The key idea
Electrolysis uses electricity to decompose an ionic compound.At the cathode, cations are reduced (gain electrons); at the anode, anions are oxidised (lose electrons).
cathode: Cu²⁺ + 2e⁻ → Cu
Use the labels to explain the scientific relationship shown.
The bit that matters
Keep the idea tight, then use the worked example to practise the exact exam wording.
Principles of electrolysis
Electrolysis requires a molten ionic compound or an aqueous solution of an ionic compound, connected to a direct current supply via two electrodes.Positive ions (cations) move to the negative cathode, where they gain electrons (reduction).Negative ions (anions) move to the positive anode, where they lose electrons (oxidation).The mnemonic AN OX RED CAT (ANOde-OXidation, REDuction-CATHode) helps recall this.
Electrolysis of aqueous solutions
When an ionic substance is dissolved in water, both the solute ions and water molecules (H⁺ and OH⁻) are present.The ions discharged depend on their relative positions in the electrolytic series: at the cathode, the less reactive metal or H⁺ is preferentially reduced; at the anode, Cl⁻ is preferentially discharged if concentrated, otherwise OH⁻ gives oxygen.
Industrial electrolysis — aluminium
Aluminium is extracted by electrolysin of aluminium oxide dissolved in molten cryolite.Cryolite lowers the melting point from about 2000°C to about 850°C, greatly reducing energy costs.Aluminium forms at the cathode and oxygen at the anode; the carbon anodes are gradually oxidised and need regular replacement.
Electroplating
Electroplating deposits a thin layer of one metal on another to improve appearance or resistance to corrosion.The object to be plated is the cathode; the plating metal is the anode; the electrolyte contains ions of the plating metal.Silver electroplating is used for jewellery; chromium plating is used on car parts.
Definitions to learn
Electrolysis
The decomposition of an ionic compound by passing electricity through it in molten or aqueous form.
Cathode
The negative electrode in electrolysis where reduction (gain of electrons) occurs.
Anode
The positive electrode in electrolysis where oxidation (loss of electrons) occurs.
Electrolyte
A molten or dissolved ionic compound that conducts electricity by ion movement.
Half-equation
An equation showing either the oxidation or the reduction half of an electrode reaction, including electrons.
Electroplating
Using electrolysis to deposit a thin layer of metal onto another object.
State the products at each electrode when aqueous copper sulfate is electrolysed with copper electrodes.
At the cathode (negative): Cu²⁺ ions are reduced → copper metal is deposited.
At the anode (positive): copper electrode dissolves → Cu → Cu²⁺ + 2e⁻.
The concentration of copper sulfate stays constant.
Cathode: copper deposited; Anode: copper dissolves.
For electrolysis questions, always state which electrode is positive (anode) and which is negative (cathode), then link each to oxidation or reduction.
Do not confuse cathode (negative, reduction) and anode (positive, oxidation). Use the mnemonic AN OX RED CAT.
How to score full marks
- 1Use AN OX RED CAT: ANOde = OXidation (loss of e⁻); CATHode = REDuction (gain of e⁻).
- 2In half-equations, electrons (e⁻) appear on the left for reduction (cathode) and on the right for oxidation (anode).
- 3For aqueous solution electrolysis, always consider whether H⁺/OH⁻ from water or the solute ions are discharged — concentration matters.
Try these yourself
Open each answer only after you have explained the full chemical process.
1Define electrolysis.[1 mark]
- 1.Describe the process in terms of current and compound.
2State which electrode is the cathode and what process occurs there.[2 marks]
- 1.Recall cathode = negative electrode.
3Write the half-equation for the formation of copper at the cathode.[1 mark]
- 1.Show electrons being gained by Cu²⁺.
4State the products at the cathode and anode when dilute sulfuric acid is electrolysed.[2 marks]
- 1.Consider which ions are discharged preferentially from aqueous solution.
5Explain why hydrogen is produced at the cathode during the electrolysis of dilute hydrochloric acid rather than sodium (from NaOH solution).[3 marks]
- 1.Compare reactivity of ions and position in the electrolytic series.
6Write the half-equation for the production of chlorine at the anode from concentrated hydrochloric acid.[1 mark]
- 1.Show electrons lost by chloride ions.
7Describe the industrial production of aluminium by electrolysis of aluminium oxide, including why cryolite is used.[5 marks]
- 1.State the role of cryolite and identify the electrodes.
8Explain the role of electrolysis in electroplating a steel object with nickel.[4 marks]
- 1.Identify the anode, cathode and electrolyte.