Practice questions: Redox Equilibrium

Attempt every question before reading the answer. These are original questions written in SPM style on the same content standards; none is copied from a past-year paper. For the multiple-choice items, write your letter first, then check the one-line reason.

Paper 1 style: multiple-choice questions

1. What is the oxidation number of chromium in the dichromate ion, Cr₂O₇²⁻? A. +3 B. +6 C. +7 D. +12 Answer: B. Seven oxygen at −2 give −14; the ion is −2, so the two chromium atoms total +12, i.e. +6 each.

2. In the reaction Zn + Cu²⁺ → Zn²⁺ + Cu, which statement is correct? A. Zinc is reduced B. Copper(II) ion is oxidised C. Zinc is the reducing agent D. Copper is the reducing agent Answer: C. Zinc loses electrons (is oxidised), so it is the reducing agent.

3. According to OIL RIG, oxidation is the A. gain of electrons B. loss of electrons C. gain of hydrogen D. loss of oxygen Answer: B. Oxidation Is Loss of electrons.

4. Molten lead(II) bromide is electrolysed with carbon electrodes. The product at the anode is A. lead B. bromine C. hydrogen D. oxygen Answer: B. Bromide ions are oxidised at the anode: 2Br⁻ → Br₂ + 2e⁻.

5. Dilute sodium chloride solution is electrolysed with carbon electrodes. The product at the cathode is A. sodium B. hydrogen C. chlorine D. oxygen Answer: B. Hydrogen is lower in the electrochemical series than sodium, so H⁺ is discharged.

6. Concentrated sodium chloride solution is electrolysed with carbon electrodes. The product at the anode is A. oxygen B. hydrogen C. chlorine D. sodium Answer: C. A high chloride concentration causes Cl⁻ to be discharged in preference to OH⁻.

7. Copper(II) sulfate solution is electrolysed using copper electrodes. At the anode, A. oxygen is released B. the copper anode dissolves C. hydrogen is released D. copper is deposited Answer: B. An active copper anode dissolves: Cu → Cu²⁺ + 2e⁻.

8. A simple cell is made from magnesium and copper. The negative terminal is A. copper, because it is less reactive B. magnesium, because it is more reactive C. copper, because it gains electrons D. magnesium, because it is reduced Answer: B. The more reactive metal is oxidised and forms the negative terminal.

9. Which pair of conditions is necessary for iron to rust? A. water and carbon dioxide B. oxygen and nitrogen C. water and oxygen D. oxygen only Answer: C. Rusting needs both water and oxygen.

10. Which metal is extracted from its ore by electrolysis rather than by carbon reduction? A. iron B. tin C. lead D. aluminium Answer: D. Aluminium is above carbon in the reactivity series, so carbon cannot reduce its oxide.

Structured questions

Structured 1, Oxidation numbers and agents. Consider the reaction: MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O. (a) State the oxidation number of manganese in MnO₂ and in MnCl₂. (b) State the oxidation number of chlorine in HCl and in Cl₂. (c) Identify the oxidising agent and the reducing agent.

Model answer. (a) In MnO₂, oxygen is −2 and there are two, so manganese is +4; in MnCl₂, chlorine is −1 and there are two, so manganese is +2. (b) Chlorine is −1 in HCl and 0 in Cl₂. (c) Manganese falls from +4 to +2 (reduced), so MnO₂ is the oxidising agent. Chlorine rises from −1 to 0 (oxidised), so HCl is the reducing agent.

Structured 2, Electrolysis of copper(II) sulfate. Copper(II) sulfate solution is electrolysed using carbon electrodes. (a) List all the ions present in the solution. (b) Name the product at each electrode and write the half-equation. (c) State one observation at the cathode.

Model answer. (a) Cu²⁺, H⁺, SO₄²⁻ and OH⁻. (b) Cathode: copper is deposited because Cu²⁺ is below H⁺ in the electrochemical series, Cu²⁺ + 2e⁻ → Cu. Anode: oxygen is released because OH⁻ is discharged with inert electrodes, 4OH⁻ → 2H₂O + O₂ + 4e⁻. (c) A brown (pink) solid coats the cathode, and the blue colour of the solution fades as Cu²⁺ is removed.

Structured 3, Rusting and prevention. An iron nail is wrapped partly in zinc and left in damp air; another is wrapped in tin. (a) State the two substances that must be present for rusting. (b) Explain why the nail wrapped in zinc does not rust even where the zinc is scratched. (c) Explain why the nail wrapped in tin rusts faster once the tin is scratched.

Model answer. (a) Water and oxygen. (b) Zinc is more reactive than iron, so it loses electrons in preference to iron (sacrificial protection); the zinc corrodes and the iron is protected even at a scratch. (c) Tin is less reactive than iron, so at a scratch the iron loses electrons in preference to the tin, and the iron rusts faster than unprotected iron.

Structured 4, Voltaic cell. A simple cell is set up with a zinc plate and a silver plate dipping into dilute sulfuric acid, joined by a wire and a bulb. (a) State which plate is the negative terminal and explain why. (b) State the direction of electron flow in the external wire. (c) Predict, with a reason, how the reading would change if the silver were replaced by copper.

Model answer. (a) The zinc plate is the negative terminal because zinc is more reactive than silver, so it is oxidised and releases electrons (Zn → Zn²⁺ + 2e⁻). (b) Electrons flow from the zinc plate to the silver plate through the external wire. (c) The reading (voltage) would be smaller, because copper is closer to zinc in the reactivity series than silver is, so the difference in reactivity is smaller and the cell produces a lower voltage.

Marking yourself

Give yourself the mark only when your working matches the reason, not just the final letter or word. If a half-equation or an oxidation number is wrong, redo it from the rules before moving on. A one-to-one teacher can mark your structured answers the way an examiner would, which is where the technique for SPM Chemistry is really built. Then revisit the worked examples for anything you missed.