23.9: Electrolysis of Water
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- Mar 21, 2025
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Electrolysis of Water
The electrolysis of water produces hydrogen and oxygen gases. The electrolytic cell consists of a pair of platinum electrodes immersed in water to which a small amount of an electrolyte such as \ce{H_2SO_4} has been added. The electrolyte is necessary because pure water will not carry enough charge due to the lack of ions. At the anode, water is oxidized to oxygen gas and hydrogen ions. At the cathode, water is reduced to hydrogen gas and hydroxide ions.
\begin{array}{lll} \text{oxidation (anode):} & 2 \ce{H_2O} \left( l \right) \rightarrow \ce{O_2} \left( g \right) + 4 \ce{H^+} \left( aq \right) + 4 \ce{e^-} & E^0 = -1.23 \: \text{V} \\ \text{reduction (cathode):} & 2 \ce{H_2O} \left( l \right) + 2 \ce{e^-} \rightarrow \ce{H_2} \left( g \right) + 2 \ce{OH^-} \left( aq \right) & E^0 = -0.83 \: \text{V} \\ \hline \text{overall reaction:} & 2 \ce{H_2O} \left( l \right) \rightarrow \ce{O_2} \left( g \right) + 2 \ce{H_2} \left( g \right) & E^0_\text{cell} = -2.06 \: \text{V} \end{array}\nonumber
In order to obtain the overall reaction, the reduction half-reaction was multiplied by two to equalize the electrons. The hydrogen ion and hydroxide ions produced in each reaction combine to form water. The \ce{H_2SO_4} is not consumed in the reaction.
