Electrolysis
Principles of Chemistry
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
Common exam question
Explaining how an element forms at an electrode
Question: Explain how sodium, copper, hydrogen or bromine forms at the named electrode during the electrolysis (2–4 marks).
Asked in 4 of the 23 papers, always as a chain of marks that follows one ion to the product. Give the steps in order: the ion's charge (copper ions are positive, bromide ions negative); so it is attracted to the oppositely charged electrode, which you name; there it gains electrons (cathode, reduction) or loses electrons (anode, oxidation); and becomes an atom. For a gas, add that the atoms join in pairs to form molecules. For hydrogen, say first that the water supplies the hydrogen ions.
A correct half-equation always earns the electron-transfer mark and usually one or two of the others, but never the attraction mark, so write the sentences too. Start from the ion, not the element: "sodium atoms are attracted to the cathode" is rejected, and muddling "bromide" with "bromine" is penalised.
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 (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 (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) |
Common exam question
Why hydrogen and not sodium forms at the cathode
Question: Give a reason why hydrogen, rather than sodium, is produced at the negative electrode when sodium chloride solution is electrolysed (1 mark).
Asked in 2 of the 23 papers. The mark is a comparison of reactivity, either way round: sodium is more reactive than hydrogen, or hydrogen is less reactive and below sodium in the reactivity series. Answering with the ions is also accepted: hydrogen ions are more easily reduced than sodium ions. One scheme also lets "hydrogen ions are discharged" through, but the reactivity comparison is credited by both.
The discharged hydrogen ions come from the water, so hydroxide ions are left behind and the solution near the cathode turns alkaline: a follow-up asks why universal indicator goes blue or purple there, one mark for the colour, one for the hydroxide ions (or sodium hydroxide) present. In a solution of a salt or an acid, the hydroxide ions come from the water; "because the solution is alkaline" is rejected as their source.