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4.3.2Chemical Changes

Electrolysis of Molten Ionic Compounds — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions

5 min read 7 parts Lesson 14 of 18

📚 AQA GCSE Combined Science Trilogy 8464 - Chemistry Hub / Topical Revision
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Part 1 of 5Rules for molten binary compounds

Only two kinds of ion
  • Only two kinds of ion A binary compound contains just two elements, such as lead(II) bromide, PbBr2, so only two ions are present.
Cathode product
  • Cathode product The positive metal ions move to the negative electrode and form the metal.
Anode product
  • Anode product The negative non-metal ions move to the positive electrode and form the non-metal element.
Why molten
  • Why molten Melting frees the ions to move, so the compound can conduct and be decomposed.

Part 2 of 5Electrolysis of molten lead(II) bromide

1
Heat lead(II) bromide in a crucible until it melts completely.
2
Place two graphite rods in the melt as electrodes and connect them to a power supply.
3
Switch on the current in a fume cupboard, because bromine vapour is toxic.
4
Pb2+ ions move to the cathode, gain two electrons and form a bead of grey lead metal.
5
Br− ions move to the anode, lose electrons and form brown bromine gas, seen as bubbles and brown fumes.

Part 3 of 5Products at each electrode

ElectrodeIon attractedProductObservation
Cathode (−)Pb2+LeadSilvery-grey metal deposit
Anode (+)Br−BromineBrown gas

Part 4 of 5Predicting other products

Molten binary ionic compound
Molten NaCl
  • Molten NaCl Sodium forms at the cathode and chlorine gas at the anode.
Molten ZnCl2
  • Molten ZnCl2 Zinc forms at the cathode and chlorine at the anode.
Molten KI
  • Molten KI Potassium forms at the cathode and iodine at the anode.
General rule
  • General rule Metal at the cathode, non-metal at the anode, whatever the compound.

Part 5 of 5Choice of electrodes

1Electrodes must be inert
  • Electrodes must be inert Graphite or platinum is used because they conduct but do not react.
2Reactive electrodes
  • Reactive electrodes A metal electrode could react with the products or the melt.
3Side reactions
  • Side reactions Extra products would form and the results would not show the true products.
4Clean outcome
  • Clean outcome Inert electrodes make sure only the electrolyte's ions are discharged.

Key terms

TermMeaning
MoltenIn the liquid state after being heated above the melting point.
Binary compoundA compound made of just two elements.
Inert electrodeAn electrode that does not take part in the reaction.
DischargeThe process by which an ion gains or loses electrons at an electrode and becomes neutral.
Cation and anionA positive ion and a negative ion respectively.

Avoid these mistakes

1
Writing that hydrogen is produced; there is no water in a molten electrolyte, so the cathode product is the metal.
2
Forgetting to mention the fume cupboard when bromine or chlorine is produced.
3
Using a reactive metal for the electrodes.
4
Saying the lead appears at the anode; metals always form at the cathode.

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