Electrolysis of Water: Diagram, Electrodes and Half-Reactions
Follow electrons, ions and gas formation through a labeled water-electrolysis diagram and understand the 2:1 gas-volume ratio.
Quick answer
Electrolysis uses electrical energy to split water into hydrogen and oxygen. Reduction produces hydrogen at the cathode, oxidation produces oxygen at the anode, and the overall reaction 2H₂O → 2H₂ + O₂ predicts twice as much hydrogen gas as oxygen gas by amount—and approximately by volume under the same conditions.
Key relationships
Read the apparatus before the equations
Two inert electrodes dip into conducting water and connect to a direct-current power source. The negative electrode is the cathode, where electrons enter the solution interface. The positive electrode is the anode, where electrons leave.
Gas-collection tubes over the electrodes make the reaction visible. The tube at the cathode fills with hydrogen faster because the balanced reaction forms two moles of hydrogen for every mole of oxygen.
Track reduction and oxidation
At the cathode, a species gains electrons: this is reduction. In neutral or alkaline descriptions, water gains electrons to form hydrogen gas and hydroxide ions. At the anode, hydroxide or water loses electrons: this is oxidation, producing oxygen gas.
Half-reactions must contain the same total number of electrons before they are added. Canceling those electrons and any species appearing on both sides produces the overall decomposition equation.
Avoid the common electrode-sign confusion
In an electrolytic cell, the external power supply pushes electrons toward the cathode, making it negative, and pulls them from the anode, making it positive. The names cathode and anode are defined by reduction and oxidation, not permanently by sign.
That distinction becomes important when comparing electrolysis with a galvanic cell, where a spontaneous reaction gives the electrodes different signs while reduction still occurs at the cathode.
What to remember
- The cathode is the site of reduction; the anode is the site of oxidation.
- Hydrogen forms at the cathode and oxygen at the anode.
- Pure water conducts poorly, so a suitable electrolyte is normally added.
- The balanced equation predicts a 2:1 hydrogen-to-oxygen ratio.
Frequently asked questions
Why is an electrolyte added to water?
Pure water contains very few mobile ions and conducts electricity poorly. A suitable electrolyte increases conductivity without introducing unwanted electrode products.
Why is there twice as much hydrogen?
The balanced equation produces two moles of H₂ for every mole of O₂, so their volumes are approximately 2:1 at the same temperature and pressure.
Is the cathode always negative?
It is negative in an electrolytic cell, but positive in a galvanic cell. It is always the electrode where reduction occurs.
Watch water electrolysis drawn with both half-reactions
Play the complete lesson free—drawn and narrated from start to finish, with no account needed.
