O-LevelChemistryElectrolysis5070/21 · Oct/Nov 2020 · Q313 Marks

This question is about copper and copper compounds.

(a)Copper is a metal. Explain why copper conducts electricity.[1]
(b)Describe a test for copper(II) ions. test observations[2]
(c)(i)Aqueous copper(II) sulfate is electrolysed using graphite electrodes. Describe what is observed during this electrolysis: • at the positive electrode • at the negative electrode • in the electrolyte.[3]
(c)(ii)Graphite conducts electricity. Give one other reason why graphite electrodes are used in electrolysis.[1]
(d)Aqueous copper(II) sulfate reacts with magnesium. CuSO₄ + Mg → Cu + MgSO₄ Construct the ionic equation, including state symbols, for this reaction.[2]
(e)A 2.25 g sample of an oxide of copper contains 0.250 g of oxygen. Deduce the empirical formula of this oxide of copper.[3]
(f)There are several commonly used alloys of copper. What is the meaning of the term alloy?[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

[1] 3(a) (some of the) electrons can move / (some of the) electrons are mobile
[2] 3(b) (aqueous) sodium hydroxide (1) (light) blue precipitate (insoluble in excess) (1) OR (aqueous) ammonia (1) (light) blue precipitate soluble in excess / dark blue solution formed in excess ammonia (1)
[3] 3(c)(i) positive electrode: bubbles / effervescence / fizzes (1) negative electrode: turns pink (1) electrolyte: blue colour (of solution) fades (1)
[1] 3(c)(ii) inert/ (relatively) unreactive
[2] 3(d) Cu2+(aq) + Mg(s) → Cu(s) + Mg2+(aq) correct formulae and balancing (1) correct state symbols - dependent on correct formulae (1)
[3] 3(e) mass of copper = 2.00 g (1) mol copper = 2.00/64 and mol oxygen = 0.25/16 OR mol copper = 0.0313 and mol oxygen = 0.0156 (1) empirical formula is Cu2O (1)
[1] 3(f) mixture of metals / mixture of a metal and another element

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