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  4. In electrolysis of NaCl when Pt electrode is taken then H2 is liberated at cathode while with Hg cathode it forms sodium amalgam :-

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Q. In electrolysis of $NaCl$ when $Pt$ electrode is taken then $H_2$ is liberated at cathode while with $Hg$ cathode it forms sodium amalgam :-

AIPMTAIPMT 2002Electrochemistry

Solution:

Sodium chloride in water dissociates as
$ NaCl \rightleftharpoons Na ^{+}+ Cl ^{-} $
$H _{2} O \rightleftharpoons H ^{+}+ OH ^{-}$
When electric current is passed through this solution using platinum electrodes, $Na ^{+}$and $H ^{+}$ move towards cathode whereas $Cl ^{-}$and $OH ^{-}$ions move towards anode.
At cathode
$H ^{+}+ e ^{-} \longrightarrow H$
$H + H \longrightarrow H _{2}$
At anode
$Cl ^{-} \longrightarrow Cl +e^{-}$
$Cl + Cl \longrightarrow Cl _{2}$
If mercury is used as cathode $H ^{+}$ions are not discharged at mercury cathode because mercury has a high hydrogen over voltage. $Na ^{+}$ions are discharged at cathode in preference of $H ^{+}$ions yielding sodium, which dissolves in mercury to form sodium amalgam.