Electrolysis: predicting the products
Electrolysis uses electricity to break a compound down. Positive ions move to the negative electrode (cathode) and negative ions to the positive electrode (anode) — and a few simple rules let you predict what forms at each.
Key facts
| Topic | Redox Chemistry |
| Cathode (–) | attracts cations; reduction |
| Anode (+) | attracts anions; oxidation |
| Aqueous | water competes with the ions |
The set-up
An electrolyte is a molten or dissolved ionic compound whose ions are free to move. Direct current passes through two electrodes: the cathode (negative) attracts positive ions and the anode (positive) attracts negative ions. At the cathode ions gain electrons (reduction); at the anode they lose electrons (oxidation).
Molten compounds
For a molten ionic compound there are only two ions present, so prediction is simple: the metal forms at the cathode and the non-metal at the anode. Molten lead bromide, for example, gives lead at the cathode and bromine at the anode.
Aqueous solutions
In solution, water adds extra possibilities (H⁺ and OH⁻), so the ions compete. At the cathode, hydrogen is usually released unless the metal is unreactive (like copper), which is deposited instead. At the anode, a halide gives the halogen; otherwise oxygen is released. Concentration and the electrode material can change the outcome, so read the conditions.
Why it matters
Electrolysis is how reactive metals like aluminium are extracted and how metals are purified or electroplated. Exam questions ask you to name the products, write what happens at each electrode, and link the result to the ions present and their reactivity.
Frequently asked questions
What happens at the cathode and anode?
The cathode is negative and attracts positive ions, which gain electrons (reduction); the anode is positive and attracts negative ions, which lose electrons (oxidation).
How do you predict products of a molten compound?
Only two ions are present, so the metal forms at the cathode and the non-metal at the anode.
Why are aqueous products different?
Water provides H⁺ and OH⁻ that compete with the compound's ions, so the products depend on reactivity, concentration and the electrode material.
Where is electrolysis used?
To extract reactive metals such as aluminium, and to purify metals or electroplate them.
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