AQA GCSE Chemistry Chemistry Paper 2 (Higher), November 2020: Question 6

Mark scheme

Show the mark scheme

Question 6

AO /

Question Answers Extra information Mark

Spec. Ref.

max 1 mark if soot wrongly

06.1 identified AO1

4.9.3.1

incomplete combustion 1

(because of) insufficient oxygen 1

allow SO2 for sulfur dioxide

06.2

sulfur reacts with oxygen to form allow sulfur burns to form sulfur 1 AO1

sulfur dioxide dioxide

(so) less sulfur dioxide emitted 1 AO1

(so) less acid rain 1 AO2

(so less) limestone reacts with 1 AO2

acid rain

4.9.3.1

4.9.3.2

06.3 (car engines work at) high 1 AO1

temperatures 4.9.3.1

(so in the engine) nitrogen (from 1

air) reacts with oxygen (from air)

Total 8

How to answer it

Atmospheric Pollutants: Soot, Sulfur Dioxide & Nitrogen Oxides

What this question tests

Incomplete combustion (AO1): Explaining the formation of carbon particulates (soot) when oxygen is limited.
Acid rain formation & consequences (AO1/AO2): Constructing a cause-and-effect logical chain from fuel desulfurisation to reduced damage on limestone (calcium carbonate).
Oxides of nitrogen in vehicle engines (AO1): Recognising that high temperatures inside internal combustion engines cause nitrogen and oxygen from atmospheric air to react.

Part 06.1 • 2 Marks

Formation of Soot from Burning Fossil Fuels

Explain why soot is formed when some fossil fuels are burned.

✅ Model Answer & Mark Breakdown

Mark 1: Incomplete combustion occurs...
Mark 2: ...because of an insufficient / limited supply of oxygen.

💡 Key Knowledge

Complete combustion: Fuel + plenty of O₂ → CO₂ + H₂O
Incomplete combustion: Fuel + limited O₂ → CO (toxic gas) + C (solid carbon particulates / soot) + H₂O
Soot: Fine unburned carbon particles that cause global dimming and respiratory problems in humans.

🧠 Exam Technique

This is a 2-mark "Explain" question: give the chemical process (incomplete combustion) and the condition causing it (lack of oxygen).
Always link "soot / carbon particulates" to "limited oxygen".

❌ Common Errors

Confusing soot with carbon monoxide or carbon dioxide (maximum 1 mark awarded if soot is wrongly identified).
Saying "there is no oxygen" — combustion cannot happen at all without oxygen; it must be insufficient or limited.

Mark Scheme Note: Max 1 mark if soot is wrongly identified. Full credit requires both "incomplete combustion" and "(because of) insufficient / limited oxygen".

Part 06.2 • 4 Marks

Reducing Sulfur in Fuels to Protect Limestone

Explain how reducing the amount of sulfur in fossil fuels reduces the erosion of limestone.

✅ Model Answer & Step-by-Step Chain

1 Sulfur reacts with oxygen when burned to produce sulfur dioxide (SO₂).

2 Less sulfur in fuels means less sulfur dioxide is produced / emitted into the atmosphere.

3 Less sulfur dioxide results in less acid rain.

4 Therefore, less acid rain reacts with the limestone (reducing erosion).

💡 Key Knowledge

Fossil fuels often contain sulfur impurities.
Combustion equation: S + O₂ → SO₂
Sulfur dioxide dissolves in atmospheric water/clouds to form acid rain (dilute sulfurous/sulfuric acid).
Limestone consists of calcium carbonate (CaCO₃), which reacts with acid in a neutralization reaction: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂ (causing erosion).

🧠 Exam Technique

For 4 marks, follow the chemical pathway start to finish:
Fuel → Pollutant gas (SO₂) → Atmosphere (Acid rain) → Surface impact (Limestone reaction).
Never skip a link in the chain! Many students state "less sulfur means less acid rain" but fail to mention the formation of sulfur dioxide.

❌ Common Errors

Saying sulfur itself directly dissolves or falls as acid rain (sulfur must first be oxidised to SO₂).
Confusing sulfur dioxide (acid rain) with carbon dioxide (global warming) or soot (global dimming).
Forgetting to mention the reaction between the acid rain and the limestone statue itself.

Mark Scheme Note: Allow "SO₂" for sulfur dioxide. Allow "sulfur burns to form sulfur dioxide". 1 mark per distinct linked point.

Part 06.3 • 2 Marks

Formation of Oxides of Nitrogen in Car Engines

Explain why oxides of nitrogen are formed in car engines.

✅ Model Answer & Mark Breakdown

Mark 1: Car engines operate at very high temperatures.
Mark 2: High temperatures provide enough activation energy for nitrogen and oxygen (from the air) to react together.

💡 Key Knowledge

Nitrogen gas (N₂) has a very strong triple covalent bond (N≡N) and is normally unreactive.
Inside a car engine cylinder, temperatures reach well over 1000 °C during spark ignition.
Equation: N₂ + O₂ → 2NO (and further oxidation to NO₂).
Both reactants originate from the ambient air drawn into the engine, not the fuel!

🧠 Exam Technique

State both high temperature AND name the two reactants: nitrogen and oxygen.
Always be specific about the source: specify that nitrogen comes from the air.

❌ Common Errors

Major misconception: Claiming nitrogen is present in the fuel (hydrocarbons contain carbon and hydrogen, not nitrogen).
Vaguely stating "high pressure" alone without mentioning the essential high temperature.

Mark Scheme Note: 1 mark for high temperatures; 1 mark for nitrogen reacting with oxygen (from the air).

Topics

Chemistry · C9: Chemistry of the Atmosphere