Electrolysis
Principles of Chemistry · 2 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 7% of your exam marks.
Electrode products, half-equations and OILRIG tested in most series.
Electrolysis of molten binary ionic compounds
- A binary ionic compound contains just two elements joined by ionic bonding (NaCl, PbBr2, MgO)
- When melted, all ions become free to move, so the molten compound is an
- During electrolysis of any molten binary compound, the products are simply the elements of the compound
- The metal cation is reduced at the → the metal is deposited there
- The non-metal anion is oxidised at the → the non-metal is released there
More molten examples
- Molten potassium iodide, KI: potassium metal at the cathode, purple iodine vapour at the anode
- Molten magnesium oxide, MgO: magnesium metal at the cathode, colourless oxygen gas at the anode
- Molten aluminium oxide, Al2O3 (dissolved in cryolite to lower the melting point): aluminium at the cathode, oxygen at the anode — the basis of industrial aluminium extraction
Electrolysis of aqueous solutions
- An aqueous solution contains the dissolved compound's ions plus ions from water
- Water self-ionises slightly: H2O ⇌ H+ + OH−
- The cell contains four kinds of ion (metal cation, non-metal anion, H+, OH−) so which ion is discharged at each electrode has to be worked out
- At the cathode (positive ion arrives), the choice is between the metal ion and H+
- The less reactive of the two is discharged
- If the metal is above hydrogen in the reactivity series, hydrogen gas is produced and the metal ion stays in solution
- If the metal is below hydrogen (Cu, Ag), the metal is deposited
- At the anode (negative ion arrives), the choice is between the non-metal anion and OH−
- If the solution contains a halide ion (Cl−, Br−, I−), the halide is discharged and the corresponding halogen (Cl2, Br2, I2) is released
- If no halide ion is present, oxygen is produced from water (sulfate, nitrate and similar polyatomic anions stay in solution)
Predicting products for the three required aqueous examples
| Solution | Cathode product | Anode product |
|---|---|---|
| Sodium chloride, NaCl(aq) | Hydrogen (Na is above H) | Chlorine (halide present) |
| Dilute sulfuric acid, H2SO4(aq) | Hydrogen | Oxygen (no halide) |
| Copper(II) sulfate, CuSO4(aq) | Copper (Cu is below H) | Oxygen (no halide) |
Predicting the products of electrolysing an aqueous solution
What comes up: Name the product formed at each electrode when a named aqueous solution is electrolysed, or explain why a particular product forms.
Write (cathode): If the dissolved metal is above hydrogen in the reactivity series, hydrogen gas is produced (the H⁺ ions from water are discharged instead). If the metal is below hydrogen (for example copper), the metal is deposited at the cathode.
Write (anode): If the solution contains a halide ion (Cl⁻, Br⁻, I⁻), the corresponding halogen is released. If no halide is present, oxygen is produced from water (sulfate and nitrate ions are not discharged).
Watch out: For the cathode choice the mark scheme accepts "sodium is more reactive than hydrogen" as an explanation for why hydrogen is produced rather than sodium — state the reactivity series comparison explicitly, don't just name the product.