Edexcel A-Level Chemistry Paper 1, June 2019: Question 5

✅ Correct Answer

The two reacting ions both possess negative charges ( I⁻ and S₂O₈²⁻ ) and therefore experience electrostatic repulsion when they approach one another.

💡 Key Knowledge

For any reaction to occur, particles must collide with sufficient energy (greater than or equal to the activation energy). Because both species are anions, significant energy is required to overcome the repulsive forces between their electron clouds before bond rearrangement can happen.

❌ Common Errors

• Stating only that "ions repel" without specifying that they are both negative.
• Vague references to general collision theory without mentioning charge or repulsion.

✅ Correct Answer

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

Reasoning: Catalysis is possible because iron exhibits variable oxidation states (+2 and +3), and breaking the reaction into two steps involves oppositely charged ions in each step, avoiding direct repulsion.

🧠 Exam Technique

State symbols are explicitly not required by the prompt, so save time by omitting them. Ensure your equations balance stoichiometrically. You may write them in reverse order, but keeping oxidation first mirrors the logical flow of the catalytic cycle.

❌ Common Errors

• Forgetting to state why the catalyst works (mentioning variable oxidation states or opposite charges is vital for the 3rd mark).
• Balancing errors with sulfate or iron species.

✅ Correct Answer

Step 1: 2Co²⁺ + 0.5O₂ + 2H⁺ → 2Co³⁺ + H₂O
Step 2: 2Co³⁺ + SO₃²⁻ + H₂O → 2Co²⁺ + SO₄²⁻ + 2H⁺

💡 Key Knowledge

Just like iron, cobalt demonstrates variable oxidation states ( Co²⁺ / Co³⁺ ). Because the medium is acidic, H⁺ ions and H₂O molecules will frequently appear in the half-equations to balance oxygen and hydrogen atoms.

🧠 Exam Technique

Check your species carefully: ensure the catalyst ( Co species) is regenerated at the end of the second step. Multiples of these equations are fully accepted by examiners.

✅ Correct Answer

C: Mn²⁺

Explanation: The reaction starts slowly because it involves two negative ions repelling each other ( MnO₄⁻ and C₂O₄²⁻ ). As the reaction proceeds, Mn²⁺ ions are formed as a product, which then act as an autocatalyst, speeding up the rate dramatically.

❌ Common Errors

• Selecting H⁺ (Option B), confusing the role of the acid medium with catalytic product generation.
• Selecting CO₂ (Option A) due to misinterpreting gas evolution as the catalytic driver.

✅ Correct Answer

Tungsten is unsuitable because adsorption is too strong, meaning desorption of the products would be far too slow.

💡 Key Knowledge

In heterogeneous catalysis, a delicate balance is required: adsorption must be strong enough to hold reactants on the surface and weaken their internal covalent bonds, but weak enough that the final products can detach (desorb) to free up active sites for the next catalytic cycle.

❌ Common Errors

• Using the spelling "absorption" instead of "adsorption" (penalised by examiners as it refers to a different bulk physical process).
• Discussing reactivity series, melting points, or cost instead of focusing strictly on the surface adsorption/desorption mechanism.