6.2.14Additional lessons
Electrolysis of Aqueous Solutions — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 145 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Competing Species
With several ions present, each electrode discharges the species that is easiest to reduce or oxidise.
Water ions
- Water ions An aqueous solution contains H⁺ and OH⁻ from water as well as the ions of the dissolved salt.
Cathode choice
- Cathode choice The species with the more positive E° is reduced, so unreactive metals form but reactive metals like sodium do not.
Anode choice
- Anode choice Halide ions are usually oxidised in preference to water when concentrated, while water forms oxygen otherwise.
Water half-equations
- Water half-equations 2H₂O + 2e⁻ → H₂ + 2OH⁻ at the cathode, and 2H₂O → O₂ + 4H⁺ + 4e⁻ at the anode.
Part 2 of 4Worked Cases
1
Concentrated aqueous sodium chloride gives hydrogen at the cathode, because sodium ions are too hard to reduce.
2
The same solution gives chlorine at the anode: 2Cl⁻ → Cl₂ + 2e⁻, because of the high chloride concentration.
3
The solution left behind contains Na⁺ and OH⁻, so it becomes alkaline sodium hydroxide.
4
Dilute sodium chloride or aqueous sodium sulfate gives oxygen at the anode, as water is oxidised instead.
5
Copper(II) sulfate with inert electrodes gives copper at the cathode and oxygen at the anode, making the solution more acidic.
Part 3 of 4Effect of the Electrode and Concentration
Effect of the Electrode and Concentration
Copper electrodes
- Copper electrodes With a copper anode in copper(II) sulfate, the anode itself dissolves: Cu → Cu²⁺ + 2e⁻.
Concentration effect
- Concentration effect A high concentration of chloride can make chloride oxidation win over water oxidation despite the potentials.
Colour change
- Colour change As copper ions are removed with inert electrodes the blue colour of the solution fades.
Gas tests
- Gas tests Hydrogen gives a squeaky pop with a lit splint, and chlorine bleaches damp blue litmus paper.
| Electrolyte | Cathode product | Anode product |
|---|---|---|
| Concentrated NaCl(aq) | H₂ | Cl₂ |
| Dilute NaCl(aq) | H₂ | O₂ (and some Cl₂) |
| CuSO₄(aq), inert electrodes | Cu | O₂ |
| CuSO₄(aq), Cu electrodes | Cu | Cu dissolves |
Part 4 of 4Exam Checklist & Tips
6.2.14 – Key Points for the Exam
- Preferential discharge The selection of one ion over another at an electrode during electrolysis.
- Inert electrode An electrode such as graphite or platinum that does not take part in the reaction.
- Active electrode An electrode such as copper that can be oxidised during electrolysis.
- Electrode potential A measure of the tendency of a species to be reduced, in volts.
Common Mistakes
- Predicting that sodium metal forms at the cathode in aqueous sodium chloride.
- Forgetting that water can be oxidised or reduced and giving only the salt ions.
- Ignoring the electrode material, which can dissolve if it is active.
- Writing electrons on the wrong side of the half-equations.
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