Reducing Agents
Cambridge IGCSE Chemistry 0620 · Paper 4 · Definitions, Electron Transfer & Oxidation Numbers
🔑 OIL RIG — The Golden Rule
Oxidation Is Loss
Reducing agent LOSES electrons
Oxidation number INCREASES ↑
Reduction Is Gain
Oxidising agent GAINS electrons
Oxidation number DECREASES ↓
Section A: Definitions
5 × Definition QuestionsDefine the term reducing agent.
Which of the following best defines a reducing agent? Choose A, B, C or D and justify your answer. A) A substance that gains electrons B) A substance that accepts protons C) A substance that causes another substance to be oxidised D) A substance that loses electrons and causes another substance to be reduced
State what happens to a reducing agent during a redox reaction. Use the terms 'oxidation' and 'electrons' in your answer.
In the reaction between magnesium and oxygen: 2Mg(s) + O₂(g) → 2MgO(s) Identify the reducing agent and justify your answer using the definition of a reducing agent.
Explain why hydrogen gas (H₂) can act as a reducing agent. Include a reference to electrons in your answer.
Section B: Electron Transfer
5 × Electron Transfer QuestionsIn terms of electrons, what does a reducing agent do during a redox reaction?
Consider the reaction: Fe²⁺(aq) + Ce⁴⁺(aq) → Fe³⁺(aq) + Ce³⁺(aq) Identify the reducing agent and write a half-equation to show the electron transfer.
In the reaction: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) Write the oxidation half-equation for the reducing agent and state how many electrons are transferred.
Carbon monoxide (CO) reduces iron(III) oxide in the blast furnace: Fe₂O₃(s) + 3CO(g) → 2Fe(l) + 3CO₂(g) Explain, in terms of electron transfer, why CO is the reducing agent.
A student says: 'A reducing agent gains electrons from another substance.' Evaluate this statement and correct it if necessary.
Section C: Oxidation Numbers
5 × Oxidation Number QuestionsWhat happens to the oxidation number of a reducing agent during a redox reaction?
In the reaction: 2Fe³⁺(aq) + Sn²⁺(aq) → 2Fe²⁺(aq) + Sn⁴⁺(aq) a) Identify the reducing agent. b) State the change in oxidation number of the reducing agent.
Determine the oxidation number of sulfur in SO₂ and H₂SO₄. Hence, explain whether SO₂ could act as a reducing agent when it reacts with a strong oxidising agent to form H₂SO₄.
The reaction between potassium manganate(VII) (KMnO₄) and iron(II) sulfate (FeSO₄) in acidic solution is shown below: MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O a) Identify the reducing agent. b) State the oxidation number change of the reducing agent.
A mystery substance X has the following properties: • It reacts with an acidified solution of potassium dichromate(VI), turning it from orange to green. • In doing so, substance X undergoes a change in oxidation number from –1 to 0. Deduce whether substance X is a reducing agent or oxidising agent. Fully justify your answer using oxidation number changes.
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