AQA GCSE Combined Science (8464), Higher tier · Chemistry › Chemistry of the atmosphere › Common atmospheric pollutants and their sources
Practise Atmospheric pollutants from fuels. 12 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.
Grade by grade
What you need to be able to do, from the first marks up to the top 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.
4
Name the pollutants released by burning fuelsCarbon dioxide, water vapour, carbon monoxide, sulfur dioxide, oxides of nitrogen and particulates.
5
Describe how carbon monoxide and soot formIncomplete combustion in a limited supply of oxygen produces carbon monoxide and carbon particles (soot).
5
Describe how sulfur dioxide formsSulfur in the fuel reacts with oxygen when the fuel burns.
6
Describe how oxides of nitrogen formAt the high temperatures in engines, nitrogen and oxygen from the air react together.
7
Write balanced equations for combustionFor example, 2CH4 + 3O2 → 2CO + 4H2O for incomplete combustion of methane.
8
Predict combustion products from fuel informationUse the elements in the fuel and the conditions (oxygen supply, temperature) to predict and explain every product.
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.
Examples: 2CH4 + 3O2 → 2CO + 4H2O and CH4 + O2 → C + 2H2O. grade 7+
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.
Examples: N2 + O2 → 2NO, and then 2NO + O2 → 2NO2. grade 7+
Predicting the products grade 7+
Go element by element through the fuel: C → CO2 (or CO and soot if oxygen is limited); H → H2O; S → SO2.
Then check the conditions: limited air means incomplete combustion; a high temperature means oxides of nitrogen.
A fuel with no carbon, such as hydrogen, forms only water, although oxides of nitrogen can still form from the air at high temperatures.
Cheatsheet
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
2CH4 + 3O2 → 2CO + 4H2O grade 7+
How to answer each type of question
Describe how a pollutant is produced
2 to 4 marks5
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 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).
Explain why incomplete combustion happens
2 marks6
Say the supply of oxygen (air) is limited.
Say that some carbon forms carbon monoxide (or soot) instead of carbon dioxide.
Example. A gas heater is used in a small room with poor ventilation. The heater produces carbon monoxide. Explain why.
Show the model answer
There is a limited supply of oxygen in the room (1), so incomplete combustion occurs and some of the carbon in the fuel forms carbon monoxide instead of carbon dioxide (1).
Write a balanced combustion equation
2 marks7
Write the correct formulae: the fuel and O2 on the left; CO2 or CO (or C) and H2O on the right.
Balance C first, then H, then O last.
If you need half an O2, double every number.
Example. Propane, C3H8, burns in a limited supply of oxygen to form carbon monoxide and water only. Write a balanced equation for this reaction.
Show the model answer
Correct formulae: C3H8 + O2 → CO + H2O (1) Balanced: 2C3H8 + 7O2 → 6CO + 8H2O (1) (C3H8 + 3½O2 → 3CO + 4H2O is also accepted.)
Predict the products of burning a fuel
3 to 6 marks8
List the elements in the fuel and give the product of each.
Use the oxygen supply to decide between complete and incomplete combustion.
Use the temperature to decide whether oxides of nitrogen form.
Explain each product, not just name it.
Example. A fuel oil contains compounds of carbon and hydrogen, and a small amount of sulfur. It is burned in a power station furnace at a very high temperature with a plentiful supply of air. Predict the gases released and explain why each one forms.
Show the model answer
Carbon dioxide, because the carbon reacts completely with the plentiful oxygen (1) Water vapour, because the hydrogen reacts with oxygen (1) Sulfur dioxide, because the sulfur in the fuel reacts with oxygen (1) Oxides of nitrogen, because nitrogen and oxygen from the air react at the very high temperature (1)
Shortcuts and 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.
Where marks are lost
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.
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.
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
Question 3Hard4 marks
Modern petrol car exhaust systems include a catalytic converter, which uses a catalyst to convert some pollutant gases into less harmful ones, including converting carbon monoxide and nitrogen monoxide into carbon dioxide and nitrogen. 2CO + 2NO → 2CO2 + N2
(a) Balance the equation for this reaction if it were written using nitrogen dioxide, NO2, instead. ....CO + ....NO2 → ....CO2 + ....N2[1]
(b) Evaluate the use of catalytic converters as a solution to air pollution from cars.[3]
Show the answer and mark scheme
(a)Answer: 4CO + 2NO2 → 4CO2 + N2
4CO + 2NO2 → 4CO2 + N2
(b)
catalytic converters reduce emissions of carbon monoxide and oxides of nitrogen, so they reduce the associated health and environmental (acid rain) problems
the reaction converts these pollutants into carbon dioxide, so it does not reduce (and may even slightly increase) the car's emissions of carbon dioxide, a greenhouse gas
catalytic converters do not remove particulates or reduce fuel / energy use, so they only partly solve the pollution problem from vehicles