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Electrolysis of Water

Discusses how water can be turned into hydrogen and oxygen gas using an electrical current.

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Electrolysis of Water

Photoelectrolysis is being explored as a method of generating power

Credit: Courtesy of Steve Hillebrand, US Fish and Wildlife Service
Source: http://commons.wikimedia.org/wiki/File:Evening_sun_reflection.jpg
License: CC BY-NC 3.0

More energy from the sun?

With fossil fuels becoming more expensive and less available, scientists are looking for other energy sources. Hydrogen has long been considered an ideal source, since it does not pollute when it burns. The problem has been finding ways to generate hydrogen economically. One new approach that is being studied is photoelectrolysis – the generation of electricity using photovoltaic cells to split water molecules. This technique is still in the research stage, but appears to be a very promising source of power in the future.

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 H2SO4 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{align*}& \text{oxidation (anode):} && 2\text{H}_2\text{O}(l) \rightarrow \text{O}_2(g)+4\text{H}^+(aq)+4e^- && E^0 = -1.23 \text{ V} \\ & \text{reduction (cathode):} && 2\text{H}_2\text{O}(l)+2e^- \rightarrow \text{H}_2(g)+2\text{OH}^-(aq) && E^0 = -0.83 \text{ V} \\ \hline & \text{overall reaction:} && 2\text{H}_2\text{O}(l) \rightarrow \text{O}_2(g)+2\text{H}_2(g) && E^0{_\text{cell}} = -2.06 \text{ V} \end{align*}

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 H2SO4 is not consumed in the reaction.

Apparatus for the hydrolysis of water

Credit: Students of the University of Siegen, User:Grimlock/Wikimedia Commons
Source: http://commons.wikimedia.org/wiki/File:Elektrolyse1.jpg
License: CC BY-NC 3.0

Apparatus for the production of hydrogen and oxygen gases by the electrolysis of water.[Figure2]


  1. What are the electrodes used in the reaction?
  2. Why is sulfuric acid used?
  3. At which electrode does oxygen appear?

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  1. What are the electrodes?
  2. What is the power source?
  3. What is put in the water to facilitate flow of electricity?
  4. Which test tube contains hydrogen gas?

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Image Attributions

  1. [1]^ Credit: Courtesy of Steve Hillebrand, US Fish and Wildlife Service; Source: http://commons.wikimedia.org/wiki/File:Evening_sun_reflection.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: Students of the University of Siegen, User:Grimlock/Wikimedia Commons; Source: http://commons.wikimedia.org/wiki/File:Elektrolyse1.jpg; License: CC BY-NC 3.0

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