GCSE · Chemistry · AQA · Spec 8462
Atmospheric pollutants from fuels
A car engine burns petrol with plenty of air going in. Surely nothing nasty comes out? Not quite, and three questions will show you why.
Chemistry · Burning fuels
Three questions decide what comes out
Pick a fuel's conditions at each fork, and predict the list before you open the end.
Oxygen supply → Sulfur impurities → Temperature
8 possible lists of pollutants.
A burning fuel doesn't just "burn". How much oxygen it gets, what is in it and how hot it gets all change what it puts into the air.
Chemistry · Follow the atoms
Where did that atom come from?
Place each pollutant by where its non-oxygen element came from. One of them will surprise you.
Still to sort
Carbon in the fuel (0)
Carbon atoms that were already part of the petrol, diesel or other fuel
Where the line is: If the pollutant is made of carbon (with or without oxygen), the carbon came from the fuel, however much oxygen the fuel gets.
Sulfur impurity in the fuel (0)
Sulfur atoms present in the fuel as impurities
Where the line is: Only a fuel that contains sulfur impurities can send anything here.
Nitrogen in the air (0)
Nitrogen that comes in with the air, not with the fuel
Where the line is: The fuel isn't the source here: the air supplies both the nitrogen and the oxygen it reacts with.
Every pollutant here has an element in it besides oxygen. Decide where that element started its journey.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Work out what a burning fuel puts into the air from what's in the fuel and how it burns.
What you need to know
- Burning fuels in power stations, industrial processes and vehicle engines puts pollutants into the atmosphere; volcanic eruptions and forest fires are natural sources.
- Petrol and diesel contain carbon and hydrogen atoms, and may contain sulfur atoms as impurities.
- Complete combustion (sufficient oxygen) gives carbon dioxide and water. Incomplete combustion (insufficient oxygen) gives carbon monoxide, soot and water.
- Sulfur impurities give sulfur dioxide. At high temperatures, such as inside a car engine, nitrogen and oxygen from the air react to give oxides of nitrogen.
The big picture
Burning fuels in power stations, industrial processes and vehicle engines puts pollutants into the atmosphere. Which ones depends on the oxygen supply, on any sulfur impurities in the fuel and on how hot the burn is. Sufficient oxygen gives carbon dioxide (itself a pollutant) and water; insufficient oxygen gives carbon monoxide and soot instead. Sulfur impurities give sulfur dioxide, and high temperatures make nitrogen and oxygen in the air react to form oxides of nitrogen.
Key points
Worked example
Problem
A lorry's diesel has no sulfur impurities. Its engine is very hot inside, but the oxygen supply is insufficient. Name every pollutant that can be released.
⚠ Watch out
Mixing up carbon monoxide and carbon dioxide. Carbon dioxide is what complete combustion makes; carbon monoxide appears when the oxygen supply is insufficient. Check the name before you write it.
Memory hook
Ask three questions of any burning fuel: Enough air? Any sulfur? Hot enough? The fuel brings the carbon and the sulfur, but only the hot air brings the nitrogen.
Check yourself
Without looking back, describe the oxygen fork of the tree: what does the fuel's carbon become when the oxygen supply is sufficient, and what changes when it is insufficient?
Flashcards
(12)Which human and which natural processes put pollutants into the atmosphere?
Which atoms do fuels such as petrol and diesel contain?
What is complete combustion?
Is the carbon dioxide from complete combustion a pollutant?
What is incomplete combustion?
What is soot?
When does burning a fuel produce sulfur dioxide (SO2)?
How are oxides of nitrogen formed?
Name the two oxides of nitrogen and their shared label.
Petrol containing sulfur burns in plenty of oxygen inside a hot car engine. Which pollutants are produced?
Which extra pollutants appear if the oxygen supply is insufficient?
Word equation: how does a sulfur impurity make a pollutant?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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