Question

Give reason for the following :

The volume of hydrogen and oxygen formed at the cathode and anode are 2:1 by volume respectively.

Answer 1

During the electrolysis of water :
$2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\stackrel{\mathrm{Electrolysis}}{\to }2{\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)$
2 vol. 2 vol. 1 vol. (By Avogadro's law)

At anode :
$2{\mathrm{O}}^{2-}\left(\mathrm{aq}\right)-4{\mathrm{e}}^{-}\to {\mathrm{O}}_2\left(\mathrm{g}\right)$

At cathode :
$4{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+4{\mathrm{e}}^{-}\to 2{\mathrm{H}}_2\left(\mathrm{g}\right)$

• Hence, at the anode, 1 molecule of oxygen produces 4 electrons that are consumed by 2 molecules of hydrogen at the cathode.
• Therefore, the volume of hydrogen and oxygen formed at the cathode and anode are 2:1 by volume respectively.