AQA A-Level Chemistry Paper 3, June 2018: Question 29
1 mark · Medium difficulty · Multiple Choice
Identify which metal ion cannot catalyse the reaction between iodide and peroxodisulfate using standard electrode potential data.
Practise this questionQuestion
Question text
29 Which ion cannot catalyse the reaction between iodide (I–) and peroxodisulfate (S O 2–)?
Use the data below to help you answer this question.
[1 mark]
O
Half-equation E / V
2– – 2–
S2O8 + 2e 2SO4 +2.01
3+ – 2+
Co + e Co +1.82
3+ – 2+
Fe + e Fe +0.77
– –
I2 + 2e 2I +0.54
3+ – 2+
Cr + e Cr –0.41
A Co2+
B Cr2+
C Fe2+
D Fe3+
Mark scheme
Show the mark scheme
29 B
How to answer it
Homogeneous Catalysis & Electrode Potentials
What this question tests
This question assesses your ability to apply standard electrode potentials (E°) to explain the mechanism of homogeneous catalysis involving transition metal ions. Specifically, it tests:
- Understanding how transition metal ions act as homogeneous catalysts via variable oxidation states.
- Evaluating thermodynamic feasibility (E°cell > 0) for each individual step in a two-stage catalytic cycle.
- Applying the rule that the redox potential of an effective catalyst couple must lie between the electrode potentials of the two reacting systems.
Question 29: Identifying the Non-Catalytic Ion
✅ Correct Answer
B — Cr²⁺
Cr²⁺ cannot catalyse the reaction because the regeneration step (oxidation of I⁻ by Cr³⁺) has a negative cell potential (E°cell = -0.95 V) and is thermodynamically non-feasible.
💡 Key Rule for Catalytic Couples
For a metal redox couple Mⁿ⁺ / M⁽ⁿ⁺¹⁾⁺ to catalyse the reaction between peroxodisulfate and iodide:
E°(I₂ / 2I⁻) < E°(Catalyst couple) < E°(S₂O₈²⁻ / 2SO₄²⁻)
+0.54 V < E°(Catalyst) < +2.01 V
Both half-steps must have positive E°cell values so the catalyst can be oxidised/reduced and then regenerated.
📐 Step-by-Step Feasibility Analysis
The overall uncatalysed reaction is:
S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂
Both reacting ions are negatively charged, leading to a high activation energy due to electrostatic repulsion. A catalyst provides an alternative route involving ions of opposite charge.
| Catalyst Option | Step 1: Reaction with S₂O₈²⁻ (+2.01 V) | Step 2: Reaction with I⁻ / I₂ (+0.54 V) | Can it act as catalyst? |
|---|---|---|---|
| A: Co²⁺ (E° = +1.82 V) | S₂O₈²⁻ oxidises Co²⁺ to Co³⁺ E°cell = +2.01 - (+1.82) = +0.19 V (> 0) | Co³⁺ oxidises I⁻ to I₂ E°cell = +1.82 - (+0.54) = +1.28 V (> 0) | Yes |
| B: Cr²⁺ (E° = -0.41 V) | S₂O₈²⁻ oxidises Cr²⁺ to Cr³⁺ E°cell = +2.01 - (-0.41) = +2.42 V (> 0) | Cr³⁺ oxidises I⁻ to I₂ E°cell = -0.41 - (+0.54) = -0.95 V (< 0) | No ❌ |
| C: Fe²⁺ (E° = +0.77 V) | S₂O₈²⁻ oxidises Fe²⁺ to Fe³⁺ E°cell = +2.01 - (+0.77) = +1.24 V (> 0) | Fe³⁺ oxidises I⁻ to I₂ E°cell = +0.77 - (+0.54) = +0.23 V (> 0) | Yes |
| D: Fe³⁺ (E° = +0.77 V) | Fe³⁺ first oxidises I⁻ to I₂ (Step 2 above) E°cell = +0.23 V (> 0) | Formed Fe²⁺ is re-oxidised by S₂O₈²⁻ (Step 1) E°cell = +1.24 V (> 0) | Yes |
🧠 Exam Technique: Fast Elimination
- Recall the syllabus standard: Fe²⁺ and Fe³⁺ are standard textbook examples of homogeneous catalysts for this reaction. You can instantly eliminate C and D!
- Spot the outlier: The E° value for Cr³⁺/Cr²⁺ is negative (-0.41 V), which falls well outside the range between +0.54 V and +2.01 V.
- Because -0.41 V < +0.54 V, Cr³⁺ is a weaker oxidising agent than I₂, meaning it can never oxidise I⁻ to regenerate Cr²⁺.
❌ Common Misconceptions
- Only checking the first step: Cr²⁺ reacts very favourably with S₂O₈²⁻ (E°cell = +2.42 V). Students who only check if the added ion reacts forget that a catalyst must be regenerated!
- Thinking only Fe²⁺ works, not Fe³⁺: Because the cycle is closed, adding either oxidation state (Fe²⁺ or Fe³⁺) initiates the reaction.
- Sign convention errors: Calculating E°cell = E°(reduction) - E°(oxidation). If E°cell is negative, the reaction is not feasible under standard conditions.
Topics
Inorganic Chemistry · Physical Chemistry · 3.2.5 Transition Metals · 3.1.11 Electrode Potentials
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.