AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › Chemistry of the atmosphere › Common atmospheric pollutants and their sources
Practise Atmospheric pollutants from fuels. 7 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Which pollutants are released when fuels burn and how each one forms: carbon dioxide, water vapour, carbon monoxide, soot and other particulates, sulfur dioxide and oxides of nitrogen. Expect 'describe how X is produced' questions, combustion equations, and questions where you predict the products from information about a fuel and how it is burned.
Key facts
Complete combustion: C → CO2, H → H2O
Incomplete combustion (limited oxygen): CO and C (soot), as well as H2O
Sulfur in the fuel + oxygen → sulfur dioxide: S + O2 → SO2
Oxides of nitrogen: nitrogen and oxygen from the air react at high temperatures (e.g. in engines)
Particulates = solid particles (e.g. soot) and unburned hydrocarbons
CH4 + 2O2 → CO2 + 2H2O
Notes
What is in fuels
Burning fuels is a major source of the pollutants in the atmosphere.
Most fuels, coal included, are made mainly of carbon and/or hydrogen, and some also contain a little sulfur.
Combustion is an oxidation reaction: the elements in the fuel react with oxygen from the air.
Complete and incomplete combustion
Complete combustion (plenty of oxygen): carbon forms carbon dioxide and hydrogen forms water vapour, e.g. CH4 + 2O2 → CO2 + 2H2O.
Incomplete combustion (limited supply of oxygen): some of the carbon forms carbon monoxide, CO, and some forms solid carbon particles (soot). Water is still formed.
Solid particles (such as soot) and unburned hydrocarbons can also be released. These form particulates in the atmosphere.
Sulfur dioxide and oxides of nitrogen
Sulfur dioxide: sulfur in the fuel reacts with oxygen as the fuel burns: S + O2 → SO2.
Oxides of nitrogen (NOx): at the high temperatures in engines, nitrogen and oxygen from the air react together. The nitrogen comes from the air, not from petrol or diesel.
How to answer each type of question
Describe how a pollutant is produced
2 to 4 marksGrade 5
Say where the element comes from: the fuel (carbon, sulfur) or the air (nitrogen).
Give the condition: limited oxygen for carbon monoxide and soot, high temperature for oxides of nitrogen.
Name the product.
Example. Describe how sulfur dioxide and oxides of nitrogen are produced when diesel fuel is burned in a lorry engine.
Show the model answerHide the model answer
The diesel contains sulfur (1), which reacts with oxygen to form sulfur dioxide (1). Nitrogen and oxygen from the air (1) react at the high temperature inside the engine to form oxides of nitrogen (1).
Don’t lose marks
Saying oxides of nitrogen come from nitrogen in petrol or diesel. They form from nitrogen and oxygen in the air at high temperatures.
Saying sulfur dioxide comes from sulfur in the air. The sulfur is in the fuel.
Writing 'carbon' when you mean carbon monoxide, or CO2 when you mean CO.
Forgetting that incomplete combustion still produces water.
Saying incomplete combustion is caused by 'not enough fuel'. It is caused by not enough oxygen.
More tips
Memory tricks
Element → product: C → CO2, H → H2O, S → SO2. Oxides of nitrogen come from the AIR, not the fuel.
Less oxygen, less oxidised carbon: CO2 (plenty of oxygen) → CO (less) → C, soot (even less).
Hot engine → NOx. Sulfur in the fuel → SO2.
Balancing combustion: C, then H, then O, and double everything if you get a half.
Exam technique
For 'describe how X is produced', give the source (fuel or air) AND the condition (limited oxygen or high temperature).
In prediction questions, go through the fuel element by element, then check the conditions.
Write 'a limited supply of oxygen', not 'no oxygen': with no oxygen, nothing burns.
Use correct formulae in equations: CO, CO2, SO2, NO2, H2O.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Name the products of complete combustionA fuel containing carbon and hydrogen forms carbon dioxide and water when it burns in plenty of oxygen.
Grade 4
Name the pollutants released by burning fuelsCarbon dioxide, water vapour, carbon monoxide, sulfur dioxide, oxides of nitrogen and particulates.
Grade 5
Describe how carbon monoxide and soot formIncomplete combustion in a limited supply of oxygen produces carbon monoxide and carbon particles (soot).
Grade 5
Describe how sulfur dioxide formsSulfur in the fuel reacts with oxygen when the fuel burns.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Name the toxic gas produced by the incomplete combustion of fuels.
carbon monoxide
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) Which pollutant is produced when a fuel containing sulfur is burned? Tick (✓) one box.[1]
Carbon monoxide
Oxides of nitrogen
Soot
Sulfur dioxide
(b) Name the toxic gas produced by the incomplete combustion of fuels.[1]
(c) Name the two products of the complete combustion of a hydrocarbon.[2]
Show the answer and mark scheme
(a)Answer: Sulfur dioxide
(b)Answer: carbon monoxide
carbon monoxide
(c)
carbon dioxide
water (vapour)
Question 2Medium6 marks
Petrol and diesel are fuels used in car engines. The exhaust gases contain pollutants.
(a) Explain how oxides of nitrogen are produced in a car engine.[2]
(b) Balance the equation for the formation of nitrogen dioxide. ....N2 + ....O2 → ....NO2[1]
(c) Incomplete combustion of the fuel produces carbon monoxide and soot. Explain why incomplete combustion happens.[1]
(d) Some fuels also produce sulfur dioxide when they burn. Explain why.[2]
Show the answer and mark scheme
(a)
nitrogen and oxygen from the air react
at the high temperature in the engine
(b)Answer: N2 + 2O2 → 2NO2
N2 + 2O2 → 2NO2
(c)
there is not enough / a limited supply of oxygen (for all the carbon to form carbon dioxide)
(d)
the fuel contains sulfur (compounds) as an impurity
the sulfur reacts with / is oxidised by oxygen when the fuel burns