3.5.3Movement and Interactions
Electrolysis Of Aqueous Solutions — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions
5 min read 7 parts Lesson 56 of 59
Part 1 of 5Ions in solution
Ions from the compound
- Ions from the compound The dissolved compound supplies positive and negative ions that are free to move.
Ions from water
- Ions from water Water ionises slightly: H2O ⇌ H+ + OH−.
- So there are always two extra ions present, hydrogen and hydroxide.
Only one product at each electrode
- Only one product at each electrode At each electrode there is a choice of two ions, and only one is discharged.
- The other ion stays in solution.
Part 2 of 5Cathode (negative electrode)
1Positive ions attracted
- Positive ions attracted Metal ions and H+ move to the cathode.
2Compare reactivity
- Compare reactivity The less reactive element is the one discharged.
3Product
- Product If the metal is more reactive than hydrogen, hydrogen gas forms: 2H+ + 2e− → H2.
- If less reactive, the metal is deposited on the cathode.
Part 3 of 5Anode (positive electrode)
Anode products
Halide ions present
- Halide ions present Chloride, bromide or iodide ions are discharged, forming chlorine, bromine or iodine.
- Example2Cl− → Cl2 + 2e−.
No halide ions present
- No halide ions present Hydroxide ions are discharged and oxygen forms (for example from sulfate or nitrate solutions).
- 4OH− → O2 + 2H2O + 4e−.
Remaining ions
- Remaining ions The ion not discharged stays in the solution, for example sulfate ions.
Part 4 of 5Predicting products
| Solution | Cathode | Anode |
|---|---|---|
| Sodium chloride, NaCl(aq) | Hydrogen (Na is more reactive than H) | Chlorine |
| Copper sulfate, CuSO4(aq) | Copper (less reactive than H) | Oxygen |
| Magnesium iodide, MgI2(aq) | Hydrogen | Iodine |
| Sodium nitrate, NaNO3(aq) | Hydrogen | Oxygen |
| Copper chloride, CuCl2(aq) | Copper | Chlorine |
Part 5 of 5Gas tests
Hydrogen
- Hydrogen A lit splint gives a squeaky pop.
Oxygen
- Oxygen A glowing splint relights.
Chlorine
- Chlorine Damp blue litmus paper turns red and is then bleached white.
Key terms
| Term | Meaning |
|---|---|
| Electrolyte | A liquid or solution containing free ions that conducts electricity and is broken down by it. |
| Inert electrode | An electrode, such as graphite or platinum, that does not react during electrolysis. |
| Anode | The positive electrode, where negative ions lose electrons. |
| Cathode | The negative electrode, where positive ions gain electrons. |
| Discharged | Describes an ion that has gained or lost electrons to form a neutral atom or molecule. |
Avoid these mistakes
1
Forgetting to list the ions from water, so hydrogen and hydroxide are never considered.
2
Thinking sodium forms at the cathode in a sodium chloride solution. Hydrogen forms because sodium is more reactive.
3
Writing that oxygen comes from the sulfate ion. It comes from the hydroxide ions of water.
4
Mixing up anode and cathodenegative ions go to the positive anode.
5
Not stating that the unreactive ion that is not discharged stays in solution.
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