Manganate(VII) — Colours, Roles & Oxidation Numbers
Cambridge IGCSE Chemistry 0620 • Papers 4 & 6 • Total: 30 marks
State the colour change observed when acidified potassium manganate(VII) solution is added to a solution containing a reducing agent.
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A student adds acidified KMnO₄ to iron(II) sulfate solution. Describe the colour change that confirms iron(II) ions are present.
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Acidified potassium manganate(VII) is added dropwise to a solution of hydrogen peroxide (H₂O₂). What colour change is observed? Explain why the colour change eventually stops when excess KMnO₄ is added.
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Explain why acidified potassium manganate(VII) does NOT change colour when added to a solution containing only oxidising agents.
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A chemist tests an unknown solution with acidified potassium manganate(VII). The solution turns from purple to a pale brown colour instead of colourless. Suggest why the expected colour change did not fully occur.
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What type of reagent is acidified potassium manganate(VII)?
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In the reaction between acidified KMnO₄ and iron(II) ions (Fe²⁺), identify the substance that is oxidised and the substance that is reduced.
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In a redox reaction, acidified KMnO₄ acts as the oxidising agent. What does this mean in terms of electron transfer?
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Why must potassium manganate(VII) be acidified before use as a test reagent for reducing agents?
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Ethanol (C₂H₅OH) is added to acidified potassium manganate(VII). State what role ethanol plays in this reaction and predict what you would observe.
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State the oxidation number of manganese in the manganate(VII) ion (MnO₄⁻).
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State the oxidation number of manganese in the Mn²⁺ ion (the product of the reduction of MnO₄⁻).
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Explain, using oxidation numbers, why the manganate(VII) ion (MnO₄⁻) is reduced during its reaction with a reducing agent.
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In the reaction between acidified KMnO₄ and iron(II) ions, the half-equation for the oxidation of iron is: Fe²⁺ → Fe³⁺ + e⁻. Using oxidation numbers, confirm that this is oxidation.
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The overall ionic equation for the reaction of acidified KMnO₄ with Fe²⁺ is shown below. Assign oxidation numbers to all species and identify which species is oxidised and which is reduced. MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺
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📊 Score Summary
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🔑 The 3 Golden Rules for KMnO₄ Questions
- Always write purple to colourless — not "clear".
- KMnO₄ is an oxidising agent — it is itself reduced (+7 → +2).
- Mn oxidation number decreases from +7 to +2 — always show the working.
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