

Observations at the anode:

• Migration of ${{\mathrm{SO}}_4}^{2-}$and ${\mathrm{OH}}^{-}$ takes place towards the anode, but neither of them is discharged due to the active nature of the anode.
• Copper ions are formed when copper loses its two electrons.
• Thus, the number of Copper ions produced by the anode is equal to the number of copper ions discharged at the cathode.
• The reaction at the anode is:

$\mathrm{Cu}\left(\mathrm{s}\right)-2{\mathrm{e}}^{-}\to {\mathrm{Cu}}^{2+}\left(\mathrm{aq}\right)$