4.3.4Chemical Changes
Electrolysis of Aqueous Solutions — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 16 of 18
Part 1 of 5Two sets of ions
Ions from the compound
- Ions from the compound The dissolved compound supplies its own positive and negative ions.
Ions from water
- Ions from water Water ionises slightly: H2O ⇌ H+ + OH−.
- So H+ and OH− are always in the solution as well.
Competition
- Competition At each electrode two ions compete, but only one is discharged.
- Write out every ion first, then decide.
Part 2 of 5Rules for each electrode
| Electrode | Ions competing | What is discharged |
|---|---|---|
| Cathode (−) | Metal ion and H+ | Hydrogen if the metal is more reactive than hydrogen; the metal if it is less reactive |
| Anode (+) | Halide ion and OH− | The halide, forming chlorine, bromine or iodine |
| Anode (+) | Sulfate or nitrate and OH− | OH−, forming oxygen and water |
Part 3 of 5Predicting products
1
List all the ions present, including H+ and OH− from water.
2
At the cathode compare the metal with hydrogen in the reactivity series.
3
If the metal is higher than hydrogen, hydrogen gas is released; if lower, the metal is deposited.
4
At the anode check for a halide ion: if present it forms the halogen; if absent, oxygen forms.
5
State that the ions not discharged stay in the solution.
Part 4 of 5Named solutions
| Solution | Anode product | Cathode product |
|---|---|---|
| Copper(II) chloride | Chlorine | Copper |
| Sodium chloride | Chlorine | Hydrogen |
| Sodium sulfate | Oxygen | Hydrogen |
| Copper(II) sulfate | Oxygen | Copper |
| Water acidified with sulfuric acid | Oxygen | Hydrogen |
Part 5 of 5Gas tests
Identifying the products
Hydrogen
- Hydrogen A lit splint makes a squeaky pop.
Oxygen
- Oxygen A glowing splint relights.
Chlorine
- Chlorine Damp blue litmus paper turns red and is then bleached white.
Copper
- Copper A pink-brown solid coats the cathode.
Key terms
| Term | Meaning |
|---|---|
| Aqueous solution | A solution in which the solvent is water. |
| Discharged | Describes an ion that has gained or lost electrons at an electrode. |
| Halide ion | A negative ion of a halogen such as Cl−, Br− or I−. |
| Inert electrode | An electrode, usually graphite, that does not react. |
| Reactivity series | A list used to predict whether a metal or hydrogen is discharged at the cathode. |
Avoid these mistakes
1
Forgetting that water supplies H+ and OH− and so predicting sodium at the cathode.
2
Giving oxygen at the anode for a chloride solution; the chloride is discharged in preference.
3
Saying all the ions are discharged; only one ion at each electrode is.
4
Not identifying the gas with the correct test.
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