4.4 Oxidation and reduction in terms of electrons
5 exam questions worth 10 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Which statement describes reduction?
Choose one answer.
Correct Not quite: the answer is Gain of electrons
Mark scheme
C
Model answer
C: reduction is gain of electrons (RIG).
Zinc reacts with copper sulfate solution: Zn + CuSO4 → ZnSO4 + Cu
Write the ionic equation for this reaction.
Mark scheme
- Zn + \mathrm{Cu^{2+}} → \mathrm{Zn^{2+}} + Cu (2)
- (1 mark if sulfate ions are included but otherwise correct)
Model answer
Zn + \mathrm{Cu^{2+}} → \mathrm{Zn^{2+}} + Cu
Write the half equation for the oxidation of iron atoms to iron(II) ions, \mathrm{Fe^{2+}}.
Mark scheme
- Fe → \mathrm{Fe^{2+}} + 2\mathrm{e^-} (1)
Model answer
Fe → \mathrm{Fe^{2+}} + 2\mathrm{e^-}
Magnesium reacts with dilute hydrochloric acid: Mg + 2\mathrm{H^+} → \mathrm{Mg^{2+}} + H2
Explain, in terms of electrons, which substance is oxidised and which is reduced.
Mark scheme
- magnesium is oxidised (1)
- because it loses (2) electrons (1)
- hydrogen ions are reduced (1)
- because they gain electrons (1)
Model answer
Magnesium atoms lose two electrons to form \mathrm{Mg^{2+}}, so magnesium is oxidised. Hydrogen ions gain these electrons to form hydrogen molecules, so they are reduced.
What is a spectator ion? Name the spectator ion in the reaction between magnesium and copper sulfate solution.
Mark scheme
- an ion that does not change during the reaction (1)
- sulfate ion / \mathrm{SO_4^{2-}} (1)
Model answer
A spectator ion is present but unchanged by the reaction. Here it is the sulfate ion.
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