Question

Electrolysis of concentrated aqueous $NaCl$ is carried out using $Pt$ electrodes. As the electrolysis progresses:

• A. $pH$ decreases
• B. $pH$ increases
• C. $pH$ first decreases and then increases
• D. $pH$ remains constant

Answer 1

The correct option is B $pH$ increases

Electrolysis of concentrated $NaCl\left(aq\right)$ carried out using inert electrodes:

Since ,
$E_{H_{2}O/H_2}^0>E_{N{a}^{+}/Na}^0$

Reaction at cathode:
$2H_{2}O\left(l\right)+2e^{-}\rightleftharpoons H_2\left(g\right)+2O{H}^{-}\left(aq\right)$

Reaction at anode:
Even if $E_{C{l}^{-}/C{l}_2}^0<E_{H_{2}O/O_2}^0$
Kinetically rate of oxidation of water is very slow. To increase its rate, the greater potential difference is applied called as overpotential. Because of this overpotential, oxidation of $C{l}^{–}$ ions becomes more feasible
The reaction that occurs at anode is :
$2C{l}^{-}\rightleftharpoons C{l}_2\left(g\right)+2e^{-}$

The release of $O{H}^{-}$ ions at the cathode results in a basic solution. Thus the $pH$ increases