Edexcel A-Level Chemistry Paper 2, June 2017: Question 5

6 marks · Medium difficulty · Short Open Response

Describe catalytic converter reactions, identify the Maxwell-Boltzmann distribution area representing the effect of a catalyst, and calculate the maximum number of carbon monoxide molecules in a given mass.

Practise this question

Question

Question 5 about catalytic converters in car exhaust systems. Part (a) asks for the equation for the reaction between carbon monoxide and nitrogen monoxide on a catalytic converter, and to describe the stages of this reaction. Part (b) shows a Maxwell-Boltzmann distribution curve with two activation energy lines, Ea and Ea(catalyst), marking regions A and B under the curve, and multiple-choice options A, B, C, D to identify the increase in particles with sufficient energy to react. Part (c) is a multiple-choice question asking for the maximum number of carbon monoxide molecules emitted per kilometre given a limit of 1.00 g.
Question text

5 This is a question about catalytic converters in car exhaust systems.

(a) When petrol is burnt in a car engine, pollutant gases including carbon monoxide

and nitrogen monoxide are formed.

(i) Write the equation for the reaction between these two polluting gases that takes

place on the surface of a catalytic converter. State symbols are not required.

(1)

(ii) Describe the stages by which the reaction in (a)(i) occurs in a catalytic converter.

(3)

(b) Which area in the Maxwell-Boltzmann distribution diagram represents the

increase in the number of particles with sufficient energy to react in the presence

of a catalyst?

Ea (catalyst)

Ea

Number of

particles (with a

given kinetic energy)

A B

Energy, E

(1)

A area A

B area B *P48059A0824*

C area A − area B

D area A + area B

(c) In the UK, the exhaust emissions of a petrol-fuelled vehicle must be less than

1.00 g of carbon monoxide per kilometre.

What is the maximum number of carbon monoxide molecules that can be

emitted per kilometre for a vehicle to meet this regulation?

(1)

A 1.37 × 1022

B 2.15 × 1022

C 6.02 × 1023

D 1.69 × 1025

(Total for Question 5 = 6 marks)

Mark scheme

Show the mark scheme Mark scheme for Question 5 showing acceptable answers, guidance, and reasoning for parts a(i), a(ii), b, and c.

Question

Answer Additional Guidance Mark

Number

5(a)(i) Correct equation 2NO + 2CO → N2 + 2CO2 (1)

Accept multiples

Ignore catalysts and conditions if stated

Question

Answer Additional Guidance Mark

Number

5(a)(ii) A description that makes reference to the following points: Absence of reference to the catalytic (3)

surface results in a deduction of one

mark

adsorption of gases to catalytic surface (1) Do not award absorption or “stick”

weakening of bonds (and chemical reaction) on Allow bonds break (and reaction occurs)

catalytic surface (1) on catalytic surface

Ignore the type of interaction referred

to between the reactants and the

catalytic surface

desorption of products from catalytic surface (1) Allow ‘release’ of products from catalytic

surface

Allow de-adsorbed

Question

Answer Mark

Number

5(b) The only correct answer is A (area A) (1)

B is not correct because this is the area representing the number of particles with sufficient energy

to react in the absence of a catalyst

C is not correct because this area subtraction does not represent the increase in the number of

particles with sufficient energy to react

D is not correct because this sum of areas represents the total number of particles with

sufficient energy to react in the presence of a catalyst

Question

Answer Mark

Number

5(c) The only correct answer is B (2.15 x 1022) (1)

A is not correct because the molar mass of carbon dioxide has been used in the calculation

instead of that of carbon monoxide

C is not correct because this is the number of molecules that are in one mole and not one gram of

carbon monoxide

D is not correct because this is the result of incorrectly using the molar mass of carbon monoxide

rather than the number of moles of carbon monoxide

(Total for Question 5 = 6 marks)

How to answer it

Study Guide: Catalytic Converters & Gas Calculations

What this question tests

This question assesses your knowledge of heterogeneous catalysis (specifically in car exhaust systems), interpretation of Maxwell-Boltzmann distribution curves when a catalyst is introduced, and stoichiometric calculations involving moles, Avogadro's constant, and mass conversions.

Question 5 (a) (i)

Heterogeneous Catalysis Equation

Write the equation for the reaction between carbon monoxide and nitrogen monoxide.

✅ Correct Answer

2NO + 2CO → N₂ + 2CO₂

(Multiples of this equation are also accepted).

❌ Common Errors

  • Balancing errors (forgetting to scale up NO and CO to balance the oxygen and nitrogen atoms).
  • Including state symbols when the question explicitly states they are not required.
Mark breakdown: 1 mark for a fully balanced chemical equation.
Question 5 (a) (ii)

Stages of Heterogeneous Catalysis

Describe the stages by which the reaction occurs in a catalytic converter.

💡 Key Knowledge: The 3-Step Process

  1. Adsorption: Reactant molecules (NO and CO) arrive at and bind to the active surface of the heterogeneous catalyst.
  2. Reaction / Bond weakening: Bonds within the reactants are weakened, and chemical reaction takes place on the catalytic surface.
  3. Desorption: The product molecules (N₂ and CO₂) detach (de-sorb) and leave the catalyst surface, freeing up active sites.

❌ Common Errors & Examiner Pitfalls

  • Terminology trap: Using the word "absorption" instead of "adsorption" loses you the mark. Absorption means soaking into the bulk material; adsorption means sticking to the surface.
  • Failing to mention the catalyst surface at all will result in an automatic 1-mark deduction.
Mark breakdown: 3 marks total (1 mark for adsorption, 1 mark for reaction/weakening of bonds on the surface, 1 mark for desorption of products).
Question 5 (b)

Maxwell-Boltzmann Distribution & Catalysts

Which area represents the increase in the number of particles with sufficient energy to react in the presence of a catalyst?

✅ Correct Answer

A (area A)

When the activation energy is lowered from Ea to Ea (catalyst) , the threshold moves to the left. The additional particles now possessing enough energy to react correspond strictly to area A.

🧠 Exam Technique

Carefully distinguish between total areas: Area B represents particles that had enough energy even without the catalyst. Area A is the extra cohort enabled by lowering the activation energy.

Mark breakdown: 1 mark for selecting option A.
Question 5 (c)

Stoichiometry & Avogadro Calculation

Calculate the maximum number of carbon monoxide molecules emitted per kilometre given a limit of 1.00 g of CO per km.

📐 Step-by-Step Calculation

  1. Find moles of CO:
    Mass = 1.00 g
    Molar mass of CO (C = 12.0, O = 16.0) = 28.0 g mol⁻¹
    Moles = 1.00 / 28.0 = 0.03571 mol
  2. Convert moles to number of molecules:
    Multiply moles by Avogadro's constant (6.02 × 10²³)
    Number of molecules = 0.03571 × 6.02 × 10²³ = 2.15 × 10²²

Correct Option: B

❌ Common Calculation Traps

  • Distractor A (1.37 × 10²²): Caused by using the molar mass of CO₂ (44.0 g mol⁻¹) instead of CO.
  • Distractor C (6.02 × 10²³): Forgetting to divide by the molar mass entirely, simply using Avogadro's number as a standalone value for 1 gram.
  • Distractor D (1.69 × 10²⁵): Dividing the mass by the molar mass incorrectly or applying reciprocal ratios.
Mark breakdown: 1 mark for selecting option B.

Topics

Physical Chemistry · Inorganic Chemistry · Topic 15: Transition Metals · Topic 9: Kinetics I · Topic 5: Formulae, Equations and Amounts of Substance

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.