Quick answer: Redox reactions transfer electrons: oxidation is loss of electrons and an increase in oxidation number, reduction is gain of electrons and a decrease. This Chemistry 11-12 test on redox reactions has 8 questions worth 20 marks, to do in 45 minutes: oxidation numbers of sulfur, ostwald process and chlorate half-equation. The mark scheme is on a separate page; free printable PDF.
This test covers oxidation numbers, oxidising and reducing agents, half-equations, overall redox equations and a titration calculation. You have 45 minutes and may use a calculator. Show all working and write charges clearly.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R3.2
Question 1 Oxidation numbers of sulfur (3 marks)
Give the oxidation number of sulfur in each species.- H₂S (1 mark)
- SO₂ (1 mark)
- H₂SO₄ (1 mark)
Question 2 Ostwald process (2 marks)
In the manufacture of nitric acid, ammonia is burned in oxygen: 4NH₃ + 5O₂ → 4NO + 6H₂O.- Give the oxidation numbers of nitrogen and oxygen before and after. (1 mark)
- State which element is oxidised, which is reduced, and name the reducing agent. (1 mark)
Question 3 Chlorate half-equation (3 marks)
Write the half-equation for the reduction of chlorate(V) ions, ClO₃⁻, to chloride ions in acidic solution.Question 4 Chlorate and iron(II) (2 marks)
Using the half-equation Fe²⁺ → Fe³⁺ + e⁻ and your answer to the previous question, write the overall equation for the reaction of chlorate(V) with iron(II) in acid.Question 5 Dichromate titration (4 marks)
A 25.0 cm³ sample of iron(II) solution is titrated with 0.0100 mol/dm³ potassium dichromate(VI) in acid. The mean titre is 19.40 cm³. Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 7H₂O + 6Fe³⁺. Calculate the concentration of iron(II).Question 6 Hydrogen peroxide as two agents (2 marks)
Explain why hydrogen peroxide can act as both an oxidising agent and a reducing agent. Use oxidation numbers in your answer.Question 7 Copper(I) oxide in acid (2 marks)
Copper(I) oxide reacts with dilute sulfuric acid: Cu₂O + H₂SO₄ → Cu + CuSO₄ + H₂O. Show that this is a disproportionation reaction.Question 8 Zinc and acid (2 marks)
Zinc reacts with dilute hydrochloric acid: Zn + 2HCl → ZnCl₂ + H₂. State the changes in oxidation number and identify the reducing agent.
