(N/A) Dihydrogen is prepared by the electrolysis of acidified or alkaline water using platinum electrodes. Generally,$15-20 \%$ of an acid $(H_2SO_4)$ or a base $(NaOH)$ is used.
Reduction of water occurs at the cathode as:
$2H_2O + 2e^- \longrightarrow H_2 + 2OH^-$
At the anode,oxidation of $OH^-$ ions takes place as:
$2OH^- \longrightarrow H_2O + \frac{1}{2}O_2 + 2e^-$
Net reaction can be represented as:
$H_2O_{(l)} \longrightarrow H_{2(g)} + \frac{1}{2}O_{2(g)}$
Electrical conductivity of pure water is very low owing to the absence of ions in it. Therefore,electrolysis of pure water takes place at a very low rate. If an electrolyte such as an acid or a base is added,the rate of electrolysis increases. The addition of the electrolyte provides ions,which facilitate the conduction of electricity and allow the electrolysis process to proceed efficiently.