Potassium Iodide as a Reducing Agent
Cambridge IGCSE Chemistry (0620) · Papers 4 & 6 · 15 Questions · 29 Marks Total
Key Reaction: KI + Cl₂ → KCl + I₂ (colourless → brown)
Colour Changes
What do you see in the test tube?
Aqueous potassium iodide is added to aqueous chlorine. State the colour change observed.
Aqueous potassium iodide is added to acidified aqueous hydrogen peroxide (H₂O₂). State the colour change observed.
A few drops of starch solution are added to a mixture of potassium iodide and an oxidising agent. State the colour change observed.
Aqueous potassium iodide is added to acidified aqueous potassium manganate(VII) (KMnO₄). State the colour change observed.
Aqueous potassium iodide is added to aqueous iron(III) chloride (FeCl₃). State the colour change observed.
Identifying the Role
Oxidising or reducing agent?
In the reaction between aqueous potassium iodide and aqueous chlorine, what role does the iodide ion (I⁻) play?
In the reaction: 2I⁻ + Cl₂ → I₂ + 2Cl⁻, what is the role of chlorine (Cl₂)?
Hydrogen peroxide (H₂O₂) reacts with acidified potassium iodide solution. Identify the oxidising agent in this reaction and explain your reasoning.
In the reaction between iron(III) ions and iodide ions, identify which species undergoes reduction and explain what 'reduction' means in terms of electrons.
Define 'oxidising agent' in terms of electron transfer. Using the reaction 2I⁻ + Cl₂ → I₂ + 2Cl⁻ as your example, identify the oxidising agent.
Half-Equations
Write & balance ionic half-equations
Write the ionic half-equation for the OXIDATION of iodide ions.
Write your half-equation:
Write the ionic half-equation for the REDUCTION of chlorine gas.
Write your half-equation:
Write the ionic half-equation for the reduction of iron(III) ions (Fe³⁺) to iron(II) ions (Fe²⁺).
Write your half-equation:
Combine the two half-equations below to write the OVERALL ionic equation for the reaction of potassium iodide with chlorine: (i) 2I⁻ → I₂ + 2e⁻ (ii) Cl₂ + 2e⁻ → 2Cl⁻
Write your half-equation:
Write the overall ionic equation for the reaction of iodide ions with acidified manganate(VII) ions. Use the half-equations: (i) 2I⁻ → I₂ + 2e⁻ (ii) MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
Write your half-equation:
🧠 The Limitless Professor's Cheat Sheet
The Golden Rule of Redox
OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons)
The Identity Paradox
The REDUCING agent is OXIDISED. The OXIDISING agent is REDUCED.
Colour Certainties for Paper 6
Half-Equation Electron Rules
Oxidation: electrons on RIGHT → 2I⁻ → I₂ + 2e⁻
Reduction: electrons on LEFT → Cl₂ + 2e⁻ → 2Cl⁻
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