Electrolysis explained
How electrolysis splits ionic compounds, which ions go where, and OIL RIG.
What is electrolysis?
Using electricity to break down an ionic compound that is molten or dissolved (so its ions can move). The liquid is called the electrolyte.
Where the ions go
Positive ions (cations) move to the negative electrode (cathode) and gain electrons: this is reduction. Negative ions (anions) move to the positive electrode (anode) and lose electrons: this is oxidation. Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain.
In solutions
At the cathode, hydrogen forms unless the metal is less reactive than hydrogen. At the anode, oxygen forms unless the solution contains halide ions (then the halogen forms).
✏️ Worked example
What forms at each electrode when molten lead bromide is electrolysed?
Cathode: Pb²⁺ gains electrons to make lead. Anode: Br⁻ loses electrons to make bromine.
🧠 Test yourself
Why must the compound be molten or dissolved?
So the ions are free to move to the electrodes.
Is gaining electrons oxidation or reduction?
Reduction.
What is made at the anode in the electrolysis of brine (NaCl solution)?
Chlorine.
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