4.2.2Core Organic Chemistry
Reactions of Alkanes — OCR A Level Chemistry A H432 Revision Notes & Topical Questions
5 min read 6 parts Lesson 8 of 29
Part 1 of 6Complete Combustion
Balancing combustion equations is done in the order carbon, then hydrogen, then oxygen.
Conditions
- With plenty of oxygen, an alkane burns completely to form carbon dioxide and water only.
Methane
- CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) releases a large amount of energy.
Propane
- C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l) is the reaction in many portable gas heaters.
Fuel use
- The energy released makes alkanes useful as fuels for heating, cooking and transport.
Part 2 of 6Incomplete Combustion
Incomplete Combustion
Limited oxygen
- If oxygen is restricted, carbon monoxide or carbon is formed instead of carbon dioxide.
Carbon monoxide
- CH₄ + 1½O₂ → CO + 2H₂O; CO is toxic because it binds to haemoglobin and prevents oxygen transport.
Soot
- Carbon particles are produced when there is little oxygen, giving a smoky yellow flame and sooty deposits.
Safety
- Faulty gas appliances and blocked flues can cause carbon monoxide to build up unnoticed since it is colourless and odourless.
Part 3 of 6Pollutants from Fuels
1
Fuels contain small amounts of sulfur, which burns to form sulfur dioxide: S + O₂ → SO₂.
2
In high temperature engines nitrogen and oxygen from the air react to form oxides of nitrogen, NOₓ.
3
Sulfur dioxide and nitrogen oxides dissolve in rainwater, forming acid rain that damages buildings and aquatic life.
4
Unburnt hydrocarbons and NOₓ react in sunlight to form photochemical smog and ground-level ozone.
5
Carbon dioxide from complete combustion is a greenhouse gas that contributes to global warming.
Part 4 of 6Reducing the Impact
Catalytic converter
- A honeycomb coated with platinum, palladium and rhodium converts harmful exhaust gases into less harmful ones.
Converter reaction
- 2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g) removes two pollutants in one step.
Hydrocarbon removal
- Unburnt hydrocarbons are also oxidised to carbon dioxide and water in the converter.
Desulfurisation
- Removing sulfur from fuels or scrubbing flue gases with calcium compounds reduces sulfur dioxide emissions.
| Pollutant | Cause | Effect |
|---|---|---|
| CO | Incomplete combustion | Toxic gas |
| SO₂ | Sulfur impurities in fuel | Acid rain |
| NOₓ | Engines at high temperature | Acid rain and smog |
| Unburnt hydrocarbons | Incomplete combustion | Photochemical smog |
Part 5 of 6Worked Example
Question
- 10 cm³ of propane gas is burned completely in excess oxygen.
- All volumes are measured at the same temperature and pressure.
- Calculate the volume of oxygen used and of carbon dioxide formed.
- Write the equationC₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l).
- Gas volumes at the same temperature and pressure are in the same ratio as the moles, so 1 : 5 : 3.
- Oxygen used = 10 × 5 = 50 cm³.
- Carbon dioxide formed = 10 × 3 = 30 cm³ (the water is liquid so it does not count).
Answer 50 cm³ of O₂ is used and 30 cm³ of CO₂ is formed.
Part 6 of 6Exam Checklist & Tips
4.2.2 – Key Points for the Exam
- Complete combustion Burning in excess oxygen to give carbon dioxide and water.
- Incomplete combustion Burning in limited oxygen to give carbon monoxide or carbon along with water.
- Catalytic converter A device that uses metal catalysts to change pollutant gases into harmless products.
- Acid rain Rain made acidic by dissolved sulfur dioxide and nitrogen oxides.
Common Mistakes
- Writing carbon monoxide as the product of complete combustion.
- Forgetting to balance oxygen last in a combustion equation, leaving halves unresolved or hydrogen unbalanced.
- Saying sulfur dioxide comes from the alkane itself; it comes from sulfur impurities in the fuel.
- Claiming a catalytic converter removes carbon dioxide;
- it produces carbon dioxide while removing carbon monoxide and NOₓ.
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