Balance fuel equations, use their mole ratios and explain how pollution-control reactions reduce particular emissions.
Balance carbon and hydrogen before oxygen
Complete combustion of a hydrocarbon with sufficient oxygen produces carbon dioxide and water. First balance C with CO₂, then H with H₂O; count the oxygen atoms on the product side and divide by two to find the O₂ coefficient. Fractional coefficients are valid unless integer coefficients are requested.
For CₙH₂ₙ₊₂ the complete-combustion products are nCO₂ and (n + 1)H₂O, requiring (3n + 1)/2 molecules of O₂ per molecule of fuel. This expression is for acyclic alkanes. In a flame water is initially vapour; use the states specified by the question, especially for enthalpy data.
Combustion releases energy overall because forming bonds in the products releases more energy than breaking reactant bonds requires. Breaking a bond itself always requires energy.
Incomplete combustion does not have one unique equation
Limited oxygen or poor fuel–air mixing can produce carbon monoxide and solid carbon as well as water. Real exhausts may contain a mixture of CO₂, CO, soot and unburned fuel. Balance the equation for the particular products requested. Incomplete combustion releases less energy per mole of fuel than complete combustion.
Carbon monoxide is toxic because it binds strongly to haemoglobin and reduces oxygen transport. Carbon particulates can damage respiratory health. These are distinct explanations: do not attribute CO poisoning to soot.
Link the pollutant to its source and effect
Nitrogen oxides can form when nitrogen and oxygen from air react at the high temperatures in an engine. Nitrogen does not need to be present in the fuel. Sulfur-containing impurities form SO₂ when oxidised. A pure alkane contains no sulfur.
| Pollutant | Source | Relevant consequence |
|---|---|---|
| CO₂ | Complete combustion of carbon-containing fuels | Enhanced greenhouse effect |
| CO | Incomplete combustion | Toxicity through reduced oxygen transport |
| NOₓ | High-temperature reactions involving air | Respiratory harm, acid deposition and photochemical smog |
| SO₂ | Oxidation of sulfur impurities | Respiratory irritation and acid deposition |
| Unburned hydrocarbons | Fuel escaping complete combustion | Photochemical smog formation |
| Carbon particulates | Incomplete combustion | Respiratory harm; reduced sunlight reaching the surface in polluted air |
What a catalytic converter changes
A three-way catalytic converter helps oxidise CO and unburned hydrocarbons and reduce nitrogen oxides. Platinum, palladium and rhodium are used on a support with a large surface area. Molecules adsorb at active sites, react by an alternative pathway with lower activation energy, and products leave the surface. The catalyst is regenerated.
The following equations illustrate pollutant conversion. In the first, CO is oxidised and NO is reduced. Oxygen can also oxidise CO and residual hydrocarbon fuel. Conversion depends on operating temperature and exhaust composition, so a cold converter is less effective.
Removing SO₂ with basic calcium compounds
Sulfur dioxide is an acidic oxide. Calcium oxide and calcium carbonate neutralise it, so they can remove SO₂ from flue gases. The simplified reactions below produce calcium sulfite. Industrial systems may then oxidise sulfite to sulfate.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Balance the complete combustion of pentane using the smallest integer coefficients.Show answer
C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O. Five carbons give five CO₂; twelve hydrogens give six H₂O; sixteen oxygen atoms require eight O₂.
Q2. Find the mass of CO₂ produced by complete combustion of 2.90 g butane. Use Mᵣ(butane) = 58.0 and Mᵣ(CO₂) = 44.0.Show answer
Amount butane = 2.90 ÷ 58.0 = 0.0500 mol. Amount CO₂ = 4 × 0.0500 = 0.200 mol. Mass CO₂ = 0.200 × 44.0 = 8.80 g. The extra mass comes from oxygen in the air.
Q3. A sulfur-free fuel is burned in air, but NO is detected. Explain its source.Show answer
Nitrogen and oxygen from the air react at the high engine temperature: N₂ + O₂ → 2NO. Removing sulfur from fuel reduces sulfur dioxide, not this nitrogen-oxide formation route.
Q4. Calculate the minimum mass of pure CaCO₃ needed to remove 3.20 kg SO₂ by the sulfite-forming reaction. Use Mᵣ values 100.0 and 64.0.Show answer
SO₂ amount = 3200 ÷ 64.0 = 50.0 mol. The CaCO₃:SO₂ ratio is 1:1, so 50.0 mol CaCO₃ is required. Mass = 50.0 × 100.0 = 5000 g = 5.00 kg. This is a theoretical minimum assuming complete reaction.
Q5. Why does fitting a catalytic converter not make a petrol vehicle carbon-neutral?Show answer
The converter changes some carbon-containing pollutants into CO₂. Burning the fossil fuel still adds CO₂ to the atmosphere; the converter does not provide an equal atmospheric CO₂ removal.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- Chemrevise: Alkanes — Coverage checklist, pp1–5. Original teaching examples and practice.
- AQA 7405 specification — 3.3.2: separation, cracking, combustion and chlorination.
- AQA June 2023 AS Paper 2 mark scheme — Q02.1–02.6 pp13–14: fractions, catalyst, combustion and pollutants.
- AQA June 2023 AS Paper 2 examiner report — Q02 p3 and Q07.1 p4: fuel questions and condenser understanding.
- NASA: Global Warming — Solar heating, outgoing infrared radiation and greenhouse-gas absorption.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.