Edexcel IGCSE Chemistry · Spec 4.12-4.16
Combustion, Pollutants & Acid Rain
Complete and incomplete combustion, the pollutants produced, and acid rain.
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Combustion, Pollutants & Acid Rain
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Explained
Burning fuels, and the three pollutants that follow
When a hydrocarbon burns, what comes out depends on how much oxygen was available.
Complete combustion, with plenty of oxygen, gives carbon dioxide and water only, and releases the most energy.
Incomplete combustion, with a limited supply, gives carbon monoxide and carbon as soot as well, and releases less energy. A yellow smoky flame is the sign of it; a clear blue flame means the supply is good.
Carbon monoxide
Toxic, and dangerous because it is colourless and has no smell, so there is no warning.
It binds to haemoglobin in place of oxygen, and binds more strongly, so the blood carries less oxygen to the tissues. The compound formed is carboxyhaemoglobin.
Soot
Solid carbon particles. They blacken buildings, block flues and boiler parts, and cause breathing problems when inhaled. They also settle on surfaces and reduce how much light reaches them.
Sulfur dioxide and acid rain
Fossil fuels contain sulfur as an impurity. When the fuel burns, the sulfur burns too and forms sulfur dioxide.
That gas rises, dissolves in the water in clouds and reacts to form an acidic solution, which falls as acid rain. It kills trees, lowers the pH of lakes and rivers so fish die, and corrodes limestone buildings and statues.
Nitrogen oxides do the same by a different route. Inside a hot engine, the nitrogen and oxygen already in the air react together, and the oxides formed dissolve to give nitric acid.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme sets out acid rain from sulfur as a three step chain: the sulfur impurity in the fuel burns, sulfur dioxide forms, and it dissolves in or reacts with rain water. The third mark depends on one of the first two.
Two instructions sit beside it. The first says not to allow acid rain alone, because that phrase was already in the question. The second says to ignore any mention of nitrogen, nitrogen dioxide or carbon dioxide.
The second one surprises people, because nitrogen oxides genuinely do cause acid rain. But that question asked about sulfur, so correct material about a different pollutant earns nothing. Answer the pollutant named.
On carbon monoxide, the same series credits that it reduces the capacity of the blood to transport oxygen, accepts correct references to haemoglobin, and allows carboxyhaemoglobin. For the other product of incomplete combustion it allows carbon, soot or C.
One more note from the same paper, on a neighbouring question, reads simply: alkanes not alkenes. Those two words differ by one letter and belong to different families, and an examiner marks what is written.
Reducing the pollutants
Sulfur can be removed from the fuel before it is burned, and flue gas desulfurisation removes sulfur dioxide from power station exhaust by reacting it with calcium oxide or calcium carbonate.
Catalytic converters in vehicles convert carbon monoxide and nitrogen oxides into carbon dioxide and nitrogen. Ensuring a good air supply, as the air hole on a Bunsen burner does, prevents incomplete combustion in the first place.
Spec 4.12-4.16
What you need to know
- Compare complete and incomplete combustion
- Name the main pollutants
- Explain how acid rain forms
Active recall
Quick check
Answer each question before opening the answer.
What are the products of complete and incomplete combustion of a hydrocarbon?
Complete: carbon dioxide + water. Incomplete: carbon monoxide and/or soot (carbon) + water.
How is acid rain formed?
Sulfur dioxide (and nitrogen oxides) from burning fuels dissolve in rain water to form acids.
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