WJEC A-Level Chemistry Unit 3, June 2025: Question 3

✅ Correct Answer

The catalyst is in a different physical state (or different phase) to the reactants.

💡 Key Knowledge

• Physical states: Solid, liquid, or gas.
• In the majority of industrial heterogeneous reactions, the catalyst is a solid while the reactants are gases or liquids.
• Catalysis occurs at active sites on the solid surface via adsorption, reaction, and desorption.

🧠 Exam Technique

Always compare the catalyst specifically to the reactants, not the products. Stating only "they are in different states" is ambiguous and may not be credited.

❌ Common Errors

• Writing "the catalyst is a solid" — this gives an example, not a definition.
• Confusing state with chemical composition (e.g., "different elements").
• Comparing the catalyst to the products or the mixture as a whole.

✅ Accepted Examples

Award 1 mark for any correct pairing of catalyst and reaction:

• Iron (Fe) in the Haber process (synthesis of NH₃ from N₂ and H₂)
• Vanadium(V) oxide (V₂O₅) in the Contact process (oxidation of SO₂ to SO₃)
• Nickel (Ni) or Platinum (Pt) in the hydrogenation of alkenes (e.g. margarine manufacture)
• Platinum (Pt) / Rhodium (Rh) / Palladium (Pd) in catalytic converters

💡 Chemical Equations for Context

• Haber Process:
  N₂(g) + 3H₂(g) ⇌ 2NH₃(g) [Fe catalyst (s)]
• Contact Process:
  2SO₂(g) + O₂(g) ⇌ 2SO₃(g) [V₂O₅ catalyst (s)]
• Hydrogenation:
  R-CH=CH-R'(g/l) + H₂(g) → R-CH₂-CH₂-R'(g/l) [Ni catalyst (s)]

🧠 Exam Technique

This is a two-part requirement packed into 1 mark. You must clearly state both:

1. The specific catalyst (name or correct chemical formula).
2. The named reaction or industrial process.

❌ Common Errors

• Naming a catalyst without stating the reaction (e.g. writing just "Iron" or "Vanadium oxide").
• Naming a homogeneous system by mistake (e.g. Fe²⁺/Fe³⁺ in the peroxodisulfate-iodide reaction, or concentrated H₂SO₄ in esterification).
• Giving incorrect oxidation states (e.g. writing V₂O₃ instead of V₂O₅).