Community Q&A
A-Level Chemistry October/November 2024 Q3(b)(ii): Iron acts as a heterogeneous catalyst in the Haber process. Describe the mode of action…
A-Level Chemistry · Paper 9701/41 · October/November 2024 · Question 3(b)(ii) · [2 marks]
Iron acts as a heterogeneous catalyst in the Haber process. Describe the mode of action of this iron catalyst.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
The iron catalyst provides a surface with active sites for the reaction to occur. The mode of action involves three main stages:
- Adsorption: Reactant molecules, nitrogen (N₂) and hydrogen (H₂), are adsorbed onto the active sites on the surface of the iron catalyst.
- Bond weakening: This adsorption process weakens the strong covalent bonds within the reactant molecules (the N≡N triple bond and H-H single bonds), lowering the activation energy of the reaction.
- Desorption: After reacting on the surface to form ammonia (NH₃), the product molecules desorb from the catalyst's surface, leaving the active sites free for more reactant molecules.
How the marks are awarded
- B1 — The first mark is awarded for correctly stating any two of the three key stages of heterogeneous catalysis. The model answer includes 'adsorption', 'bond weakening', and 'desorption'.
- B1 — The second mark is awarded for correctly stating all three key stages. The model answer clearly describes adsorption of reactants, subsequent weakening of their bonds, and the final desorption of the product.
Common mistakes
- Only mentioning two of the three key stages (e.g., forgetting the crucial 'bond weakening' step), which limits the score to 1 mark.
- Confusing 'adsorption' (molecules binding to the surface) with 'absorption' (molecules entering the bulk of the material).
- Giving a generic answer like 'it provides an alternative reaction pathway with lower activation energy' without describing the specific steps for a heterogeneous catalyst.
- Stating that the reactants are 'absorbed' by the catalyst, which is incorrect terminology.
Examiner tip: For heterogeneous catalysis, always remember and describe the three-step sequence: Adsorption (of reactants), Reaction (involving bond weakening), and Desorption (of products).
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
Your answer
Sign in to answer this question.