Oxidation numbersEdexcel A-Level Chemistry: Subtopic test

1. 1
   A technician is relabelling reagent bottles in a school store room. The bottles are marked only with the formulae KMnO₄, Fe₂O₃, H₂O₂ and CaH₂, and she must add the correct oxidation numbers to each label.

   (a)

   What is the oxidation number of manganese in KMnO₄?

   [1 mark]

   • A+7
   • B+5
   • C+3
   • D+1

   (b)

   Which formula is correct for iron(III) oxide?

   [1 mark]

   • AFeO
   • BFe₃O₂
   • CFeO₃
   • DFe₂O₃

   (c)

   Explain why the oxidation numbers of oxygen in H₂O₂ and of hydrogen in CaH₂ differ from the values these elements usually have.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Sulfur forms many compounds and ions in which its oxidation number ranges from −2 to +6.

   (a)

   Which species contains sulfur with an oxidation number of +4?

   [1 mark]

   • AH₂S
   • BSO₄²⁻
   • CSO₃²⁻
   • DS₈

   (b)

   In which change is sulfur oxidised?

   [1 mark]

   • ASO₄²⁻ to SO₂
   • BSO₃²⁻ to SO₄²⁻
   • CS₈ to H₂S
   • DSO₂ to S₈

   (c)

   Calculate the oxidation number of sulfur in the thiosulfate ion, S₂O₃²⁻. Show your working.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Nitrogen forms compounds and ions in which its oxidation number ranges from −3 to +5. An environmental chemist is studying nitrogen species found in air and in rainwater.

   (a)

   Calculate the oxidation number of nitrogen in NH₄⁺, in N₂O and in NO₃⁻.

   [3 marks]

   (b)

   Nitrogen dioxide dissolves in water according to the equation 2NO₂ + H₂O → HNO₂ + HNO₃. Use oxidation numbers to show that this reaction is a disproportionation.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A teacher gives a student a set of reactions and compounds to analyse using oxidation numbers.

   (a)

   Use oxidation numbers to classify each of these reactions as redox, disproportionation or neither. Reaction 1: 2Mg + O₂ → 2MgO. Reaction 2: 2H₂O₂ → 2H₂O + O₂. Reaction 3: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.

   [6 marks]

   (b)

   Vanadium forms the ions VO₂⁺ and VO²⁺ and the compounds vanadium(V) oxide and vanadium(III) chloride. Deduce the oxidation number of vanadium in VO₂⁺ and in VO²⁺, write the formulae of the two compounds, and explain whether the change from VO₂⁺ to VO²⁺ is oxidation or reduction.

   [6 marks]

   Total for question 4: 12 marks

End of questions