Edexcel A-Level Chemistry AS Paper 2, June 2025: Question 8

8 marks · Medium difficulty · Open Response

Determine the equilibrium constant expression, calculate product yield difference with a catalyst, define heterogeneous catalysis, explain its mechanism, and identify exhaust pollutants and products.

Practise this question

Question

Question 8 about catalysts. Part (a) shows the reaction H2O + CH4 ⇌ 3H2 + CO over a nickel catalyst. (a)(i) is a multiple-choice question asking for the equilibrium constant Kc expression among four options. (a)(ii) asks how many more tonnes of product can be produced in an 8-hour period if catalyst reduces time per tonne from 4 hours to 1 hour (options: 2, 4, 6, 8). Part (b) covers vehicle exhaust catalysts: (i) define heterogeneous catalyst (1 mark), (ii) describe how a heterogeneous catalyst increases reaction rate (2 marks), and (iii) identify three major pollutants and the products they form over the catalyst (3 marks).

Mark scheme

Show the mark scheme Mark scheme for Question 8. 8(a)(i) gives correct answer C ([H2]^3[CO]/[H2O][CH4]). 8(a)(ii) gives correct answer C (6). 8(b)(i) awards 1 mark for stating the catalyst is in a different state or phase to the reactants. 8(b)(ii) awards 2 marks: provides a surface for reaction/adsorption, and lowers activation energy or weakens bonds. 8(b)(iii) awards 3 marks for identifying three pollutant-product pairs from: nitrogen oxides to nitrogen, carbon monoxide to carbon dioxide, unburnt hydrocarbons to carbon dioxide and water, or particulates/carbon/soot to carbon dioxide.

How to answer it

Catalysts, Equilibrium Constants & Catalytic Converters

Edexcel AS Chemistry • Topic 8 • Total: 8 Marks

What this question tests

  • Writing homogenous/heterogeneous equilibrium constant expressions ( Kc ) with correct powers.
  • Industrial rate and batch yield calculations over time.
  • Defining heterogeneous catalysis in terms of physical states/phases.
  • Explaining the mechanism of heterogeneous catalysis (adsorption, activation energy, surface effects).
  • Environmental chemistry of car exhausts: identifying exhaust pollutants and their catalytic reduction/oxidation products.

Part (a)(i) — Expression for Equilibrium Constant Kc

Reaction: H₂O(g) + CH₄(g) ⇌ 3H₂(g) + CO(g)

✅ Correct Answer

Option C: Kc = [H₂]³[CO] / ([H₂O][CH₄])

💡 Key Knowledge

  • Kc = [products] / [reactants] at equilibrium.
  • Stoichiometric balancing coefficients become powers (exponents), NOT multiplying coefficients inside or outside brackets.
  • Square brackets [ ] denote equilibrium concentrations in mol dm⁻³.

❌ Common Errors & Distractor Analysis

  • Option A: Multiplied concentration by 3 ( 3[H₂] ) instead of raising to the 3rd power ( [H₂]³ ).
  • Option B & D: Wrote the reciprocal expression [reactants] / [products] .
Mark scheme: 1 mark for Option C.

Part (a)(ii) — Industrial Rate & Batch Output Calculation

Time reduced from 4 hours to 1 hour per tonne in an 8-hour period

✅ Correct Answer

Option C: 6

📐 Step-by-Step Calculation

  1. Without catalyst: 8 hours ÷ 4 hours/tonne = 2 tonnes
  2. With catalyst: 8 hours ÷ 1 hour/tonne = 8 tonnes
  3. Additional ("more") tonnes produced: 8 tonnes − 2 tonnes = 6 tonnes

🧠 Exam Technique Alert

Always re-read the exact question phrasing! The exam specifically highlighted the word "more" in bold. Students choosing Option D (8) correctly calculated total production but failed to subtract the original output.

Mark scheme: 1 mark for Option C.

Part (b)(i) — Defining Heterogeneous Catalysts

State what is meant by a heterogeneous catalyst. [1 Mark]

✅ Correct Answer

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

💡 Contextual Example

In a vehicle exhaust system, the catalytic converter contains solid metals (Pt, Pd, Rh), whereas the incoming pollutant exhaust gases are in the gas phase.

❌ Examiner Trap

Stating only that the catalyst is a "solid" does not score the mark. You must explicitly reference that it is a solid while the reactants are gases, or use the formal comparative definition: different state / phase to reactants.

Mark scheme: 1 mark for reference to different state/phase to reactants (allow solid catalyst with gaseous reactants).

Part (b)(ii) — Mechanism of a Heterogeneous Catalyst

Describe how a heterogeneous catalyst increases reaction rate in an exhaust. [2 Marks]

✅ Correct Answer (Scoring 2/2)

  • Point 1 (Surface/Adsorption): The catalyst provides a surface for the reaction to take place (reactants are adsorbed onto the active sites of the surface). [1 mark]
  • Point 2 (Bonds/Activation Energy): Adsorption weakens chemical bonds in the reactant molecules (or holds them in favorable orientations), lowering the activation energy ( Ea ). [1 mark]

🧠 Strict Marking Dependency

The mark scheme specifies: writing just "provides an alternative pathway with lower activation energy" scores 0 marks unless the surface adsorption/action (Point 1) is also credited!

For heterogeneous catalysts, always describe the surface mechanism first.

Mark scheme: M1 = Provides surface / reactants adsorb onto surface. M2 = Lowers activation energy / weakens bonds / holds reactants in place.

Part (b)(iii) — Exhaust Pollutants and Their Products

Identify three major pollutants and the products they form. [3 Marks]

✅ Accepted Pollutant-Product Pairs (Choose any 3)

Pollutant (Reactant) Product Formed Process
Nitrogen oxides (NO / NO₂) Nitrogen (N₂) [and oxygen] Reduction
Carbon monoxide (CO) Carbon dioxide (CO₂) Oxidation
Unburnt hydrocarbons (e.g. C₈H₁₈) Carbon dioxide (CO₂) and water (H₂O) Combustion / Oxidation
Carbon / Soot / Particulates (C) Carbon dioxide (CO₂) Oxidation

🧠 Chemical Equation Alternative

You can score both M1 and M2 simultaneously by giving the balanced catalytic converter redox equation:

2NO + 2CO → N₂ + 2CO₂

This shows NO → N₂ (1 mark) and CO → CO₂ (1 mark).

❌ Common Misconceptions

  • Writing "N" instead of "nitrogen" or "N₂". Nitrogen gas is diatomic.
  • Forgetting to name the products: listing three pollutants without their products only scores max 1 mark under special allowance.
  • Confusing catalytic converters with power station flue gas desulfurisation: SO₂ is not a primary target of car catalytic converters.
Mark scheme: 1 mark for each correctly matched pollutant and its product(s). Maximum 3 marks. Chemical formulas or names accepted.

Topics

Physical Chemistry · Organic Chemistry · Inorganic Chemistry · Topic 11: Equilibrium II · Topic 9: Kinetics I · Topic 6: Organic Chemistry I · Topic 15: Transition Metals

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.