Ch 3 · Air Quality and Climate · O Level Chemistry 5070
Topic 3 of 3 in Chemistry of the Environment — O Level Chemistry 5070 — 45 lessons.
Introduction
Clean air is a mixture with a fixed composition, but the burning of fossil fuels adds pollutants that damage health, buildings and the climate. Cambridge asks for each pollutant's source and effect as a matched pair, the strategies that reduce them, and, for candidates, the chemistry of catalytic converters and the mechanism of the greenhouse effect. Learn the pollutants as a table and the strategies as cause-and-effect sentences.
Key ideas — Composition of clean, dry air
Clean, dry air is approximately 78% nitrogen, 21% oxygen, and the remaining roughly 1% is a mixture of noble gases (mostly argon, about 0.9%) and carbon dioxide (about 0.04%). "Dry" matters: water vapour varies from place to place and is excluded from the figures.
🧑🏫 Teacher's explanation
Think of the atmosphere like a giant soup. In a perfectly clean, dry soup, you have a huge amount of one ingredient (nitrogen), a significant amount of another (oxygen), and just a tiny pinch of everything else (noble gases and carbon dioxide).
📖 Definitions
🔍 Worked example — Easy
Calculate the volume of oxygen in a 2.0 liter sample of clean, dry air.
🔍 Worked example — Standard
A sample of air contains 0.04\% carbon dioxide. If the total volume is 500 cm^3, calculate the volume of carbon dioxide.
🔍 Worked example — Exam-style
In a 100 mL sample of clean, dry air, find the combined volume of nitrogen and oxygen.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) State the approximate percentage of oxygen in clean, dry air. [1] (b) The remaining 1% of clean, dry air consists of noble gases and carbon dioxide. State the approximate percentage of carbon dioxide in clean, dry air. [1] [2 marks]
Key ideas — Air pollutants: sources and effects
Acid rain forms when sulfur dioxide and nitrogen dioxide dissolve in rainwater to form sulfuric and nitric acids. It kills fish and trees, corrodes limestone buildings and metal structures, and leaches nutrients from soil.
🧑🏫 Teacher's explanation
Think of the Earth's atmosphere like a giant greenhouse. In a real greenhouse, the glass lets sunlight in but traps the heat inside, keeping the plants warm. In our atmosphere, certain gases like CO_2 and CH_4 act like that glass. They allow solar energy to pass through but prevent the heat (thermal energy) from escaping back into space, causing the planet to warm up.
📖 Definitions
🔍 Worked example — Standard
A car engine produces carbon monoxide due to incomplete combustion. If the combustion reaction is 2C_8H_18 + 17O_2 arrow 16CO + 18H_2O, how many moles of CO are produced from 1 mole of octane (C_8H_18)?
🔍 Worked example — Exam-style
A power station uses flue gas desulfurisation to reduce SO_2 emissions. The reaction is CaO + SO_2 arrow CaSO_3. If 100 g of CaO reacts completely with SO_2, what mass of CaSO_3 is produced? (A_r: Ca = 40, O = 16, S = 32)
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) State one adverse effect of sulfur dioxide on the environment. [1] (b) The factory installs a desulfurisation unit using calcium oxide (CaO). Write a balanced chemical equation for the reaction between SO_2 and CaO. [2] (c) Explain how the use of catalytic converters in cars helps reduce pollution. [2] [5 marks]
Key ideas — Strategies to reduce pollution
catalytic converters. Vehicle exhausts pass through a catalytic converter containing a honeycomb coated with platinum, palladium and rhodium. On the hot catalyst surface the pollutants react with each other to form less harmful gases. Carbon monoxide and nitrogen monoxide react to give carbon dioxide and nitrogen: 2CO(g) + 2NO(g) arrow 2CO_2(g) + N_2(g) Nitrogen dioxide is similarly reduced: 2NO_2(g) + 4CO(g) arrow N_2(g) + 4CO_2(g), and unburnt hydrocarbons are oxidised to carbon dioxide and water. Carbon monoxide is oxidised, the oxides of nitrogen are reduced, so the converter carries out a redox reaction. It removes toxic and acid rain gases but still releases carbon dioxide.
🧑🏫 Teacher's explanation
Think of the atmosphere like a giant, delicate balance scale. On one side, we have natural processes like photosynthesis that pull carbon out of the air, and on the other, we have human activities like burning fossil fuels that dump carbon in. When the 'dumping' side becomes too heavy, the scale tips, leading to climate change.
📖 Definitions
🔍 Worked example — Easy
A power station uses flue gas desulfurisation. If the waste gas contains SO_2, calculate the mass of CaO required to react completely with 32 g of SO_2. (Ar: Ca=40, O=16, S=32)
🔍 Worked example — Standard
In a catalytic converter, the reaction 2CO(g) + 2NO(g) arrow 2CO_2(g) + N_2(g) occurs. If 10 cm^3 of CO reacts completely, what volume of N_2 is produced at constant temperature and pressure?
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) State one adverse effect of SO_2 on the environment. [1] (b) The power station uses calcium oxide (CaO) to remove SO_2 from the flue gases. (i) Write a balanced chemical equation for the reaction between SO_2 and CaO. [1] (ii) Identify the type of reaction occurring in (i). [1] (c) Explain how planting trees helps to reduce the impact of climate change. [2] [5 marks]
Key ideas — The greenhouse effect and photosynthesis
how greenhouse gases warm the Earth. Energy from the Sun reaches the Earth mainly as visible light and short-wavelength radiation, which passes through the atmosphere and is absorbed by the Earth's surface. The warm surface then emits thermal energy (infrared radiation) back towards space. Greenhouse gases such as carbon dioxide and methane absorb some of this thermal energy and then re-emit it in all directions, so part of it is reflected back to the surface instead of escaping to space. The more greenhouse gas in the atmosphere, the more thermal energy is trapped and the higher the average temperature of the Earth: this is global warming. Its consequences include melting ice caps, rising sea levels, more extreme weather and changes to where crops can grow.
🧑🏫 Teacher's explanation
Imagine the Earth is like a car parked in the sun on a summer day. The sunlight passes through the glass windows easily, heating up the seats and the dashboard. However, the heat trying to escape from the warm interior gets trapped by the glass, making the inside much hotter than the air outside. This is exactly how the greenhouse effect works on a planetary scale.
📖 Definitions
🔍 Worked example — Easy
A plant undergoes photosynthesis. If it consumes 180 g of CO_2, calculate the mass of glucose produced. (Relative atomic masses: C=12, O=16, H=1)
🔍 Worked example — Standard
In the reaction 6CO_2 + 6H_2O arrow C_6H_12O_6 + 6O_2, how many moles of O_2 are produced from 2 moles of CO_2?
🔍 Worked example — Exam-style
A sample of air contains 0.04\% CO_2 by volume. If the total volume of air is 2.0 dm^3, calculate the volume of CO_2 in cm^3.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. Petrol contains small amounts of sulfur compounds. When petrol burns in a car engine, the exhaust gases include carbon dioxide, carbon monoxide, sulfur dioxide and nitrogen dioxide. (a) Explain how nitrogen dioxide is formed in the engine. [2] (b) State one adverse effect of sulfur dioxide and one way its emission from power stations is reduced. [2] (c) Write an equation for a reaction that removes carbon monoxide and nitrogen monoxide in a catalytic converter and state why the reaction is described as redox. [3]
🔍 Worked example — Standard
A car engine produces nitrogen dioxide (NO_2) due to high temperatures. Write a balanced equation for the formation of nitrogen monoxide (NO) from nitrogen (N_2) and oxygen (O_2) from the air.
🔍 Worked example — Exam-style
(Supplement) In a catalytic converter, carbon monoxide is converted to carbon dioxide. Write the balanced equation for the reaction between CO and O_2 and explain why this is a redox reaction.
⚠️ Common errors
Common mistakes
Practice
Answers 78% nitrogen [B1]; 21% oxygen [B1]; any two of argon (or noble gases) and carbon dioxide [B1]. Carbon monoxide [B1]; it is toxic because it binds to haemoglobin in red blood cells and prevents oxygen being carried [B1]. Calcium oxide is a base [B1]; it neutralises the acidic sulfur dioxide in the flue gases (forming calcium sulfite/sulfate) so less sulfur dioxide is released to form acid rain [B1]. Planting trees [B1], which remove carbon dioxide by photosynthesis [B1]; reducing fossil fuel use, for example by using renewable energy [B1], so less carbon dioxide is produced by combustion [B1]. Thermal energy (infrared) emitted from the Earth's surface [B1] is absorbed by methane molecules [B1] and re-emitted back towards the surface, so less energy escapes to space and the Earth warms [B1].