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Six original Cambridge-style questions on oxidation numbers: calculating them, naming compounds, and using changes in oxidation number to identify redox processes.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
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Sum to zero, or to the charge.

01
Calculation
[4 marks]

Give the oxidation number of the underlined element: (a) NO3−, (b) VO2+, (c) H2O2, (d) PCl5.

  • (a) N + 3(−2) = −1, N = +5. ✓
  • (b) V + 2(−2) = +1, V = +5. ✓
  • (c) peroxide: O = −1. ✓
  • (d) P + 5(−1) = 0, P = +5. ✓
02
Recall
[2 marks]

Write the formulae of (a) copper(I) oxide and (b) potassium manganate(VII).

  • (a) Cu is +1, so Cu2O. ✓
  • (b) Mn is +7, so the ion is MnO4− and the salt is KMnO4. ✓
03
Analysis
[3 marks]

For Cu + 2Ag+ → Cu2+ + 2Ag, state which species is oxidised, which is reduced, and identify the oxidising agent.

  • Cu is oxidised: 0 → +2. ✓
  • Ag+ is reduced: +1 → 0. ✓
  • Ag+ is the oxidising agent. ✓
04
Analysis
[3 marks]

Chlorine reacts with hot aqueous sodium hydroxide: 3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O. Use oxidation numbers to explain why this is disproportionation.

  • Cl in Cl2 is 0. ✓
  • In NaCl, Cl is −1 (reduced); in NaClO3, Cl is +5 (oxidised). ✓
  • The same element is simultaneously oxidised and reduced, which is disproportionation. ✓
05
Analysis
[2 marks]

In SO2 + 2H2S → 3S + 2H2O, give the change in oxidation number of sulfur from each reactant.

  • In SO2, S is +4 and goes to 0: reduced, a change of −4. ✓
  • In H2S, S is −2 and goes to 0: oxidised, a change of +2 (so two H2S are needed per SO2). ✓
06
Explain
[2 marks]

Explain, in terms of electrons, why an oxidising agent is itself reduced.

  • An oxidising agent oxidises another species by removing electrons from it. ✓
  • It gains those electrons, so it is reduced and its oxidation number decreases. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.