This topic accounts for approximately 7% of your exam marks.
stable
Low
Stable7%
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)
Exam tip
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.