Two unlabelled aqueous solutions, R and S, are magnesium chloride, MgCl2(aq)\mathrm{MgCl_2(aq)}MgCl2(aq), and magnesium nitrate, Mg(NO3)2(aq)\mathrm{Mg(NO_3)_2(aq)}Mg(NO3)2(aq).
A student electrolyses each solution using inert electrodes. The gases are collected and measured under the same conditions.
| Solution | Observation at the cathode (-) | Observation at the anode (+) |
|---|---|---|
| R | 24 cm3 of a colourless gas | 24 cm3 of a pale green gas |
| S | 24 cm3 of a colourless gas | 12 cm3 of a colourless gas |
Identify solutions R and S and the product at each electrode.
Explain how the ions present and the volumes of gas collected support your identifications. Include half equations in your answer.
A pure sample of magnesium chloride is heated until it is molten and is then electrolysed using inert electrodes.
Predict the product at each electrode. Explain why the product at the cathode differs from the product formed from magnesium chloride solution.
Include a half equation for the cathode reaction.
98 exam-style questions on OCR GCSE Chemistry 3.4 Electrolysis, covering 3.4.1 Products formed at the electrodes in electrolysis and 3.4.2 The technique of electrolysis. Each one has a worked solution and a mark scheme showing where the marks go.