3.2.3Physical Chemistry
Combustion of Alkanes — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 71 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Complete and Incomplete Combustion
Alkanes are used as fuels because they release large amounts of heat when burned in oxygen.
Complete combustion
- Complete combustion A plentiful supply of oxygen gives carbon dioxide and water, for example C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
Incomplete combustion
- Incomplete combustion A limited supply of oxygen gives carbon monoxide or carbon (soot) as well as water.
Carbon monoxide equation
- Carbon monoxide equation CH₄ + 1½O₂ → CO + 2H₂O describes incomplete combustion of methane to form carbon monoxide.
Soot equation
- Soot equation CH₄ + O₂ → C + 2H₂O shows carbon forming as a solid when oxygen is even more limited.
Part 2 of 5Pollutants From Fuels
Pollutants From Fuels
Carbon monoxide
- Carbon monoxide This colourless, odourless gas is dangerous because it reduces the ability of blood to carry oxygen.
Nitrogen oxides
- Nitrogen oxides High temperatures in engines cause N₂ + O₂ → 2NO, and NO can oxidise further to NO₂.
Carbon dioxide
- Carbon dioxide Complete combustion still releases CO₂, a greenhouse gas that contributes to global warming.
| Pollutant | Source | Problem |
|---|---|---|
| CO | Incomplete combustion | Toxic: binds to haemoglobin |
| NOₓ | Nitrogen and oxygen reacting at high engine temperature | Acid rain, smog |
| SO₂ | Sulfur impurities in fuels | Acid rain |
| Unburnt hydrocarbons | Fuel not burned fully | Smog |
| Particulates | Incomplete combustion | Respiratory problems |
Part 3 of 5Reducing the Impact
1
Catalytic converters in car exhausts contain platinum, palladium and rhodium on a ceramic honeycomb surface.
2
They convert harmful gases into less harmful ones, for example 2CO + 2NO → 2CO₂ + N₂.
3
Sulfur can be removed from fuels before burning, and sulfur dioxide can be removed from power-station flue gases.
4
Burning fuel with plenty of air ensures more complete combustion and reduces carbon monoxide and soot.
5
Carbon dioxide emissions can only be reduced by burning less fossil fuel or by using alternative energy sources.
Part 4 of 5Worked Example
Question
- 25 cm³ of ethane burns completely in excess oxygen: 2C₂H₆(g) + 7O₂(g) → 4CO₂(g) + 6H₂O(l).
- All volumes are measured at the same temperature and pressure.
- What volume of oxygen is used and what volume of carbon dioxide forms?
- At constant temperature and pressure, gas volumes are in the same ratio as the moles in the equation.
- The ratio ethaneoxygen is 2 : 7, so oxygen used = 25 × 7/2 = 87.5 cm³.
- The ratio ethanecarbon dioxide is 2 : 4, so CO₂ formed = 25 × 4/2 = 50 cm³.
- Water is a liquid at room temperature, so it does not add to the gas volume.
Answer 87.5 cm³ of O₂ is used and 50 cm³ of CO₂ is produced
Part 5 of 5Exam Checklist & Tips
3.2.3 – Key Points for the Exam
- Complete combustion Burning in excess oxygen, giving carbon dioxide and water.
- Incomplete combustion Burning in limited oxygen, giving carbon monoxide or carbon as well as water.
- Greenhouse gas A gas that absorbs infrared radiation and contributes to global warming.
- Catalytic converter A device in exhaust systems that converts CO, NOₓ and unburnt hydrocarbons into less harmful products.
Common Mistakes
- Writing incomplete combustion equations that produce only CO₂ and H₂O.
- Forgetting to balance oxygen atoms across both the carbon dioxide and water products.
- Saying NOₓ comes from nitrogen in the fuel; in engines it comes from nitrogen and oxygen in air.
- Describing carbon monoxide as an acidic gas that causes acid rain, when it is toxic.
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