Electrolysis of Aqueous Solutions
Section: 4. Electrochemistry | Syllabus: Cambridge IGCSE Chemistry (0620)
Competition in Aqueous Electrolysis
Water itself provides a small number of H⁺ and OH⁻ ions (from H₂O → H⁺ + OH⁻). So when an ionic compound is dissolved in water, four types of ion are present, and there is competition for discharge at each electrode.
| At the cathode (reduction) | At the anode (oxidation) |
|---|---|
| the metal ion from the compound, or H⁺ from water | the non-metal ion from the compound, or OH⁻ from water |
The Discharge Rules
- Cathode: if the metal is less reactive than hydrogen, the metal ion is discharged and the metal deposits; if the metal is more reactive than hydrogen, H⁺ is discharged instead and hydrogen gas forms
- Anode: if a halide ion (Cl⁻, Br⁻, I⁻) is present, the halogen forms; if no halide is present (e.g. SO₄²⁻, NO₃⁻), OH⁻ is discharged and oxygen forms
Part of the reactivity series relevant to electrolysis: K, Na, Ca, Mg, Al — (carbon) — Zn, Fe — HYDROGEN — Cu, Ag, Au.
In aqueous electrolysis, ions from the dissolved compound compete with H⁺ and OH⁻ from water
Worked Example: Copper(II) Sulfate with Inert Electrodes
Aqueous CuSO₄ is electrolysed using inert graphite electrodes. Ions present: Cu²⁺, SO₄²⁻, H⁺, OH⁻.
- Question: state the products at each electrode, and describe the observations. [4]
- Answer - cathode: Cu is less reactive than hydrogen, so Cu²⁺ is discharged: Cu²⁺ + 2e⁻ → Cu; pink/brown copper coats the cathode [2]
- Answer - anode: SO₄²⁻ is not a halide, so OH⁻ is discharged: 4OH⁻ → O₂ + 2H₂O + 4e⁻; colourless gas bubbles (oxygen) [2]
As Cu²⁺ ions are removed, the blue colour of the solution fades.
Copper(II) Sulfate with Copper Electrodes
If the same CuSO₄ solution is instead electrolysed using copper electrodes, the anode is no longer inert - the result changes completely at the anode, while the cathode reaction is unaffected.
- Cathode (unchanged): Cu²⁺ + 2e⁻ → Cu - copper still deposits
- Anode (different!): instead of oxygen forming from OH⁻, the copper electrode itself dissolves: Cu → Cu²⁺ + 2e⁻
Because copper lost at the anode exactly replaces copper gained at the cathode, the blue colour of the solution and its Cu²⁺ concentration stay constant. This is the basis of both electroplating and copper purification, covered later in this section.
Common Mistakes
- ❌ Assuming the anode always releases a gas
- ✓ only with an inert anode does an ion in the electrolyte get discharged there; with an active (reactive) anode like copper, the electrode metal itself dissolves instead
Further Examples
Silver nitrate solution (AgNO₃) - ions present: Ag⁺, NO₃⁻, H⁺, OH⁻
- Cathode: Ag is less reactive than hydrogen, so Ag⁺ + e⁻ → Ag - shiny silver coats the cathode
- Anode: NO₃⁻ is not a halide, so 4OH⁻ → O₂ + 2H₂O + 4e⁻ - oxygen gas forms
Potassium iodide solution (KI) - ions present: K⁺, I⁻, H⁺, OH⁻
- Cathode: K is far more reactive than hydrogen, so 2H⁺ + 2e⁻ → H₂ - hydrogen gas forms
- Anode: I⁻ is a halide, so 2I⁻ → I₂ + 2e⁻ - the solution turns brown as iodine forms
| Solution | Cathode product | Anode product |
|---|---|---|
| CuSO₄ (inert electrodes) | Cu | O₂ |
| CuSO₄ (copper electrodes) | Cu | none - anode dissolves |
| AgNO₃ | Ag | O₂ |
| KI | H₂ | I₂ |
Effect of Concentration on Halide Solutions
For a dissolved halide compound, concentration can change which ion is discharged at the anode:
- Dilute halide solution: OH⁻ may be discharged instead of the halide, so oxygen tends to form
- Concentrated halide solution: the halide ion is present in much greater excess and is discharged, so the halogen forms
The full worked comparison for dilute and concentrated sodium chloride is covered in the next topic, Electrolysis of Aqueous NaCl & H₂SO₄.
Common Mistakes
- ❌ Comparing a metal's reactivity with water, rather than with hydrogen
- ✓ the cathode competition is always between the metal ion and H⁺ - use the reactivity series position relative to hydrogen, not to water
- ❌ Forgetting that halides always win at the anode when present
- ✓ a halide ion (Cl⁻, Br⁻, I⁻) is discharged in preference to OH⁻ whenever it is present in reasonable concentration
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