Redox Reactions: Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Redox Reactions revision notes

Tier note: questions and answers marked (0620 Extended, 5070 required) test content beyond 0620 Core. All other questions are answerable by a 0620 Core candidate.

---

Section A

1. (0620 Extended, 5070 required) Define oxidation and reduction in terms of oxygen transfer, electron transfer and oxidation number. [3]

2. (0620 Extended, 5070 required) Deduce the oxidation number of the underlined element, showing your working: SO₄²⁻, MnO₄⁻, Cr₂O₇²⁻, NH₄⁺. [4]

Section B

3. Magnesium metal reacts with copper(II) sulfate solution.

(a) Write the ionic equation, omitting spectator ions. [2] (b) Identify what is oxidised and what is reduced, giving the oxidation number changes. [4] (c) Identify the oxidising agent and explain your choice. [2]

4. Zinc metal is added separately to solutions of magnesium sulfate, copper(II) sulfate and iron(II) sulfate.

(a) Predict in which cases a reaction occurs and explain your reasoning. [3] (b) State what you would observe in the copper(II) sulfate. [2]

5. Chlorine gas is bubbled through a solution of potassium bromide, and the reactivity of chlorine and bromine as oxidising agents is being compared.

(a) Write the ionic equation. [2] (b) Explain the observation in terms of oxidising power. [3]

6. Explain why an oxidising agent is itself reduced. [2]

7. A colourless solution turns the purple colour of acidified potassium manganate(VII) colourless when added to it.

(a) State whether the colourless solution is an oxidising agent or a reducing agent, and explain your reasoning. [2] (b) Aqueous potassium iodide is added instead to a different oxidising agent, and the solution turns brown. Explain this observation. [2]

---

Answers

1. (0620 Extended, 5070 required) Oxidation is the gain of oxygen, loss of electrons, or an increase in oxidation number [1] [1]; reduction is the opposite in each case — loss of oxygen, gain of electrons or a decrease in oxidation number [1]. (OIL RIG: Oxidation Is Loss, Reduction Is Gain — of electrons.)

2. (0620 Extended, 5070 required) S in SO₄²⁻ = +6 [1]. Mn in MnO₄⁻ = +7 [1]. Cr in Cr₂O₇²⁻ = +6 [1]. N in NH₄⁺ = −3 [1].

3. (a) Mg + Cu²⁺ → Mg²⁺ + Cu [1], balanced with correct charges [1]. (b) Magnesium is oxidised, from 0 to +2 [1] [1]; copper is reduced, from +2 to 0 [1] [1]. (c) The copper(II) ion [1], because it accepts electrons from the magnesium and is itself reduced to copper metal [1].

4. (a) Zinc reacts with copper(II) sulfate and iron(II) sulfate but not magnesium sulfate [1]; a metal will only displace a metal less reactive than itself [1], and zinc is above copper and iron but below magnesium in the reactivity series [1]. (b) The blue colour of the solution fades [1] and a red-brown solid (copper) is deposited on the zinc [1]. (Also accept: the mixture warms up as the reaction is exothermic.)

5. (a) Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ [1] [1]. (b) The solution turns orange/brown as bromine is formed [1]. Chlorine is a stronger oxidising agent than bromine [1], so it removes electrons from the bromide ions, oxidising them to bromine while itself being reduced to chloride ions [1]. This same reactivity order (Cl > Br > I) is why chlorine can displace both bromide and iodide, while bromine can only displace iodide.

6. An oxidising agent works by taking electrons from another species [1]; gaining electrons is reduction, so the oxidising agent is necessarily reduced in the process [1].

7. (a) The colourless solution is a reducing agent [1], because it has decolourised the manganate(VII) ion by reducing it, which means the colourless species must itself have been oxidised, donating electrons to the manganate(VII) [1]. (b) The oxidising agent removes electrons from the colourless iodide ions, oxidising them to iodine, which turns the solution brown [1]; the oxidising agent is itself reduced in the process [1].

---

Where marks are usually lost

• Forgetting that oxidation numbers in a polyatomic ion must sum to the overall charge on that ion.
• Saying the metal is the oxidising agent in a displacement reaction — it is the metal ion in solution that oxidises the reacting metal, not the other way round.
• Leaving spectator ions in an ionic equation.
• Not balancing charge as well as atoms.
• Confusing which colour change signals oxidation and which signals reduction — acidified manganate(VII) decolourising means it has been reduced, while a solution turning brown with potassium iodide means iodide has been oxidised to iodine.
• Describing a colour-change test result without linking it explicitly to which species was oxidised and which was reduced.

Colour-change tests as a pair, worth learning together

Acidified potassium manganate(VII) — deep purple, decolourises as it is reduced; used to detect a reducing agent. Aqueous potassium iodide — colourless, turns brown as colourless iodide ions are oxidised to iodine; used to detect an oxidising agent.

These two reagents are effectively opposite tests: manganate(VII) tests for something that can reduce it (i.e. a reducing agent present in the sample), while iodide tests for something that can oxidise it (i.e. an oxidising agent present in the sample) — remembering them as a complementary pair, rather than two separate unrelated facts, makes both easier to recall correctly under exam pressure.

For condensed recall notes on this topic, see the Redox Reactions revision notes; for the full explanation with additional worked examples, see the Redox Reactions study guide.