4.1.2Electrolysis
Electrolysis of Molten Compounds — Combined Science 0653 - Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 4 of 6 Unit quiz
Part 1 of 4Key ideas
Electrolysis
- Electrolysis Electrolysis uses an electric current to break down an ionic compound that is molten or dissolved in water.
Why molten works
- Why molten works In a solid the ions are fixed in a lattice and cannot carry charge.
- When molten, the ions are free to move, so the liquid conducts and is called an electrolyte.
Inert electrodes
- Inert electrodes Graphite or platinum is used because they conduct but do not take part in any side reactions.
Binary compounds
- Binary compounds A binary compound contains just two elements, so electrolysis of the molten form gives the two elements.
Part 2 of 4Electrolysis of molten lead(II) bromide
1
Place solid lead(II) bromide in a heat-resistant container and heat strongly until it melts.
2
Dip two graphite rods into the liquid and connect them to a DC power supply.
3
Switch on; the positive lead ions are attracted to the negative electrode (cathode).
4
At the cathode lead ions gain electrons to form grey molten lead, which collects underneath.
5
The negative bromide ions move to the positive electrode (anode) where they lose electrons and form bromine; brown fumes appear and bubbles form.
6
Carry out the experiment in a fume cupboard, because bromine vapour is toxic.
Part 3 of 4Predicting the products
1Identify the ions
- Identify the ions Split the formula into a metal cation and a non-metal anion.
2Opposites attract
- Opposites attract Cations move to the negative cathode; anions move to the positive anode.
3Discharge
- Discharge The ions gain or lose electrons to become uncharged atoms or molecules.
4Products
- Products The metal forms at the cathode and the non-metal at the anode.
Part 4 of 4Examples
| Molten compound | Cathode (−) product | Anode (+) product |
|---|---|---|
| Lead(II) bromide, PbBr2 | Lead | Bromine |
| Zinc chloride, ZnCl2 | Zinc | Chlorine |
| Potassium chloride, KCl | Potassium | Chlorine |
| Aluminium oxide, Al2O3 | Aluminium | Oxygen |
Key terms
| Term | Meaning |
|---|---|
| Electrolyte | An ionic liquid or solution that conducts electricity and is decomposed by it. |
| Cathode | The negative electrode, where positive ions gain electrons. |
| Anode | The positive electrode, where negative ions lose electrons. |
| Inert electrode | An electrode made of a material that does not react, such as graphite. |
| Molten | Melted to form a liquid. |
Avoid these mistakes
1
Saying that electrons travel through the molten liquid; electrons flow in the wires, while ions carry the charge in the electrolyte.
2
Mixing up anode and cathode; remember that the cathode is negative and attracts cations.
3
Trying to electrolyse the solid compound, which does not conduct because its ions cannot move.
4
Writing that hydrogen is produced; in a molten binary compound only the two elements are formed.
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