Diesel is a hydrocarbon fuel obtained from crude oil. When diesel undergoes incomplete combustion in a car engine, several pollutants are produced. Explain why incomplete combustion occurs in a car engine and describe the harmful effects of the pollutants formed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Diesel engines are widely used in lorries and buses. Concerns have been raised about the air quality near busy roads, particularly in urban areas.
Model answer (5 marks)
Incomplete combustion occurs when the engine does not receive enough oxygen to react fully with the diesel fuel.
During this process the fuel is oxidised only partially, producing mainly carbon monoxide (CO) and unburnt carbon particles (soot).
CO is harmful because it binds to haemoglobin in the blood with a much higher affinity than oxygen, reducing the blood’s oxygen‑carrying capacity and causing hypoxia.
Soot particles are also a problem: they scatter and absorb sunlight, contributing to global dimming, and when inhaled they can irritate the respiratory tract and aggravate asthma and other lung diseases.
During this process the fuel is oxidised only partially, producing mainly carbon monoxide (CO) and unburnt carbon particles (soot).
CO is harmful because it binds to haemoglobin in the blood with a much higher affinity than oxygen, reducing the blood’s oxygen‑carrying capacity and causing hypoxia.
Soot particles are also a problem: they scatter and absorb sunlight, contributing to global dimming, and when inhaled they can irritate the respiratory tract and aggravate asthma and other lung diseases.
Examiner tips
- Use the word ‘incomplete combustion’ and explain the lack of oxygen; mention CO and soot; link CO to haemoglobin; link soot to health and climate effects
Common mistakes
- Forgetting to mention the lack of oxygen; confusing CO with CO₂; not explaining the health impact of soot
Mark scheme (5 marks)
- Incomplete combustion occurs because there is insufficient oxygen (supply) in the engine
- Carbon monoxide is produced as a pollutant
- Carbon monoxide is toxic / harmful to humans because it reduces the ability of blood to carry oxygen
- Carbon particulates (soot) are also produced
- Carbon particulates cause global dimming and/or health problems for humans
Key terms in this question
incomplete combustion · pollutants
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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