The Electrolysis of Water

The Electrolysis of Water

It is possible to split water into hydrogen and oxygen gas by electrolysis. Acids are typically added to increase the concentration of hydrogen ion in solution (see the figure below). The reactions are

\(\begin{array}{l}\underset{¯}{\begin{array}{l}\text{anode:}\phantom{\rule{3.83em}{0ex}}2{\text{H}}_{2}\text{O}(l)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{O}}_{2}(g)+{\text{4H}}^{\text{+}}(aq)+{\text{4e}}^{\text{−}}\phantom{\rule{4em}{0ex}}{E}_{\text{anode}}^{°}=\text{+1.229 V}\\ \text{cathode:}\phantom{\rule{0.3em}{0ex}}2{\text{H}}^{\text{+}}(aq)+{\text{2e}}^{\text{−}}\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{H}}_{2}(g)\phantom{\rule{4em}{0ex}}{E}_{\text{cathode}}^{°}=\text{0 V}\end{array}}\\ \hline \text{overall:}\phantom{\rule{3.45em}{0ex}}2{\text{H}}_{2}\text{O}(l)\phantom{\rule{0.2em}{0ex}}⟶\phantom{\rule{0.2em}{0ex}}{\text{2H}}_{2}(g)+{\text{O}}_{2}(g)\phantom{\rule{4em}{0ex}}{E}_{\text{cell}}^{°}=\text{−1.229 V}\end{array}\)

Note that the sulfuric acid is not consumed and that the volume of hydrogen gas produced is twice the volume of oxygen gas produced. The minimum applied voltage is 1.229 V.

Water decomposes into oxygen and hydrogen gas during electrolysis. Sulfuric acid was added to increase the concentration of hydrogen ions and the total number of ions in solution, but does not take part in the reaction. The volume of hydrogen gas collected is twice the volume of oxygen gas collected, due to the stoichiometry of the reaction.