Question

Electrolysis is carried out in three cells
(A) $1.0M$ $Cu{SO}_4$ PT electrode
(B) $1.0M$ $Cu{SO}_4$ copper electrodes
(C) $1.0M$ $KCl$ Pt electrodes
If volume of electrolytic solution is maintained constant in each of the cell, which is correct set of pH changes in (A),(B) and (C) cell respectively?

• A. Decrease in all the three
• B. Increase in all the three
• C. Decrease, constant, increase
• D. Increase, constant, increase

Answer 1

The correct option is C Decrease, constant, increase

If volume of electrolytic solution is maintained constant in each of the cell, the correct set of pH changes in (A),(B) and (C) cell is Decrease, constant, increase respectively.
(A) $1.0M$ $Cu{SO}_4$ Pt electrode
Anode:
$H_{2}O⟶2H^{+}+\frac{1}{2}{O}_2+2e^{-}$
Cathode
${Cu}^{2+}\left(aq\right)+2e^{-}⟶Cu\left(s\right)$ (pH=decrease)
Since protons are generated during reaction, $\left[H^{+}\right]$ increases and pH decreases.
(B) $1.0M$ $Cu{SO}_4$ copper electrodes
Anode:
$Cu\left(s\right)⟶{Cu}^{2+}\left(aq\right)+2e^{-}$
Cathode
${Cu}^{2+}\left(aq\right)+2e^{-}⟶Cu\left(s\right)$ (pH=constant)
Since neither protons nor hydroxide ions are generated/consumed during reaction, $\left[H^{+}\right]$ and pH remains same.
(C) $1.0M$ $KCl$ Pt electrodes
Anode:
$2{Cl}^{-}⟶{Cl}_2\left(aq\right)+2e^{-}$
Cathode
$2H_{2}O+2e^{-}⟶H_2+2{OH}^{-}\left(aq\right)$ (pH=increase)
Since hydroxide ions are generated during reaction, $\left[O{H}^{-}\right]$ increases, $\left[H^{+}\right]$ decreases and pH increases.