Chemistry · Interactive
Electrolysis of Water
Pass an electric current through water and it decomposes into hydrogen and oxygen: 2 H2O → 2 H2 + O2. Drag through the steps below to watch the O-H bonds break, the atoms regroup at each electrode, and the gases form in a 2:1 ratio, with every atom conserved. Free to use, and exportable straight into your slides.
Drag to rotate, scroll to zoom, and drag the slider to step through the reaction. Open fullscreen ↗
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What is the electrolysis of water?
Electrolysis of water is the decomposition of water into hydrogen and oxygen gas using electricity. A direct current is passed through the water between two electrodes; the energy breaks the stable O-H bonds so the hydrogen and oxygen can separate. Because it needs a constant supply of electrical energy, the reaction is endothermic and stops as soon as the current is switched off. The balanced chemical equation is 2 H2O → 2 H2 + O2.
How electrolysis of water works, step by step
- 1
Before the current
Two water molecules (H2O) sit at rest. The O-H bonds are stable, so water does not break down on its own. A continuous electric current has to be supplied to start the reaction.
- 2
Decomposition
When a direct current passes through the water, the O-H bonds break. Hydrogen and oxygen atoms migrate toward opposite electrodes and begin to recombine. This decomposition absorbs energy, so it is endothermic.
- 3
Gases form
Hydrogen atoms pair up into hydrogen gas (H2) at the cathode (negative electrode), and oxygen atoms combine into oxygen gas (O2) at the anode (positive electrode). The gases form in a volume ratio of about 2:1, and every atom is accounted for: 4 H and 2 O on each side.
Frequently asked questions
What is the equation for the electrolysis of water?
The balanced equation is 2 H2O -> 2 H2 + O2. Two water molecules decompose into two molecules of hydrogen gas and one molecule of oxygen gas when an electric current is applied.
Why is twice as much hydrogen produced as oxygen?
The balanced equation gives two H2 molecules for every one O2 molecule, so hydrogen and oxygen gas form in a volume ratio of about 2 to 1. You can see this in the simulation as four hydrogen atoms pair into two H2 molecules while two oxygen atoms make a single O2.
Which gas forms at the cathode and which at the anode?
Hydrogen gas (H2) forms at the cathode, the negative electrode, and oxygen gas (O2) forms at the anode, the positive electrode.
Is the electrolysis of water endothermic or exothermic?
It is endothermic. Breaking the stable O-H bonds in water requires a continuous input of electrical energy, which is why the reaction stops when the current is switched off.
How do I use this interactive electrolysis of water diagram?
Drag the reaction-progress slider, or click the numbered steps, to move from the intact water molecules through bond breaking to the finished hydrogen and oxygen gases. Drag to rotate the 3D view, scroll to zoom, and toggle the labels. You can also export it to use in your own slides.