NEET UG 2002 — Chemistry Physical Chemistry

Sodium chloride in water dissociates as: $$\begin{aligned} \mathrm{NaCl} & \rightleftharpoons \mathrm{Na}^{+}+\mathrm{Cl}^{-} \\ \mathrm{H}_2 \mathrm{O} & \rightleftharpoons \mathrm{H}^{+} \mathrm{OH}^{-} \end{aligned}$$NaClH2​O​⇌Na++Cl−⇌H+OH−​ When electric current is passed through this solution using platinum electrodes. $\mathrm{Na}^{+}$Na+and $\mathrm{H}^{+}$H+move towards cathode whereas $\mathrm{Cl}^{-}$Cl−and $\mathrm{OH}^{-}$OH−ions move towards anode. At cathode $$\begin{aligned} \mathrm{H}^{+}+e^{-} & \rightarrow \mathrm{H} \\ \mathrm{H}+\mathrm{H} & \rightarrow \mathrm{H}_2 \end{aligned}$$H++e−H+H​→H→H2​​ An anode $$\begin{aligned} \mathrm{Cl}^{-} & \rightarrow \mathrm{Cl}+e^{-} \\ \mathrm{Cl}+\mathrm{Cl} & \rightarrow \mathrm{Cl}_2 \end{aligned}$$Cl−Cl+Cl​→Cl+e−→Cl2​​ If mercury is used as cathode $\mathrm{H}^{+}$H+ ions are not discharged at mercury cathode because mercury has a high hydrogen over voltage. $\mathrm{Na}^{+}$Na+ ions are dischaged at cathode in preference of $\mathrm{H}^{+}$H+ions yielding sodium, which dissolves in mercury to form amalgam