Definition · Plain-language
Electrolysis
Electrolysis uses an electric current to drive a chemical change that would not happen on its own, decomposing a compound at electrodes.
The step most authors miss
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Using electricity to force a reaction
Some chemical changes do not happen on their own because they are not energetically favourable. Electrolysis supplies the energy to drive such a reaction using an electric current. A direct current is passed through an electrolyte — an ionic compound that is molten or dissolved so its ions can move — by way of two electrodes connected to a power supply. The energy from the current pushes the reaction in a direction it would not take spontaneously, which is why electrolysis is, in effect, the reverse of a spontaneous reaction in a battery.
Electrodes, ions and what happens at each
The two electrodes have opposite charges. The cathode is the negative electrode; positive ions (cations) are attracted to it and gain electrons there — a reduction. The anode is the positive electrode; negative ions (anions) are attracted to it and lose electrons there — an oxidation. In the electrolysis of molten lead bromide, lead ions gain electrons at the cathode to form lead metal, while bromide ions lose electrons at the anode to form bromine gas. The compound is thus decomposed into its elements at the two electrodes.
Why electrolysis matters
Electrolysis is industrially vital. Reactive metals such as aluminium are too tightly bound in their ores to extract by heating with carbon, so they are obtained by electrolysis of a molten compound — aluminium from molten aluminium oxide. The chlor-alkali process electrolyses brine (sodium chloride solution) to make chlorine, hydrogen and sodium hydroxide. Electroplating uses electrolysis to coat one metal with a thin layer of another, for protection or appearance. The splitting of water into hydrogen and oxygen is also a form of electrolysis.
Key facts
At a glance
- Definition: using an electric current to drive a non-spontaneous reaction
- Requires: a molten or dissolved electrolyte with mobile ions
- Cathode: negative electrode — cations gain electrons (reduction)
- Anode: positive electrode — anions lose electrons (oxidation)
- Uses: metal extraction, electroplating, making chlorine
- Example: molten lead bromide → lead + bromine
Common misconceptions
What people often get wrong
Often heard: Electrolysis works on any solid ionic compound.
Actually: The compound must be molten or dissolved so its ions are free to move and carry charge. A solid ionic compound holds its ions fixed in a lattice, so it cannot be electrolysed as a solid.
Often heard: Electrolysis produces electricity.
Actually: It is the opposite: electrolysis consumes electrical energy to force a chemical change. A battery or fuel cell produces electricity from a spontaneous reaction; electrolysis drives a non-spontaneous reaction using a current.
Often heard: The cathode is always the positive electrode.
Actually: In electrolysis the cathode is the negative electrode, where reduction occurs, and the anode is positive. Cathode means the electrode where reduction happens, regardless of the cell type.
Going deeper
