What you'll learn
This revision guide covers the major pollutants produced during combustion of fuels and their environmental and health impacts. You'll learn how these pollutants form, why they're harmful, and the chemical principles behind pollution problems like acid rain and climate change. This topic is crucial for Paper 2 and frequently appears in 6-mark extended response questions.
Key terms and definitions
Combustion — a chemical reaction in which a fuel reacts rapidly with oxygen, releasing energy as heat and light
Complete combustion — burning in excess oxygen to produce carbon dioxide and water only
Incomplete combustion — burning in limited oxygen to produce carbon monoxide and/or carbon (soot) alongside water
Pollutant — a harmful substance released into the environment
Particulates — small solid particles of unburnt carbon and other materials produced during incomplete combustion
Acid rain — rainfall made acidic (pH below 5.6) by dissolved pollutants such as sulfur dioxide and nitrogen oxides
Greenhouse gas — a gas in the atmosphere that absorbs infrared radiation and contributes to global warming
Catalytic converter — a device fitted to vehicle exhausts that converts harmful gases into less harmful ones using transition metal catalysts
Core concepts
Products of complete combustion
When hydrocarbons burn in plenty of oxygen, complete combustion occurs. The only products are carbon dioxide and water.
The general word equation is: hydrocarbon + oxygen → carbon dioxide + water
For example, methane combustion: methane + oxygen → carbon dioxide + water
CH₄ + 2O₂ → CO₂ + 2H₂O
Complete combustion releases the maximum amount of energy from a fuel. Carbon dioxide is produced, which is a greenhouse gas but not toxic to humans at normal atmospheric concentrations.
Products of incomplete combustion
When there is insufficient oxygen, incomplete combustion occurs. This produces less energy and creates harmful pollutants.
Two main pollutants form:
Carbon monoxide (CO) — a toxic, colourless, odourless gas
- Forms when oxygen supply is limited
- Binds irreversibly to haemoglobin in red blood cells
- Prevents oxygen transport around the body
- Can cause death by suffocation
- Particularly dangerous in enclosed spaces with poor ventilation
Particulates — tiny solid particles of carbon (soot) and other materials
- Cause respiratory problems when inhaled
- Damage lung tissue
- Can trigger asthma attacks
- Contribute to global dimming by reflecting sunlight
- Create visible smoke pollution
Example word equation for incomplete combustion: methane + oxygen → carbon monoxide + water
2CH₄ + 3O₂ → 2CO + 4H₂O
With even less oxygen, carbon (soot) forms: methane + oxygen → carbon + water
CH₄ + O₂ → C + 2H₂O
Sulfur dioxide formation and effects
Sulfur dioxide (SO₂) forms when fuels containing sulfur impurities burn.
sulfur + oxygen → sulfur dioxide
S + O₂ → SO₂
Many fossil fuels, especially coal and crude oil fractions, contain sulfur compounds. When burned in power stations, vehicle engines, or industrial processes, sulfur dioxide is released.
Effects of sulfur dioxide:
- Causes respiratory problems in humans
- Irritates eyes, nose and throat
- Worsens asthma and bronchitis
- Dissolves in atmospheric water to form sulfurous acid
- Reacts further with oxygen to form sulfuric acid
- Major contributor to acid rain
Nitrogen oxides formation and effects
Nitrogen oxides (NOₓ, mainly NO and NO₂) form during combustion at high temperatures.
At normal temperatures, nitrogen and oxygen in the air don't react. However, the high temperatures inside vehicle engines and furnaces provide enough energy for the reaction:
nitrogen + oxygen → nitrogen monoxide
N₂ + O₂ → 2NO
Nitrogen monoxide then reacts with more oxygen in the atmosphere:
nitrogen monoxide + oxygen → nitrogen dioxide
2NO + O₂ → 2NO₂
Effects of nitrogen oxides:
- Nitrogen dioxide is a brown toxic gas
- Causes respiratory problems
- Irritates lungs and reduces resistance to infections
- Dissolves in atmospheric moisture to form nitric acid
- Contributes significantly to acid rain
- Involved in photochemical smog formation
- Can cause acid rain to form far from emission sources
Acid rain formation and impacts
Acid rain forms when sulfur dioxide and nitrogen oxides dissolve in atmospheric water droplets.
Formation reactions:
Sulfur dioxide forms sulfurous acid: SO₂ + H₂O → H₂SO₃
This oxidises to sulfuric acid: H₂SO₃ + ½O₂ → H₂SO₄
Nitrogen dioxide forms nitric acid: 2NO₂ + H₂O + ½O₂ → 2HNO₃
Environmental effects of acid rain:
- Damages and kills trees by washing away essential nutrients from soil
- Acidifies lakes and rivers, killing aquatic life
- Fish and invertebrates cannot survive in acidic water (pH < 5)
- Corrodes limestone buildings and metal structures
- Damages stonework on historic buildings and monuments
- Harms crops and reduces agricultural yields
- Releases toxic metal ions from soil (e.g., aluminium)
Regional impact:
Acid rain can be transported hundreds of kilometres from its source. Industrial areas in one country can cause acid rain in neighbouring countries. This was a major problem in Scandinavia from UK and German emissions in the 1970s-1980s.
Methods to reduce pollution
Several strategies reduce combustion pollutants:
Reducing sulfur dioxide emissions:
- Remove sulfur from fuels before burning (desulfurization)
- Use low-sulfur or sulfur-free fuels
- Use flue gas desulfurization in power stations (limestone slurry sprays)
- Switch to renewable energy sources
Reducing nitrogen oxides:
- Catalytic converters in vehicles convert nitrogen oxides to nitrogen gas
- Lower combustion temperatures where possible
- Use lean-burn engines
- Recirculate exhaust gases to reduce combustion temperature
Reducing carbon monoxide and particulates:
- Ensure adequate oxygen supply during combustion
- Catalytic converters oxidise carbon monoxide to carbon dioxide
- Diesel particulate filters trap soot in vehicle exhausts
- Regular engine maintenance for efficient combustion
Catalytic converter reactions:
2CO + 2NO → 2CO₂ + N₂ (carbon monoxide and nitrogen monoxide removed)
The catalyst (platinum, palladium or rhodium) speeds up the reaction without being used up.
Worked examples
Example 1: Identifying combustion products (4 marks)
Question: A Bunsen burner burns natural gas (mainly methane, CH₄). Describe the products formed when the air hole is: (a) fully open [2 marks] (b) closed [2 marks]
Model answer:
(a) Complete combustion occurs [1 mark] Products are carbon dioxide and water (vapour) [1 mark]
(b) Incomplete combustion occurs [1 mark] Products are carbon monoxide and/or carbon (soot) and water [1 mark]
Mark scheme notes:
- Must specify both products for full marks
- Carbon and carbon monoxide can both form in incomplete combustion
- Don't forget water is always produced
Example 2: Explaining acid rain formation (6 marks)
Question: Explain how burning coal in a power station can lead to acid rain falling hundreds of kilometres away. Include equations in your answer.
Model answer:
Coal contains sulfur impurities [1 mark]
When coal burns, sulfur reacts with oxygen: S + O₂ → SO₂ [1 mark]
Sulfur dioxide dissolves in atmospheric water droplets [1 mark]
Forms sulfuric acid: SO₂ + H₂O + ½O₂ → H₂SO₄ (or equivalent two-step reaction) [1 mark]
Wind carries the acidic droplets away from the power station [1 mark]
Acid rain falls when these droplets precipitate [1 mark]
Mark scheme notes:
- Equations must be balanced for full credit
- Must show progression from fuel → pollutant → acid → precipitation
- Reference to transport mechanism essential for full marks
Example 3: Evaluating pollution reduction methods (4 marks)
Question: Catalytic converters are fitted to petrol cars to reduce pollution. Describe how they work and give one limitation of this technology.
Model answer:
Catalytic converters contain transition metal catalysts (platinum/palladium/rhodium) [1 mark]
They convert harmful gases into less harmful ones:
- Carbon monoxide → carbon dioxide
- Nitrogen oxides → nitrogen gas [1 mark]
Example reaction: 2CO + 2NO → 2CO₂ + N₂ [1 mark]
Limitation: They only work effectively when hot / don't reduce CO₂ emissions / expensive to manufacture / contain rare metals / can be stolen [1 mark]
Mark scheme notes:
- Any valid limitation accepted
- Must explain the conversion process, not just state "removes harmful gases"
- Chemical symbols or names both acceptable
Common mistakes and how to avoid them
Confusing complete and incomplete combustion products — Remember: complete combustion needs excess oxygen and produces only CO₂ and H₂O. Incomplete combustion (limited oxygen) produces CO and/or C plus H₂O. Never write CO₂ as a product of incomplete combustion.
Forgetting water is always a product — Both complete and incomplete combustion of hydrocarbons always produce water. Many students only write carbon dioxide or carbon monoxide in their answers.
Mixing up pollution effects — Carbon monoxide causes death by preventing oxygen transport (not acid rain). Sulfur dioxide and nitrogen oxides cause acid rain (not direct suffocation). Particulates cause respiratory problems (not acid rain directly).
Writing vague statements about acid rain — Don't just write "acid rain damages the environment." Be specific: it kills aquatic life, damages trees, corrodes buildings. Use precise pH values where relevant (normal rain ~pH 5.6, acid rain below pH 5).
Stating nitrogen oxides come from fuel — Nitrogen oxides form from nitrogen in the air reacting at high temperatures, not from nitrogen compounds in the fuel (unlike sulfur dioxide which does come from sulfur in fuel).
Forgetting catalytic converters don't reduce CO₂ — Catalytic converters convert CO to CO₂, NO to N₂, but they don't reduce carbon dioxide emissions. They may even increase CO₂ slightly by oxidizing CO.
Exam technique for "Chemistry of the atmosphere: common pollutants from combustion and their effects"
Use correct chemical terminology consistently — Distinguish clearly between carbon monoxide (CO), carbon dioxide (CO₂) and carbon (C). Examiners deduct marks for vague terms like "carbon gases" or "harmful fumes" when specific pollutants are required.
For "Explain" questions worth 4-6 marks, structure answers in logical steps: source of pollutant → how it forms → how it causes damage → specific examples. Include equations where appropriate as they often earn marks even if the written explanation is incomplete.
Learn the differences between pollutant sources — Questions often ask you to compare different fuels or combustion conditions. Know that: coal has high sulfur content, vehicle engines produce NOₓ due to high temperatures, incomplete combustion occurs with insufficient oxygen.
Practice balancing combustion equations — Both complete and incomplete combustion equations appear regularly. Start by balancing carbon atoms, then hydrogen, then oxygen. Remember that incomplete combustion equations may have multiple valid answers depending on how limited the oxygen supply is.
Quick revision summary
Combustion produces various pollutants depending on oxygen availability and fuel composition. Complete combustion yields carbon dioxide and water; incomplete combustion produces toxic carbon monoxide and particulates. Sulfur dioxide forms from sulfur impurities in fuels, while nitrogen oxides form from atmospheric nitrogen at high temperatures. Both sulfur dioxide and nitrogen oxides dissolve in atmospheric moisture to create acid rain, which damages ecosystems, buildings and human health. Catalytic converters, fuel treatment and pollution controls reduce emissions but cannot eliminate all harmful products.