AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), 2023: Question 5

12 marks · Standard Demand difficulty · Short Answer

Analyze the combustion of fuels, properties of hydrocarbons, and the environmental impact of resulting atmospheric pollutants including carbon monoxide, particulates, and sulfur dioxide.

Practise this question

Question

The image shows a multi-part GCSE chemistry exam question. It starts with balancing the combustion equation for methane. Then, a table compares petrol and diesel based on their carbon atom range (4-12 vs 12-20) and boiling point range (40-205°C vs 250-350°C). Subsequent parts ask to identify a common element in exhaust gases, the source of nitrogen in car engines, the environmental effect of particulates, properties of carbon monoxide, why water is produced as a vapour, and the environmental problems caused by sulfur impurities.
Question text

05 The combustion of fuels is a source of atmospheric pollutants.

05.1 Methane is a fuel.

Balance the equation for the combustion of methane.

[1 mark]

CH4 + O2 → CO2 + H2O

05.2 Many fuels are mixtures.

Petrol and diesel are mixtures of hydrocarbons.

Table 4 shows properties of petrol and of diesel.

Table 4

Petrol Diesel

Range of number of carbon atoms in

4 to 12 12 to 20

a hydrocarbon molecule

Range of boiling points in °C 40 to 205 250 to 350

Compare the properties of petrol and diesel.

Use Table 4.

[2 marks]

05.3 The gases released when a fuel is burned in car engines may include:

• oxides of nitrogen

• carbon monoxide

• water vapour.

Which chemical element do all these gases contain?

[1 mark]

*25* Tick ( ) one box.

Carbon

Hydrogen

Nitrogen

Oxygen

05.4 When diesel burns in car engines, oxides of nitrogen are produced.

Where does the nitrogen come from?

[1 mark]

05.5 When diesel burns, particulates may be produced.

What environmental effect do particulates from burning diesel cause?

[1 mark]

05.6 Carbon monoxide may be produced when diesel burns.

Give one reason why carbon monoxide is difficult to detect.

[1 mark]

05.7 Explain why water vapour and not liquid water is produced when diesel burns.

[2 marks]

05.8 Sulfur is a common impurity in diesel.

Explain why this causes an environmental problem.

[3 marks]

Mark scheme

Show the mark scheme The mark scheme provides the following answers: 05.1 requires balancing the equation with 2O2 and 2H2O. 05.2 awards marks for stating petrol has fewer carbon atoms and a lower boiling point range than diesel. 05.3 identifies oxygen as the common element. 05.4 identifies air or the atmosphere as the source of nitrogen. 05.5 identifies global dimming as the effect of particulates. 05.6 notes carbon monoxide is colourless or odourless. 05.7 explains that diesel burns at temperatures above 100°C, which is above the boiling point of water. 05.8 explains that sulfur dioxide is produced, causing acid rain, which damages buildings, trees, or aquatic life.

Question 5

AO /

Question Answers Extra information Mark

Spec. Ref.

05.1 CH4 + 2 O2 → CO2 + 2 H2O 1 AO2

5.7.1.3

AO /

Spec. Ref.

05.2 allow converse throughout AO3

5.7.1.3

hydrocarbon molecules in petrol 1

have fewer carbon atoms than

those in diesel

petrol has a lower boiling point 1

(range) than diesel

allow petrol is more flammable

than diesel

allow petrol is less viscous than

diesel

AO /

Spec. Ref.

05.3 oxygen 1 AO1

5.8.1.2

AO /

Spec. Ref.

05.4 air allow the atmosphere 1 AO1

5.9.1.1

allow from volcanoes 5.9.3.1

AO /

Spec. Ref.

05.5 global dimming 1 AO2

– OMBINED SCIENCE: TRILOGY – 2F – 5.9.3.1

AO /

Spec. Ref.

05.6 (carbon monoxide is) colourless ignore clear 1 AO1

or 5.9.3.2

(carbon monoxide is) odourless

AO /

Spec. Ref.

05.7 (diesel) burns at a high 1 AO2

temperature

or

(diesel) burns at a temperature

greater than 100 °C

AO3

(which is) above the boiling 1

point of water 5.2.2.1

5.9.3.1

AO /

Spec. Ref.

05.8 sulfur dioxide (is produced when 1 AO1

diesel is burnt) 5.9.3.1

5.9.3.2

(which causes) acid rain 1

(which results in)

any one from: 1

• damage to buildings /

statues / bridges

• damage to trees / plants

• damage to aquatic life

• acidification of lakes /

rivers / soil

• respiratory problems allow (which affects) asthma

Total Question 5 12

How to answer it

Atmospheric Pollutants and Fuels

What this question tests

This question assesses your ability to balance combustion equations, interpret data about hydrocarbon properties, and identify the sources and environmental impacts of atmospheric pollutants like NOx, particulates, carbon monoxide, and sulfur dioxide.

Part (a)

05.1 Balancing Combustion

Complete the equation for the combustion of methane

Correct Answer

CH₄ + 2O₂ → CO₂ + 2H₂O

Step-by-Step Balancing

  1. Carbon: 1 on left, 1 on right. Balanced.
  2. Hydrogen: 4 on left (CH₄). You need 4 on right, so add a 2 in front of H₂O (2 × 2 = 4).
  3. Oxygen: Now count the right side: 2 (in CO₂) + 2 (in 2H₂O) = 4. Add a 2 in front of O₂ on the left.
Part (b)

05.2 Comparing Properties

Compare petrol and diesel using the table provided

Correct Answer

  • Petrol molecules have fewer carbon atoms than diesel.
  • Petrol has a lower boiling point (range) than diesel.

1 mark for each comparison.

Exam Technique

When asked to "compare" using a table, make sure you mention both substances. Use comparative words like "lower," "higher," "more," or "fewer."

Part (c) & (d)

05.3 & 05.4 Sources of Gases

05.3 Common Element

Oxides of nitrogen, carbon monoxide, and water vapour all contain Oxygen.

05.4 Nitrogen Source

Correct Answer: The air (or atmosphere).

Nitrogen is not in the fuel. It is drawn into the engine from the air and reacts with oxygen due to the high heat of the engine.

Part (e) & (f)

05.5 & 05.6 Particulates and CO

05.5 Particulates

Environmental effect: Global dimming.

(Health effects like lung damage are also often accepted, but global dimming is the primary environmental answer).

05.6 Carbon Monoxide

Why is it hard to detect?

  • It is colourless.
  • It is odourless.

Common Error: Do not use the word "clear." Clear describes transparency in liquids; use "colourless" for gases.

Part (g)

05.7 State of Matter

Why is water vapour produced instead of liquid water?

Correct Answer

  • The diesel burns at a high temperature (or a temperature above 100°C).
  • This temperature is above the boiling point of water.

Logic Link

To get both marks, you must link the engine temperature to the physical property (boiling point) of water. Simply saying "it's hot" only gets 1 mark.

Part (h)

05.8 Sulfur Impurities

Explain the environmental problem caused by sulfur

The 3-Mark Chain

  1. Reaction: Sulfur reacts with oxygen to form sulfur dioxide (SO₂). [1 mark]
  2. Primary Effect: This gas dissolves in rainwater to cause acid rain. [1 mark]
  3. Environmental Damage: Mention one specific result:
    • Damage to limestone buildings/statues.
    • Damage to trees/plants.
    • Killing aquatic life (fish) in lakes.
    [1 mark]

Common Misconception

Do not confuse sulfur dioxide with carbon dioxide. Sulfur dioxide does not cause global warming; it causes acid rain. Keep these pollutants and their effects distinct in your revision!

Topics

Chemistry · C7: Organic Chemistry · C9: Chemistry of the Atmosphere · C8: Chemical Analysis · C2: Bonding, Structure and the Properties of Matter

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 2 (Foundation), 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.