Class 12 NEET Chemistry Electrochemistry Questions

When sodium chloride is dissolved in water, it ionises as $\mathrm{NaCl} \rightleftharpoons \mathrm{Na}^{+} + \mathrm{Cl}^{-}$ Water also dissociates as : $\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{H}^{+} + \mathrm{OH}^{-}$ During passing of electric current through this solution using platinum electrode, $\mathrm{Na}^{+}$ and $\mathrm{H}^{+}$ ions move towards cathode. However, only $\mathrm{H}^{+}$ ions are discharged more readily than $\mathrm{Na}^{+}$ ion because of their low discharge potential (In the electromotive series hydrogen is lower than sodium). These $\mathrm{H}^{+}$ ions gain electrons and change into neutral atoms. At cathode $\mathrm{H}^{+} + e^{-} \rightarrow \mathrm{H}, \mathrm{H} + \mathrm{H} \rightarrow \mathrm{H}_{2}$ $\mathrm{Cl}^{-}$ and $\mathrm{OH}^{-}$ ions move towards anode. $\mathrm{Cl}^{-}$ ions lose electrons and change into neutral atom. At anode, $\mathrm{Cl}^{-} - e^{-} \rightarrow \mathrm{Cl}, \mathrm{Cl} + \mathrm{Cl} \rightarrow \mathrm{Cl}_{2}$, $\mathrm{H}^{+}$ If mercury is used as cathode, $\mathrm{Na}^{+}$ ions are not discharged at mercury cathode because mercury has a high hydrogen overvoltage. $\mathrm{H}^{+}$ ions are discharged at the cathode in perference to $\mathrm{Na}^{+} + e^{-} = \mathrm{Na}$ ions, yielding sodium, which dissolves in mercury to form sodium amalgam. At cathode : $\mathrm{Hg}$