OxfordAQA International A Level Chemistry
OxfordAQA International A Level Chemistry 3.3.2 Alkanes Exam Questions
Free OxfordAQA International A Level Chemistry revision for 3.3.2 Alkanes: what you need to know, common mistakes and exam-style questions with answers. Covers fractional distillation, thermal and catalytic cracking, combustion pollutants, catalytic converters and free-radical chlorination of methane.
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About OxfordAQA International A Level Chemistry 3.3.2 Alkanes
Alkanes is a Unit 2 organic topic about the main fuels and the chemistry of saturated hydrocarbons. You need to explain how crude oil is separated by fractional distillation, compare thermal and catalytic cracking, describe complete and incomplete combustion, and explain the formation and removal of pollutants including nitrogen oxides, carbon monoxide, carbon particles and unburnt hydrocarbons. You also need to know how catalytic converters work and how sulfur dioxide is removed using calcium oxide or calcium carbonate. Finally you need the free-radical substitution mechanism for the reaction of methane with chlorine. It connects to the introduction to organic chemistry, energetics and the environmental chemistry in organic analysis.
What you need to know
- Fractional distillation Crude oil is heated and vaporised, then separated in a column with a temperature gradient. Fractions with longer chains have higher boiling points and condense lower down.
- Cracking Thermal cracking uses high temperature and high pressure and produces a high proportion of alkenes. Catalytic cracking uses a zeolite catalyst at slight pressure and high temperature and produces motor fuels and aromatic hydrocarbons.
- Combustion Complete combustion gives carbon dioxide and water. Incomplete combustion produces carbon monoxide, which is toxic, and carbon particles.
- Pollutants Nitrogen oxides form when nitrogen and oxygen react at high temperature in engines. Catalytic converters use platinum, palladium or rhodium to convert CO, NOx and unburnt hydrocarbons into CO2, N2 and H2O.
- Removing sulfur dioxide SO2 from burning fuels causes acid rain. It is removed from flue gases by reacting it with calcium oxide or calcium carbonate.
- Free-radical substitution Initiation: Cl2 → 2Cl• in UV light. Propagation: Cl• + CH4 → HCl + •CH3, then •CH3 + Cl2 → CH3Cl + Cl•. Termination: two radicals combine, for example •CH3 + Cl• → CH3Cl. Further substitution gives a mixture of products.
Common mistakes in 3.3.2 Alkanes
- Saying fractions are separated by different melting points. They are separated by boiling points.
- Writing curly arrows in free-radical mechanisms. This specification uses dot notation without curly arrows for free-radical reactions.
- Saying nitrogen oxides come from nitrogen in the fuel. They form from atmospheric nitrogen and oxygen at high temperatures.
- Forgetting UV light as the condition for initiation.
Try three 3.3.2 Alkanes exam-style questions
1. Write the two propagation steps for the formation of chloromethane from methane. (2 marks)
Show the mark scheme answer
Cl• + CH4 → HCl + •CH3; •CH3 + Cl2 → CH3Cl + Cl•.
2. Write an equation for the reaction in a catalytic converter that removes CO and NO. (1 mark)
Show the mark scheme answer
2CO + 2NO → 2CO2 + N2.
3. Write an equation for the cracking of decane, C10H22, to form octane and ethene. (1 mark)
Show the mark scheme answer
C10H22 → C8H18 + C2H4.
3.3.2 Alkanes FAQs
What does OxfordAQA International A Level Chemistry 3.3.2 Alkanes cover?
It covers fractional distillation, thermal and catalytic cracking, combustion pollutants, catalytic converters and free-radical chlorination of methane.
Which unit is 3.3.2 Alkanes examined in?
It is examined in Unit 2, the second International AS paper, which covers organic and physical chemistry. The Unit 5 practical and synoptic paper at A2 can also assess it.
What is the difference between thermal and catalytic cracking?
Thermal cracking uses high temperatures and high pressures and gives mainly alkenes. Catalytic cracking uses a zeolite catalyst at a lower pressure and gives motor fuels and aromatic hydrocarbons.
