Question

Acidified water is electrolysed using platinum electrodes. Give the electrode reactions.

Answer 1

Electrolysis is the decomposition of compound into its ions by passing electric current in its aqueous or molten state.

1. Pure water do not electrolyzed that because it contains water molecules which do not dissociatethe into ions.

2. By adding acid such as sulphuric acid $\left({\mathrm{H}}_2{\mathrm{SO}}_4\right)$ in water. It helps in ionization of water.

Chemical Equation: This acidified water dissociates into ions. ${\mathrm{H}}^{+},{\mathrm{OH}}^{-},{\mathrm{SO}}_4^{2-}$ ions are present in the solution.

$$\begin{gathered} {\mathrm{H}}_2{\mathrm{SO}}_4\rightleftharpoons 2{\mathrm{H}}^{+}+{\mathrm{SO}}_4^{2-} \\ \mathrm{sulphuric}\mathrm{acid}\mathrm{hydrogen}\mathrm{ion}\mathrm{sulphate}\mathrm{ion} \end{gathered}$$

$$\begin{gathered} {\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{H}}^{+}+{\mathrm{OH}}^{-} \\ \mathrm{water}\mathrm{hydroxide}\mathrm{ion} \end{gathered}$$

At the anode, ${\mathrm{OH}}^{-}$ ions are negatively charged, move towards the anode which is a positively charged electrode. ${\mathrm{OH}}^{-}$ ions are discharged at the anode.

Reaction :

${\mathrm{OH}}^{-}\to \mathrm{OH}+{\mathrm{e}}^{-}$

$2\mathrm{OH}+2\mathrm{OH}\to 2{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2\uparrow$

At cathode. ${\mathrm{H}}^{+}$ are moves towards the cathode which is a negatively charged electrode.

Reaction :

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

$\mathrm{H}+\mathrm{H}\to {\mathrm{H}}_2\uparrow$

Hence, during electrolysis of acidified water, oxygen is evolved at anode and hydrogen is evolved at cathode.