Edexcel A-Level Chemistry Paper 1, November 2021: Question 3

Mark scheme

Show the mark scheme

Question

Answer Additional Guidance Mark

Number

3(a) An answer that makes reference to the following points ACCEPT suitable annotated diagram (2)

• metal cations in a ‘sea’ of delocalised electrons (1) ALLOW reference to metal ions

IGNORE ‘free’ electrons/positive nucleus

• (metallic bonding is the strong) electrostatic Do not award reference to molecules

attraction between (cations and electrons) (1)

Example diagram

Approximately equal numbers of positive ions and negative

electrons

Question

Answer Additional Guidance Mark

Number

3(b) An answer that makes reference to the following points (3)

• adsorption of CO and/or NO molecules on the catalytic surface (1) Allow ‘active site’ for surface

Do not award absorption

• weakening of bonds

(and chemical reaction between CO and NO) (1) Do not award weaken the bonds between

molecules

Allow bonds break (within CO and NO)

• desorption of CO2 and/or N2 /product (molecules) from the

catalytic surface (1) Allow de-adsorption for desorption

Do not award desorption of the reactants

Do not award reference to incorrect

products such as H2/O2/C/NO2

Penalise omission of catalytic surface

once only

(Total Question 3 = 5 marks)

How to answer it

Catalytic Converters & Metallic Bonding

📌 What this question tests

This question assesses your understanding of metallic bonding (structure, particles, and electrostatic forces) and heterogeneous catalysis (the sequential physical and chemical stages that occur on a catalytic surface, specifically adsorption, bond weakening/reaction, and desorption).

Part (a): Bonding in Platinum

Describe the bonding in platinum. You may include a diagram in your answer. (2 marks)

✅ Correct Answer

Metal cations (or positive metal ions) surrounded by a "sea" of delocalised electrons.
Strong electrostatic attraction between the positive metal ions and the delocalised electrons.

💡 Key Knowledge

Platinum is a transition metal, so it forms a giant metallic lattice structure.
An annotated diagram showing alternating positive ions and negative electrons is fully accepted by examiners.

🧠 Exam Technique

Two distinct marking points: Make sure you state both the components (cations and delocalised electrons) and the nature of the force (electrostatic attraction) to secure both marks.

❌ Common Errors

Using the word "molecules" instead of cations/ions (metallic structures are giant lattices, not molecular).
Writing "free electrons" without specifying their role or nature, or referring to "positive nuclei" instead of ions.

Mark Scheme Breakdown:
• Mark 1: Reference to metal cations in a 'sea' of delocalised electrons.
• Mark 2: Reference to strong electrostatic attraction between cations and electrons.

Part (b): Stages in a Catalytic Converter

Describe the stages in a catalytic converter that result in a decrease of harmful emissions (CO and NO). No equations are required. (3 marks)

✅ Correct Answer

Stage 1 (Adsorption): Reactant molecules (CO and NO) are adsorbed onto the surface of the catalyst.
Stage 2 (Reaction): Bonds within the reactant molecules are weakened and break, allowing them to react together on the surface.
Stage 3 (Desorption): Product molecules (such as CO₂ and N₂) desorb from the catalytic surface.

💡 Key Knowledge

Heterogeneous catalysts work via surface chemistry. The three golden steps are always: Adsorb → React → Desorb.
Using the term "active sites" for the catalytic surface is fully credited.

🧠 Exam Technique

Be careful with spelling! Examiners strictly penalise mixing up adsorption with absorption.
Even though the question states "no equations are required", naming the correct incoming gases (CO, NO) and exiting products (CO₂, N₂) shows clear understanding and anchors your description.

❌ Common Errors

Writing absorption (taking into the bulk) instead of adsorption (binding to the surface).
Failing to mention the catalyst surface in multiple steps (penalised if omitted throughout).
Mentioning incorrect products like H₂, O₂, or C.

Mark Scheme Breakdown:
• Mark 1: Adsorption of CO and/or NO molecules onto the catalytic surface.
• Mark 2: Weakening of bonds / chemical reaction between CO and NO on the surface.
• Mark 3: Desorption of CO₂ and/or N₂ (products) from the catalytic surface.

Topics

Physical Chemistry · Inorganic Chemistry · Topic 2: Bonding and Structure · Topic 15: Transition Metals