Question 1Multiple choice · 1 mark
Which term is described by this statement?
“A substance that reduces another substance and is itself oxidised.”
- A amphoteric oxide
- B acid
- C oxidation
- D reducing agent
Show answer
Answer: D
Free Redox questions for IGCSE Chemistry (0620), part of Chemical reactions. Each question has the answer and the marking points an examiner would look for.
Generate a full Redox worksheet →All Chemical reactions questionsWhich term is described by this statement?
“A substance that reduces another substance and is itself oxidised.”
Answer: D
Potassium manganate(VII) is an oxidising agent.
Describe the colour change when acidified potassium manganate(VII) is added to a reducing agent.
Describe the colour change when potassium iodide is added to an oxidising agent.
Explain what is meant by an oxidation number and give the oxidation number of iron in Fe₂O₃.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | purple to colourless purple to colourless B1 | 1 |
| (b) | colourless to brown colourless to brown (iodine formed) B1 | 1 |
| (c) | charge on ion; +3 the charge an atom would have if all bonds were ionic B1 +3 B1 | 2 |
Zinc reacts with aqueous copper(II) sulfate: Zn + Cu²⁺ → Zn²⁺ + Cu.
Explain why this is a redox reaction, in terms of electron transfer.
State the change in oxidation number of copper.
Describe two observations.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | Zn loses electrons; Cu²⁺ gains zinc loses two electrons (oxidised) B1 copper(II) ions gain two electrons (reduced) B1 | 2 |
| (b) | +2 to 0 from +2 to 0 B1 | 1 |
| (c) | blue fades; brown/pink solid Any 2 of: blue colour fades; brown/pink solid forms (zinc dissolves; temperature rises) B2 | 2 |
Oxidation and reduction can be described in several ways.
Define reduction in terms of oxygen and in terms of electrons.
In the reaction CuO + H₂ → Cu + H₂O, identify the oxidising agent.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | loss of oxygen; gain of electrons loss of oxygen B1 gain of electrons B1 | 2 |
| (b) | copper(II) oxide copper(II) oxide B1 | 1 |
This question is about oxidation and reduction.
Define oxidation in terms of electrons and in terms of oxidation number.
Iron(III) oxide reacts with aluminium.
Fe₂O₃ + 2Al → Al₂O₃ + 2Fe
Give the oxidation number of iron in Fe₂O₃ and in Fe.
Identify the reducing agent. Explain your answer.
Acidified potassium manganate(VII) is used to detect reducing agents.
State the colour change when it reacts with a reducing agent.
Aqueous potassium iodide is used to detect oxidising agents. State the colour change.
In the electrolysis of molten lead(II) bromide, write the ionic half-equation at each electrode and state which is oxidation.
Explain why the reaction between hydrochloric acid and sodium hydroxide is not a redox reaction.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | loss of electrons; increase in oxidation number loss of electrons B1 increase in oxidation number B1 | 2 |
| (b)(i) | +3; 0 +3 B1 0 B1 | 2 |
| (ii) | aluminium; it is oxidised / loses electrons aluminium B1 it gains oxygen / loses electrons / its oxidation number increases (0 to +3) B1 | 2 |
| (c)(i) | purple to colourless purple to colourless B1 | 1 |
| (ii) | colourless to brown colourless to (yellow-)brown B1 | 1 |
| (d) | Pb²⁺ + 2e⁻ → Pb; 2Br⁻ → Br₂ + 2e⁻; anode reaction is oxidation cathode: Pb²⁺ + 2e⁻ → Pb B1 anode: 2Br⁻ → Br₂ + 2e⁻ B1 balanced with electrons B1 oxidation occurs at the anode (bromide ions lose electrons) B1 | 4 |
| (e) | no change in oxidation numbers no element changes its oxidation number / no electrons transferred B1 | 1 |
Iron(III) oxide reacts with carbon monoxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂.
Identify the substance that is oxidised and the substance that is reduced.
Define oxidation in terms of electrons.
Identify the reducing agent.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | CO oxidised; Fe₂O₃ reduced carbon monoxide is oxidised B1 iron(III) oxide is reduced B1 | 2 |
| (b) | loss of electrons loss of electrons B1 | 1 |
| (c) | carbon monoxide carbon monoxide B1 | 1 |
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