O-LevelChemistryElectrolysis5070/22 · Oct/Nov 2018 · Q210 Marks

Magnesium reacts with aqueous copper(II) sulfate. Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s)

(a)(i)Explain by referring to the equation, why this is a redox reaction.[2]
(a)(ii)Construct the ionic equation for this reaction.[1]
(b)Pure copper can be made by the electrolysis of aqueous copper(II) sulfate, using one pure copper electrode and one impure copper electrode. Draw a labelled diagram of this electrolysis.[3]
(c)What observations are made when adding aqueous ammonia to a solution containing copper(II) ions, slowly with mixing, until no further change occurs?[2]
(d)Blocks of magnesium are attached to underground pipes made of iron to stop them rusting. Explain how the magnesium stops the pipes rusting.[2]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2018). The mark or mark type is shown in brackets.

[1] (a)(i) magnesium loses electrons so is oxidation (1)
[1] (a)(i) copper ions gain electrons so is reduction (1)
[1] (a)(ii) Mg + Cu²⁺ → Mg²⁺ + Cu
[1] (b) workable arrangement with two electrodes dipping in liquid and connected correctly to power supply with two wires and with no big gaps in the wiring (1)
[1] (b) pure / copper is negative electrode and impure copper is positive electrode (1)
[1] (b) electrolyte is labelled copper ions OR (soluble) copper salt OR electrolyte (1)
[1] (c) (at first) (light) blue precipitate (1)
[1] (c) (dissolves in excess ammonia) to form a dark blue solution / dark blue solution (in excess ammonia) / deep blue solution (in excess ammonia) (1)
[1] (d) magnesium is more reactive than iron (1)
[1] (d) magnesium corrodes instead of iron / magnesium reacts instead of iron (1)

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