Electrolysis — splitting ionic compounds with electricity
Electrolysis uses electrical energy to break down (decompose) an ionic compound into its elements. It only works when the ions are free to move — that is, when the compound is molten or dissolved in water.
Setting up an electrolysis cell
- Electrolyte — molten or aqueous ionic compound (provides free ions).
- Electrodes — usually graphite or platinum (inert): the cathode is connected to the negative terminal; the anode to the positive terminal.
- DC power supply — pushes electrons through the external circuit.
What happens at each electrode
- Cations (+) are attracted to the cathode (−) where they gain electrons (reduction).
- Anions (−) are attracted to the anode (+) where they lose electrons (oxidation).
Why molten or in solution?
In a solid, ions are locked in a lattice — they can't move and so can't carry charge. Once melted (or dissolved), ions move toward the oppositely charged electrode.
✦Worked example— Example: molten lead bromide PbBr₂
When melted, PbBr₂ contains Pb²⁺ and Br⁻ ions:
- At the cathode (−): Pb²⁺ + 2e⁻ → Pb (silvery liquid lead pools at the bottom).
- At the anode (+): 2Br⁻ → Br₂ + 2e⁻ (orange-brown gas).
Aqueous solutions — extra rules (see C4.10)
When an ionic compound is dissolved in water, both the dissolved ions and water (H⁺/OH⁻) can be discharged. Special rules decide which is preferred at each electrode.
📖Definition— Key terminology
- Electrolyte — the ionic substance being electrolysed.
- Electrode — the conductor that delivers electric current.
- Inert electrode — graphite or platinum, doesn't react.
- Cathode — negative electrode (reduction).
- Anode — positive electrode (oxidation).
A useful mnemonic: "PANIC" — Positive = ANode = Is where Chlorine (anions) come out (oxidation).
⚠Common mistakes
- Saying solid ionic compounds conduct. They don't — ions can't move.
- Mixing up cathode and anode. Cathode = negative = cations; anode = positive = anions.
- Confusing electron flow with current direction in equations. At the cathode, ions gain electrons (reduction); at the anode, ions lose electrons (oxidation).
- Forgetting that electrolysis is non-spontaneous — you must put energy in.
Links
Sets up C4.10 (aqueous electrolysis), C4.11 (Al extraction) and C4.12 (half-equations HT).
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