Oxidation numbers and redoxEdexcel International A Level Chemistry: Mind map

Exam questions on Oxidation numbers and redox

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.
   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
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.
   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
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.
   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