Edexcel A-Level Chemistry Paper 1, June 2023: Question 8

6 marks · Hard difficulty · Extended Writing

Compare and contrast the role of platinum and manganese(II) ions as catalysts in their respective reactions.

Practise this question

Question

Exam question 8 worth 6 marks asking to compare and contrast the role of catalysts in two reactions: platinum in vehicle catalytic converters, and Mn2+(aq) ions in the oxidation of ethanedioate ions by manganate(VII) ions.
Question text

8 Transition metals and their compounds can act as catalysts in many reactions such as

the ones shown:

• platinum, Pt, in the catalytic converters of vehicles

• manganese(II) ions, Mn2+(aq), in the oxidation of ethanedioate ions, C O2–(aq),

by manganate(VII) ions, MnO–(aq).

Compare and contrast the role of the catalysts in these reactions.

(6)

Mark scheme

Show the mark scheme Mark scheme for question 8, worth 6 marks using level of response marking for indicative content (up to 4 marks) and structure/reasoning (up to 2 marks), detailing indicative points such as alternative pathways with lower activation energy, heterogeneous vs homogeneous catalysis, adsorption/desorption for platinum, and repulsion/autocatalysis for Mn2+.

Question

Acceptable Answers Additional Guidance Mark

Number

8 This question assesses the student’s ability to show a coherent and logically Guidance on how the mark scheme (6)

structured answer with linkages and fully sustained reasoning. should be applied:

The mark for indicative content

Marks are awarded for indicative content and for how the answer is structured and should be added to the mark for

shows lines of reasoning. lines of reasoning. For example, a

response with four indicative

The following table shows how the marks should be awarded for indicative content. marking points that is partially

Number of indicative marking Number of marks awarded for structured with some linkages and

points seen in answer indicative marking points lines of reasoning scores 4 marks

64 (3 marks for indicative content and

5-4 3 1 mark for partial structure and

3-2 2 some linkages and lines of

11 reasoning).

00 If there were no linkages between

the points, then the same indicative

The following table shows how the marks should be awarded for structure and lines marking points would yield and

of reasoning overall score of 3 marks (3 marks

Number of marks awarded for for indicative content and zero

structure of answer and sustained marks for linkages).

lines of reasoning

Answer shows a coherent logical structure 2 In general it would be expected

with linkages and fully sustained lines of that 5 or 6 indicative points would

reasoning demonstrated throughout get 2 reasoning marks, and 3 or 4

Answer is partially structured with some 1 indicative points would get 1 mark

linkages and lines of reasoning for reasoning, and 0, 1 or 2

Answer has no linkages between points 0 indicative points would score zero

and is unstructured marks for reasoning

Indicative Points

IP1 Similarity

both (increase the rate of a reaction) by providing an alternative mechanism / route

with lower activation energy (and are chemically unchanged at the end of the

reaction)

IP2 Heterogeneous / homogeneous

platinum is heterogenous / different physical state to reactants Allow specific states mentioned –

and Pt is a solid and the reactants are

gases and Mn2+ and the reactants

Mn2+ is homogenous / same physical state to reactants

are all in aqueous solution

Platinum Allow names / formulae of specific

IP3 reactants adsorb onto platinum (surface) reactants (CO and NO / NO /NO )

2 x

and products (CO2 and N2)

IP4 the reactant bonds are weakened and the products are desorbed Allow active sites for platinum

surface

Allow broken for weakened

Mn2+

Allow just MnO – and C O 2–ions

IP5 the negative ions repel so the reaction starts slowly 4 2 4

repel (each other)

IP6 Mn2+ acts as an autocatalyst

Allow Mn2+ reduces MnO – then

or 4

the Mn species / Mn3+ formed

the reaction then speeds up as Mn2+ catalyst forms

oxidises the C O 2– ions

or

the reaction is faster between positive ions / Mn2+ and negative ions Allow Mn2+ acts as a catalyst as

Mn has a (wide range of) variable

oxidation states

How to answer it

Comparing Catalytic Roles: Platinum vs Manganese(II)

What this question tests

This 6-mark extended-writing question tests your understanding of transition metal catalysis, specifically comparing heterogeneous catalysis (solid platinum in vehicle catalytic converters) and homogeneous catalysis (aqueous manganese(II) ions in the oxidation of ethanedioate ions by manganate(VII)). It assesses your ability to identify similarities and differences in mechanism, state clearly structured lines of reasoning, and use precise chemical terminology.

Question Breakdown

Edexcel A-Level Chemistry - Transition Metals

💡 Key Knowledge Required

  • Definition of a catalyst: provides an alternative route with lower activation energy and remains chemically unchanged.
  • Heterogeneous catalysts exist in a different phase/state from the reactants (e.g., solid Pt with gaseous reactants).
  • Homogeneous catalysts exist in the same phase/state as the reactants (e.g., aqueous Mn²⁺ with aqueous ions).
  • Surface adsorption processes for heterogeneous systems (adsorption, bond weakening, desorption).
  • Autocatalysis and ionic repulsion principles for homogeneous systems.

🧠 Exam Technique for 6-Mark Extended Answers

  • Indicative Points (IP): You must hit specific marking points (up to 4 marks available based on how many IPs you include).
  • Structure & Reasoning: An extra 2 marks are awarded for a coherent, logical structure with clear linkages between points. Avoid isolated bullet points; use comparative phrasing (e.g., "Whereas platinum is... manganese(II) is...").

✅ Model Marking Points (Indicative Content)

  • IP1 (Similarity): Both increase the rate of reaction by providing an alternative pathway with a lower activation energy, remaining chemically unchanged at the end.
  • IP2 (Phase): Platinum is heterogeneous (different physical state to reactants, i.e., solid vs gases), whereas Mn²⁺ is homogeneous (same physical state, i.e., aqueous solution).
  • IP3 (Pt - Adsorption): Reactants adsorb onto the platinum metal surface.
  • IP4 (Pt - Reaction/Desorption): Reactant bonds are weakened, reaction occurs, and products are desorbed.
  • IP5 (Mn²⁺ - Repulsion): Between negative ions (MnO₄⁻ and C₂O₄²⁻), negative charges repel, causing the reaction to start slowly.
  • IP6 (Mn²⁺ - Autocatalysis): Mn²⁺ acts as an autocatalyst, so the reaction speeds up as the catalyst forms (or via positive ion / negative ion attraction).

❌ Common Student Errors

  • Vague definitions: Stating "catalysts speed up reactions" without mentioning lower activation energy or remaining unchanged.
  • Missing states: Failing to explicitly state that platinum is a solid while reactants are gases, or missing that Mn²⁺ and reactants are all aqueous.
  • Lack of linkages: Writing random facts about both catalysts in separate paragraphs without directly comparing and contrasting them.
  • Confusing mechanisms: Describing surface adsorption for manganese(II) ions dissolved in solution.
Mark Scheme Award Rule: Indicative points map to marks as follows: 6 points = 4 marks | 5-4 points = 3 marks | 3-2 points = 1-2 marks. Structure and reasoning add up to an additional 2 marks for fully sustained linkages.

Topics

Inorganic Chemistry · Topic 15: Transition Metals

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.