Edexcel A-Level Chemistry Paper 1, June 2025: Question 8

6 marks · Medium difficulty · Extended Writing

Compare and contrast the mechanisms of heterogeneous vanadium(V) oxide and homogeneous iron(II) catalysts, illustrating with equations for the redox reactions involved.

Practise this question

Question

Question 8 asks students to compare and contrast the mechanisms of the catalysts in a provided table and illustrate their answer with equations demonstrating the redox reactions involved, worth 6 marks. The table contains two rows: Row 1 lists catalyst V2O5(s) with the catalysed reaction SO2(g) + 1/2O2(g) -> SO3(g); Row 2 lists catalyst Fe2+(aq) with the catalysed reaction S2O8^2-(aq) + 2I^-(aq) -> I2(aq) + 2SO4^2-(aq). A large lined response space follows across two pages.

Mark scheme

Show the mark scheme Mark scheme for Question 8 showing a 6-mark levels of response rubric based on 6 indicative points and structure of reasoning marks. The indicative points are: IP1 (similarity: both increase reaction rate by lowering activation energy via an alternative pathway), IP2 (similarity: both involve changes in oxidation numbers followed by return to original oxidation number), IP3 (mechanism: V2O5 involves adsorption of reactants, bond weakening, and desorption of products, or Fe2+ attracts negatively charged ions avoiding repulsion between negative ions), IP4 (difference: V2O5 is heterogeneous in a different phase to reactants, while Fe2+ is homogeneous in the same phase as reactants), IP5 (equations for V2O5: V2O5 + SO2 -> V2O4 + SO3 and V2O4 + 1/2O2 -> V2O5), IP6 (equations for Fe2+: S2O8^2- + 2Fe2+ -> 2Fe3+ + 2SO4^2- and 2Fe3+ + 2I^- -> I2 + 2Fe2+).

How to answer it

Transition Metal Catalysis: Homogeneous vs Heterogeneous Mechanisms

📌 What this question tests

This 6-mark extended-response question assesses your understanding of transition metal catalysts, specifically:

  • Heterogeneous catalysis: Mechanism of action (adsorption, bond weakening, reaction, desorption) and phase differences using V₂O₅ in the Contact Process.
  • Homogeneous catalysis: Overcoming electrostatic repulsion between negatively charged ions using Fe²⁺ in the persulfate–iodide reaction.
  • Variable oxidation states: Formulating balanced multi-step redox equations showing the transition metal's intermediate oxidation state and its regeneration.
  • Comparing and contrasting: Identifying core mechanistic similarities alongside distinct differences with a coherent line of reasoning.

Question 8 (6 Marks)

Extended Response: Compare and contrast catalytic mechanisms

💡 Key Knowledge: The Chemistry

  • Why transition metals act as catalysts: They possess partially filled d-orbitals, allowing them to exhibit variable oxidation states and exchange electrons readily.
  • Heterogeneous catalyst (V₂O₅): Operates in a different phase (solid) to reactants (gases). Active sites adsorb reactants, lowering activation energy (Ea), followed by desorption of products.
  • Homogeneous catalyst (Fe²⁺): Operates in the same phase (aqueous) as reactants. Without a catalyst, two negatively charged ions (S₂O₈²⁻ and I⁻) strongly repel each other, creating a high Ea. Fe²⁺ facilitates the reaction via oppositely charged ion interactions in two low-barrier redox steps.

🧠 Exam Technique: Structuring a 6-Mark Answer

  • Use headings: Organise into Similarities, Differences, and Reaction Steps & Equations.
  • Check both catalysts: Ensure you describe both V₂O₅ and Fe²⁺ thoroughly; omitting one caps your score significantly.
  • Level-of-Response Rule:
    • 5–6 Indicative Points (IPs) + Coherent reasoning = 6 marks
    • 3–4 IPs + Partial reasoning = 3–4 marks
    • Chemistry errors immediately deduct reasoning marks.

✅ Model Answer (All 6 Indicative Points)

1. Similarities between the catalysts:

  • Mechanism of Action: Both catalysts increase the rate of reaction by providing an alternative reaction pathway with a lower activation energy (Ea) [ IP1 ].
  • Oxidation State Changes: Both involve transition metals changing their oxidation state during an intermediate redox step and then returning to their original oxidation state upon regeneration [ IP2 ]:
    • Vanadium is reduced from +5 (in V₂O₅) to +4 (in V₂O₄), then oxidised back to +5.
    • Iron is oxidised from +2 (Fe²⁺) to +3 (Fe³⁺), then reduced back to +2.

2. Differences in catalytic mechanisms:

  • Phase / Classification: V₂O₅ is a heterogeneous catalyst because it is in a different phase (solid) to the gaseous reactants (SO₂ and O₂). In contrast, Fe²⁺ is a homogeneous catalyst because it is in the same phase (aqueous solution) as the reactants (S₂O₈²⁻ and I⁻) [ IP4 ].
  • Specific Mechanism:
    • V₂O₅: Reactant gas molecules (SO₂ and O₂) adsorb onto the solid surface, weakening reactant bonds. Following reaction, the product (SO₃) desorbs from the surface [ IP3 ].
    • Fe²⁺: Uncatalysed, the two anions (S₂O₈²⁻ and I⁻) repel each other. The positively charged Fe²⁺ and Fe³⁺ ions attract the negative anions, overcoming electrostatic repulsion [ IP3 ].

3. Redox Equations for intermediate steps:

Contact Process with V₂O₅ [ IP5 ]:

Step 1: V₂O₅(s) + SO₂(g) → V₂O₄(s) + SO₃(g)
Step 2: V₂O₄(s) + ½O₂(g) → V₂O₅(s)

Persulfate–Iodide Reaction with Fe²⁺ [ IP6 ]:

Step 1: S₂O₈²⁻(aq) + 2Fe²⁺(aq) → 2SO₄²⁻(aq) + 2Fe³⁺(aq)
Step 2: 2Fe³⁺(aq) + 2I⁻(aq) → 2Fe²⁺(aq) + I₂(aq)

❌ Common Errors & Misconceptions

  • Absorption vs Adsorption: Writing absorption (soaking in) instead of adsorption (sticking to the surface) for heterogeneous catalysis.
  • Forgetting Desorption: Mentioning reactants attaching to the surface without stating that products must desorb to free up active sites.
  • Incorrect Order/Stoichiometry for Fe²⁺: Writing Fe²⁺ + I⁻ reacting first. While Fe³⁺ can catalytically start the reaction, if Fe²⁺ is added, it must first be oxidised by S₂O₈²⁻. Ensure equations balance for charge and atoms!
  • Confusing Auto-catalysis: Calling Fe²⁺ an autocatalyst. Fe²⁺ is added as an external catalyst; autocatalysis only occurs when a reaction product acts as the catalyst (e.g., Mn²⁺ in ethanedioate titrations).

🔍 Examiner Commentary & Mark Breakdown

Indicative Content (Max 4 marks):
• 6 indicative points = 4 marks
• 4–5 indicative points = 3 marks
• 2–3 indicative points = 2 marks
• 1 indicative point = 1 mark

Structure and Reasoning (Max 2 marks):
• 2 marks: Sustained logical structure linking both similarities, differences, and equations without chemical contradictions.
• 1 mark: Partially structured response with some linkages.

Tip: State symbols are not strictly required in the equations for IP5 and IP6, but writing them proves you recognise the phase differences required for IP4!

Topics

Inorganic Chemistry · Physical Chemistry · Topic 15: Transition Metals · Topic 9: Kinetics I · Topic 3: Redox I

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.