1.9.2Principles of Chemistry
Electrolysis Experiments — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 23 of 25 Unit quiz
Section 1 of 5Molten Binary Compounds
Only the ions from the compound are present, so predicting the products is straightforward.
Rule
- A molten binary ionic compound always gives the metal at the cathode and the non-metal at the anode.
Lead(II) bromide
- Pb²⁺ ions go to the cathode and form grey lead metal; Br⁻ ions go to the anode and form brown bromine gas.
Other examples
- Molten copper chloride gives copper and chlorine, and molten magnesium oxide gives magnesium and oxygen.
Apparatus
- Heat the compound in a crucible, insert two graphite electrodes and connect them to a power supply.
Section 2 of 5Aqueous Solutions: the Rules
Aqueous Solutions: the Rules
Extra ions
- Water ionises slightly, so H⁺ and OH⁻ ions are present as well as the ions of the dissolved compound.
Cathode rule
- Hydrogen forms at the cathode unless the metal is less reactive than hydrogen, in which case the metal is deposited.
Anode rule
- A halide ion (Cl⁻, Br⁻, I⁻) is discharged to give the halogen;
- if none is present, oxygen forms from OH⁻ or water.
Order
- Write down all the ions first, then decide which ion at each electrode is discharged.
| Electrolyte | Cathode product | Anode product |
|---|---|---|
| Concentrated sodium chloride | Hydrogen | Chlorine |
| Dilute sulfuric acid | Hydrogen | Oxygen |
| Copper(II) sulfate | Copper | Oxygen |
Section 3 of 5Explaining the Three Examples
Explaining the Three Examples
Sodium chloride
- Na⁺ is more reactive than hydrogen so H₂ forms, and Cl⁻ is discharged at the anode, leaving NaOH in solution.
Dilute sulfuric acid
- H⁺ ions gain electrons to form hydrogen, and sulfate stays in solution while oxygen forms at the anode.
Copper(II) sulfate
- Copper is less reactive than hydrogen so copper coats the cathode; oxygen forms at the anode as before.
Electrodes
- Inert graphite electrodes are used so they do not react with the products.
Section 4 of 5Testing the Products
1
Collect gases in inverted test tubes placed over each electrode.
2
Hydrogen is identified because it burns with a squeaky pop when a lit splint is applied.
3
Oxygen is identified because it relights a glowing splint.
4
Chlorine is identified because it bleaches damp litmus paper.
5
A pink-brown deposit on the cathode shows that copper metal has formed.
Section 5 of 5Exam Checklist & Tips
1.9.2 – Key Points for the Exam
- Binary compound A compound made of just two elements.
- Inert electrode An electrode, such as graphite, that does not take part in the reaction.
- Discharge The gain or loss of electrons by an ion at an electrode to form a neutral species.
- Reactivity series A ranking of metals used to decide whether metal or hydrogen forms at the cathode.
Common Mistakes
- Predicting a metal at the cathode from a sodium or potassium solution; hydrogen forms instead.
- Forgetting that water supplies H⁺ and OH⁻ ions in aqueous electrolysis.
- Saying oxygen forms from sulfate ions; sulfate stays in solution.
- Mixing up which gas gives a squeaky pop and which relights a glowing splint.
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