13.5 Atmospheric Pollutants

A free GCSE Chemistry lesson for AQA (8462) from OBA School: the pollutants made when fuels burn, their effects on health and the environment, and ways to reduce them. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe how carbon monoxide, soot, sulfur dioxide and oxides of nitrogen are formed when fuels burn
• Predict the products of burning a fuel from its composition and write balanced equations
• Describe the effects of each pollutant, including acid rain and global dimming
• Describe and evaluate ways of reducing atmospheric pollution
• Calculate masses and gas volumes of pollutants from data

Key words

• Complete combustion: Burning in plenty of oxygen. A hydrocarbon gives only carbon dioxide and water.
• Incomplete combustion: Burning in a limited supply of oxygen. Carbon monoxide and/or carbon (soot) form as well as water.
• Particulates: Tiny solid particles, such as soot (carbon), released into the air.
• Acid rain: Rain with a pH lower than normal because sulfur dioxide or oxides of nitrogen have dissolved in it.
• Global dimming: Less sunlight reaching Earth’s surface because particulates reflect it.
• Catalytic converter: A device in a car exhaust that uses a catalyst to change harmful gases into less harmful ones.

What forms when fuels burn

Most fuels, such as petrol, diesel, coal and natural gas, contain carbon and hydrogen, and some contain sulfur. When they burn, the elements combine with oxygen. You can predict the products from what is in the fuel.

• carbon (plenty of oxygen); Product carbon dioxide; Example equation C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
• carbon (limited oxygen); Product carbon monoxide or carbon (soot); Example equation 2CH₄ + 3O₂ → 2CO + 4H₂O CH₄ + O₂ → C + 2H₂O
• hydrogen; Product water vapour; Example equation 2H₂ + O₂ → 2H₂O
• sulfur impurities; Product sulfur dioxide; Example equation S + O₂ → SO₂
• nitrogen from the air (high temperature); Product oxides of nitrogen (NOx); Example equation N₂ + O₂ → 2NO 2NO + O₂ → 2NO₂

Worked example: writing combustion equations with state symbols

Natural gas (methane) burning in a gas cooker flame. At flame temperature, water is a gas (steam).
Plenty of air (complete): CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)
Faulty boiler with too little air (incomplete): 2CH₄(g) + 3O₂(g) → 2CO(g) + 4H₂O(g)
Check: count each type of atom on both sides. In the second equation, C: 2 = 2, H: 8 = 8, O: 6 = 2 + 4.

Answer: Change the products to match the oxygen supply; balance; add state symbols

Key fact: Oxides of nitrogen do not come from nitrogen in the fuel. They form when nitrogen and oxygen from the air react at the very high temperatures inside engines.

The pollutants and their effects

• Carbon monoxide is toxic. It combines with haemoglobin in red blood cells in place of oxygen, so the blood carries less oxygen. A victim becomes drowsy, then unconscious, and can die. It is colourless and odourless, so it is not easily detected. Faulty gas boilers are a risk at home.
• Sulfur dioxide and oxides of nitrogen cause breathing (respiratory) problems. They dissolve in rain water to form acid rain, which damages trees and plants, makes lakes too acidic for fish, and corrodes buildings and statues made of limestone, as well as metal structures such as bridges. Oxides of nitrogen can also trigger asthma attacks.
• Particulates cause global dimming by reflecting sunlight, and can be breathed deep into the lungs, causing health problems (they may even cause cancer). Diesel engines make more particulates than petrol engines because diesel has bigger hydrocarbon molecules, which are less likely to burn completely.
• In busy cities, particulates and nitrogen dioxide (a brown gas) can build up in still weather to form smog, a haze that makes breathing difficult.

Reducing pollution

• Catalytic converters in car exhausts; How it helps convert carbon monoxide and oxides of nitrogen into carbon dioxide and nitrogen: 2CO + 2NO → 2CO₂ + N₂
• Removing sulfur from fuels; How it helps low-sulfur petrol and diesel make much less SO₂
• Flue gas desulfurisation; How it helps power station waste (flue) gases pass through a basic substance such as calcium oxide, calcium hydroxide or limestone, which reacts with the acidic SO₂
• Particulate filters; How it helps diesel vehicles trap soot in the exhaust
• Good air supply, servicing boilers, CO alarms; How it helps prevent incomplete combustion; detect CO at home
• Low-emission zones, public transport, electric vehicles; How it helps fewer exhaust gases in towns

Exam tip: In an “evaluate” question, give advantages and disadvantages. For example, catalytic converters reduce CO and NOx but still release CO₂, and only work well once hot.

Pollution calculations

Worked example: sulfur dioxide from a power station (Higher tier)

A power station burns 200 tonnes of coal containing 2.0% sulfur by mass. S + O₂ → SO₂
mass of sulfur = 2.0 ÷ 100 × 200 = 4.0 tonnes
Mr: S = 32, SO₂ = 32 + 2 × 16 = 64. So 32 t of S makes 64 t of SO₂.
mass of SO₂ = 4.0 × 64 ÷ 32 = 8.0 tonnes

Answer: 8.0 tonnes of SO₂

Worked example: volume of a gas (Higher tier)

What volume does 7.0 g of carbon monoxide occupy at room temperature and pressure?
Mr of CO = 12 + 16 = 28. Moles = 7.0 ÷ 28 = 0.25 mol
volume = 0.25 × 24 = 6.0 dm³

Answer: 6.0 dm³

Common mistakes

• Saying carbon monoxide causes acid rain or global warming. Its danger is that it is toxic.
• Saying oxides of nitrogen come from nitrogen in the fuel. They come from nitrogen in the air, at high temperature.
• Confusing global dimming with global warming. Dimming is caused by particulates reflecting sunlight.
• Writing that catalytic converters remove carbon dioxide. They make CO₂; they remove CO and NOx.

Revision questions

Try these, then check your answers below each one.

Question 1. Why is carbon monoxide particularly dangerous? (Choose from: It is toxic, and colourless and odourless so hard to detect, It causes acid rain, It causes global dimming, It is a strong greenhouse gas.)

Answer: It is toxic, and colourless and odourless so hard to detect

Working: Carbon monoxide is toxic (it reduces the oxygen carried by the blood) and is colourless and odourless, so it is not easily detected.

Question 2. Particulates reflect sunlight. What is the name of this effect?

Answer: global dimming

Working: Particulates cause global dimming; they can also damage the lungs.

Question 3. (Higher tier) What volume, in dm³, does 14 g of carbon monoxide occupy at room temperature and pressure? (Mr CO = 28; 1 mol of gas = 24 dm³)

Answer: 12 dm³

Working: Moles = 14 ÷ 28 = 0.5. Volume = 0.5 × 24 = 12 dm³.

Related lessons

• Previous lesson: 13.3–13.4 Greenhouse Gases and Global Climate Change
• Next lesson: 14.1 Finite and Renewable Resources
• In this chapter: 13.1 History of Our Atmosphere
• In this chapter: 13.2 Our Evolving Atmosphere

Continue in the full course

This lesson is section 13.5 (AGCCH1305) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.