Question

In electrolysis of NaCl when Pt electrode is taken then $H_2$ is liberated at cathode while with Hg cathode it forms sodium amalgam because :
a) Hg is more inert than Pt.
b) More voltage is required to reduce $H^{+}$ at Hg than at Pt.
c) Na is dissolved in Hg while it does not dissolve in Pt.
d) Concentration of $H^{+}$ ions is larger than Pt electrode is taken.

Answer 1

Hint: Electrolysis is the process by which ionic substances are decomposed into simpler substances when an electric current is passed through it.

Complete step by step answer:
When sodium chloride is dissolved in water, it ionizes as
$NaCl\rightleftharpoons N{a}^{+}+C{l}^{-}$
Water dissociates as ,
$H_{2}O\rightleftharpoons H^{+}+O{H}^{-}$
During passing of electric current through this solution using platinum electrode , $N{a}^{+}$ and $H^{+}$ ions move towards the cathode. However, only $H^{+}$ ions are discharged more readily than $N{a}^{+}$ ions because of their low discharge potential since , in electromotive series hydrogen is lower sodium. These $H^{+}$ ions gain electrons and change into neutral atoms.
At cathode,
$$\begin{gathered} H^{+}+e^{-}\to H \\ H+H\to H_2 \end{gathered}$$
$C{l}^{-}$ and $O{H}^{-}$ ions move towards anode.
$C{l}^{-}$ ions lose electrons and change into neutral atoms.
At anode ,
$C{l}^{-}+e^{-}\to Cl$
$Cl+Cl\to C{l}_2$
If mercury is used as cathode $H^{+}$ ions are not discharged at mercury cathode because mercury has a high hydrogen overvoltage. $N{a}^{+}$ ions are discharged at the cathode in preference to $H^{+}$ ions, yielding sodium, which dissolves in mercury to form sodium amalgam.
At cathode $N{a}^{+}+e^{-}\to Na$.
Thus, more voltage is required to reduce $H^{+}$ at amalgam cathode than a platinum cathode.

Hence , the required option for the given question is b).

Note: During passing of electric current through this solution using platinum electrode, $N{a}^{+}$ and $H^{+}$ ions move towards the cathode. However, only $H^{+}$ ions are discharged more readily than $N{a}^{+}$ ions because of their low discharge potential since, in electromotive series hydrogen is lower sodium. These $H^{+}$ ions gain electrons and change into neutral atoms.