Oxidation numbers and redoxEdexcel International A Level Chemistry: Subtopic test

1. 1
   Sulfur forms compounds and ions in which its oxidation number ranges from −2 to +6. Examples include hydrogen sulfide, H₂S, sulfur dioxide, SO₂, the sulfate ion, SO₄²⁻, and the sulfite ion, SO₃²⁻. In the contact process sulfur dioxide is oxidised to sulfur trioxide by oxygen.

   (a)

   What is the oxidation number of sulfur in the sulfate ion, SO₄²⁻?

   [1 mark]

   • A+4
   • B+8
   • C+6
   • D−2

   (b)

   What is the systematic name of sodium sulfite, Na₂SO₃?

   [1 mark]

   • ASodium sulfate(VI)
   • BSodium sulfate(II)
   • CSodium sulfate(III)
   • DSodium sulfate(IV)

   (c)

   In the reaction 2SO₂ + O₂ → 2SO₃, state which element is oxidised and which is reduced. Give the oxidation numbers to support your answer.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Hydrides and peroxides contain elements in unusual oxidation states. Sodium hydride, NaH, reacts with water: NaH + H₂O → NaOH + H₂. Hydrogen peroxide, H₂O₂, decomposes when a catalyst is added: 2H₂O₂ → 2H₂O + O₂.

   (a)

   What is the oxidation number of hydrogen in sodium hydride, NaH?

   [1 mark]

   • A+1
   • B−1
   • C0
   • D+2

   (b)

   Which statement about the decomposition of hydrogen peroxide is correct?

   [1 mark]

   • AIt is a disproportionation, because oxygen is both oxidised (−1 to 0) and reduced (−1 to −2)
   • BOxygen is oxidised only
   • COxygen is reduced only
   • DIt is not a redox reaction because there is no change of oxidation number

   (c)

   Deduce the changes in oxidation number of hydrogen in the reaction NaH + H₂O → NaOH + H₂, and state which species is the reducing agent.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Acidified potassium manganate(VII) solution is purple. When it is added to a solution containing iron(II) ions, the purple colour disappears and iron(III) ions and manganese(II) ions form.

   (a)

   Write ionic half-equations for (i) the reduction of MnO₄⁻ to Mn²⁺ in acid solution and (ii) the oxidation of Fe²⁺ to Fe³⁺.

   [3 marks]

   (b)

   Combine your half-equations to give the overall ionic equation. Hence name the oxidising agent and state the change in oxidation number of manganese.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student studies these four reactions.
   Reaction 1: Cl₂(g) + 2NaOH(aq) → NaCl(aq) + NaClO(aq) + H₂O(l), with cold dilute sodium hydroxide.
   Reaction 2: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).
   Reaction 3: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l).
   Reaction 4: 2Cu⁺(aq) → Cu(s) + Cu²⁺(aq).

   (a)

   Use oxidation numbers to explain why Reaction 1 is a disproportionation reaction, and write the ionic equation for it.

   [6 marks]

   (b)

   Classify Reactions 2, 3 and 4 as redox, disproportionation or neither, using oxidation numbers to justify each answer.

   [6 marks]

   Total for question 4: 12 marks

End of questions