Maintaining Air Quality links the combustion chemistry of Topic 11 to environmental chemistry. It begins with what clean, dry air contains and how its gases are separated by fractional distillation of liquid air. It then covers the six common air pollutants, acid rain, the carbon cycle and greenhouse gases, and the clean-up method that matches each pollutant.
The pollutant table of name, source and harmful effect is the most-examined part of the topic. Almost every school paper asks for it in exact exam phrasing. Balanced equations with state symbols, such as those for acid rain, flue gas desulfurisation and the catalytic converter, are also tested reliably.
What these notes cover
- •Composition of clean, dry air
- •Fractional distillation of liquid air
- •The six common air pollutants: sources and effects
- •Carbon monoxide, sulfur dioxide and nitrogen oxides
- •Methane, unburned hydrocarbons and ground-level ozone
- •How acid rain forms: the sulfur and nitrogen chains
- •Damage caused by acid rain and liming
- •The carbon cycle
- •Greenhouse gases and global warming
- •Catalytic converters and flue gas desulfurisation
- •Reducing the carbon footprint
- •Common mistakes and exam technique
Key definitions
- Clean, dry air
- A mixture of gases that are not chemically bonded to one another, with water vapour excluded. By volume it is about 78% N₂, 21% O₂, 1% noble gases (mainly argon) and 0.04% CO₂.
- Fractional distillation of liquid air
- The physical separation of the gases in liquid air, which is possible because they have different boiling points. Nitrogen (−196 °C) boils off first, then argon (−186 °C), and oxygen (−183 °C) is collected last.
- Acid rain
- Rain with a pH below 5. It is caused by sulfur dioxide and nitrogen dioxide reacting with oxygen and water in the atmosphere to form strong acids. Unpolluted rain is only slightly acidic, at pH about 5.6.
- Carbon cycle
- The set of natural processes that moves carbon between the atmosphere, living things and fossil fuels. Photosynthesis removes carbon dioxide, while respiration, decomposition and combustion return it.
- Global warming
- A gradual rise in the average temperature of the Earth’s atmosphere. Higher concentrations of greenhouse gases such as carbon dioxide and methane trap more heat, which may contribute to it.
Key formulas
Formation of nitrogen oxides
N₂(g) + O₂(g) → 2NO(g), then 2NO(g) + O₂(g) → 2NO₂(g)
The first step needs very high temperatures, such as those inside car engines or in lightning, because nitrogen’s triple bond makes it very unreactive. The second step happens in air without high temperature.
Formation of acid rain
2SO₂(g) + O₂(g) + 2H₂O(l) → 2H₂SO₄(aq) and 4NO₂(g) + O₂(g) + 2H₂O(l) → 4HNO₃(aq)
Both follow the pattern acidic oxide + oxygen + water → strong acid. The coefficients are 2, 1, 2 → 2 for sulfur and 4, 1, 2 → 4 for nitrogen.
Acid rain attacking limestone or marble
CaCO₃(s) + H₂SO₄(aq) → CaSO₄(aq) + H₂O(l) + CO₂(g)
Acid + carbonate → salt + water + carbon dioxide. The ionic equation is CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g), with sulfate as the spectator ion.
Photosynthesis
6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g)
This is the only major process that removes CO₂ from the air. Respiration, C₆H₁₂O₆(aq) + 6O₂(g) → 6CO₂(g) + 6H₂O(l), is its exact reverse and returns CO₂.
Catalytic converter
2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g)
This takes place over a platinum and rhodium catalyst in a car’s exhaust. Poisonous CO becomes CO₂, and NO becomes unreactive N₂.
Flue gas desulfurisation
CaO(s) + SO₂(g) → CaSO₃(s) or CaCO₃(s) + SO₂(g) → CaSO₃(s) + CO₂(g)
Power-station waste gases are sprayed with a slurry of calcium oxide or calcium carbonate. This traps the acidic SO₂ as solid calcium sulfite, CaSO₃, not calcium sulfate.
Worked example
In the reaction 2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g), identify the substance oxidised, the substance reduced, the oxidising agent and the reducing agent. Explain using oxidation states.
- 1In CO, oxygen is −2, so carbon must be +2 because the compound is neutral; in CO₂, two oxygens contribute −4, so carbon is +4.
- 2Carbon goes from +2 → +4, an increase in oxidation state, so CO is oxidised.
- 3In NO, nitrogen is +2; in N₂, nitrogen is an uncombined element, so its oxidation state is 0.
- 4Nitrogen goes from +2 → 0, a decrease in oxidation state, so NO is reduced.
- 5The oxidising agent causes oxidation and is itself reduced, so it is NO; the reducing agent causes reduction and is itself oxidised, so it is CO.
- 6Since one substance is oxidised while another is reduced in the same reaction, this is a redox reaction.
Watch out: A catalytic converter does not remove sulfur dioxide or carbon dioxide; in fact it produces CO₂. If a question asks how to reduce SO₂ or CO₂ emissions, “fit a catalytic converter” scores zero.
Common mistakes
Quoting 79% for nitrogen
Wrong: Air contains 79% nitrogen.
Right: Nitrogen alone is 78%. The 79% figure is everything that is not oxygen (nitrogen + argon + carbon dioxide), and the missing 1% is mostly argon.
Blaming carbon dioxide for acid rain
Wrong: CO₂ causes acid rain, so any rain below pH 7 is acid rain.
Right: Dissolved CO₂ forms weak carbonic acid and makes rain only slightly acidic (pH ≈ 5.6). Acid rain (pH below 5) is caused by sulfur dioxide and nitrogen dioxide, so name the specific gases.
Linking global warming to the ozone hole
Wrong: The hole in the ozone layer lets more heat in, causing global warming.
Right: Global warming is caused by greenhouse gases such as carbon dioxide and methane trapping heat. Ozone depletion is a separate problem: CFCs destroy the ozone layer, so more ultraviolet radiation reaches the Earth.
Questions students ask
What is the composition of clean dry air by volume?
Clean, dry air is about 78% nitrogen, 21% oxygen, 1% noble gases (mainly argon) and 0.04% carbon dioxide. The figures are for dry air because water vapour varies from day to day and place to place. If asked to name the third most abundant gas, answer argon, not “noble gases”.
Why is carbon monoxide dangerous to humans?
Carbon monoxide binds irreversibly with haemoglobin in red blood cells, reducing the blood’s ability to transport oxygen around the body. This causes breathing difficulties, fatigue and, at high concentrations, death. It is colourless and odourless, so victims cannot detect it.
How does a catalytic converter reduce air pollution?
Exhaust gases pass over a platinum and rhodium catalyst on a honeycomb ceramic support, which gives a large surface area. Carbon monoxide and nitrogen monoxide react to form carbon dioxide and nitrogen: 2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g), and unburnt hydrocarbons are oxidised to carbon dioxide and water. It does not remove sulfur dioxide or carbon dioxide.
Why are biofuels described as carbon-neutral?
Burning ethanol still produces carbon dioxide: C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l). However, that carbon dioxide was absorbed by the sugar cane crop during photosynthesis only a few months earlier, so there is little net addition to the atmosphere. Write “closer to carbon-neutral”, not “produces no carbon dioxide”.
In the full PDF
- •Diagrams: composition of clean, dry air, the fractional distillation of liquid air flowchart and the flue gas desulfurisation chamber
- •The full summary table of all six pollutants with their sources and harmful effects in exam phrasing
- •Further worked examples: air composition in a syringe, measuring oxygen with heated copper, a marble statue and its ionic equation, liming a lake, a CO₂ data-response question, biofuels and hydrogen fuel cells
- •Five misconceptions that cost marks, each with an exam-ready statement
- •Exam technique on state symbols, the haemoglobin sentence, conditions for nitrogen oxides and the source-and-effect checklist
- •The remaining common slips and an end-of-section summary for every sub-topic