Oxidation states and redoxAQA A-Level Chemistry: Subtopic test

1. 1
   In a titration, a solution of iron(II) ions is oxidised by acidified potassium manganate(VII). The manganate(VII) ion, MnO₄⁻, is reduced to Mn²⁺, and the iron(II) ions are oxidised to iron(III) ions, Fe³⁺.

   (a)

   What is the oxidation state of manganese in the MnO₄⁻ ion?

   [1 mark]

   • A+4
   • B+6
   • C+7
   • D−1

   (b)

   Which statement describes the role of MnO₄⁻ in the reaction?

   [1 mark]

   • AIt is a reducing agent because it gains electrons
   • BIt is an oxidising agent because it accepts electrons from the iron(II) ions
   • CIt is an oxidising agent because it loses electrons
   • DIt is a reducing agent because it is reduced

   (c)

   State the changes in oxidation state of manganese and of iron, and identify the reducing agent.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A technician is checking the oxidation states of an element in a range of compounds: sodium hydride, NaH; hydrogen peroxide, H₂O₂; potassium dichromate(VI), K₂Cr₂O₇; and sodium thiosulfate, Na₂S₂O₃.

   (a)

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

   [1 mark]

   • A−1
   • B0
   • C+1
   • D+2

   (b)

   What is the oxidation state of oxygen in hydrogen peroxide, H₂O₂?

   [1 mark]

   • A−2
   • B−½
   • C0
   • D−1

   (c)

   Deduce the oxidation state of chromium in K₂Cr₂O₇ and of sulfur in Na₂S₂O₃. Show your working.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Solid sodium iodide reacts with concentrated sulfuric acid. Iodide ions are oxidised to iodine, and sulfuric acid is reduced to hydrogen sulfide. The ionic equation is 8I⁻ + 10H⁺ + SO₄²⁻ → 4I₂ + H₂S + 4H₂O.

   (a)

   Explain, in terms of electrons and oxidation states, why iodide ions are oxidised in this reaction and identify the oxidising agent.

   [3 marks]

   (b)

   Deduce the oxidation state of sulfur in SO₄²⁻ and in H₂S, and hence show that the number of electrons gained by each sulfur atom is consistent with the number of iodide ions in the equation.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student adds excess zinc to a solution of acidified ammonium vanadate(V) containing VO₂⁺ ions. The vanadium is reduced step by step through VO²⁺ and V³⁺ to V²⁺, and the zinc is oxidised to Zn²⁺. The student used 25.0 cm³ of a solution in which the concentration of VO₂⁺ was 0.0400 mol dm⁻³.

   (a)

   Deduce the oxidation state of vanadium in each of the species VO₂⁺, VO²⁺, V³⁺ and V²⁺, and explain the roles of vanadium and zinc in terms of electrons and oxidation states.

   [6 marks]

   (b)

   Calculate the minimum mass of zinc needed to reduce all the vanadium in the 25.0 cm³ sample from VO₂⁺ to V²⁺. (Ar of Zn = 65.4)

   [6 marks]

   Total for question 4: 12 marks

End of questions