The electrolysis of molten compounds
4 marks · NormalA student investigates the electrolysis of three different molten ionic compounds using carbon electrodes. The table below shows the ions present in each compound.
| Compound | Cation(s) | Anion(s) |
|---|---|---|
| Magnesium bromide (MgBr2) | Mg^{2+} | Br^- |
| Potassium iodide (KI) | K^+ | I^- |
| Calcium chloride (CaCl2) | Ca^{2+} | Cl^- |
For magnesium bromide, state the product formed at the anode.
For potassium iodide, state the product formed at the cathode.
Write the half-equation for the reaction at the anode when calcium chloride is electrolysed.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Bromine (Br2) is formed
(b) Potassium (K) is formed
(c) 2Cl^- → Cl2 + 2e^-
Common mistakes: Stating wrong product at each electrode; Incorrect charges or formulae in equation
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States bromine (Br2) formed at the anode for MgBr2
B2 (b) · 1 mark · States potassium (K) formed at the cathode for KI
M1 (c) · 1 mark · Writes correct formula for chloride half-equation at anode
A1 (c) · 1 mark · Balances equation: 2Cl^- → Cl2 + 2e^-
Worked solution
(a) For MgBr_2, the anode product is bromine gas (Br_2), as Br^- ions are oxidised.
(b) For KI, the cathode product is potassium metal (K), as K^+ ions gain electrons.
(c) At the anode during electrolysis of CaCl_2, chloride ions are oxidised: