AQA A Level Chemistry 3.3.2 Alkanes Exam Questions

About AQA A Level Chemistry 3.3.2 Alkanes

Alkanes are saturated hydrocarbons and the main fuels obtained from crude oil. On the AQA A Level course you learn how fractional distillation separates crude oil, why and how thermal and catalytic cracking are used, the products of complete and incomplete combustion, the pollutants formed in engines and how catalytic converters and flue gas desulfurisation reduce them. You also study the free-radical substitution of alkanes by chlorine, and how chlorine radicals from CFCs catalyse the decomposition of ozone. Examiners test balanced cracking and combustion equations, full free-radical mechanisms, and environmental explanations involving named pollutants.

What you need to know

• Fractional distillation Crude oil is heated and vaporised, then fractions condense at different heights in a column with a temperature gradient. Fractions with longer chains have higher boiling points because of stronger van der Waals forces.
• Cracking Cracking breaks C-C bonds in long alkanes to give shorter, more useful alkanes and alkenes. Thermal cracking uses high temperature and high pressure and gives a high proportion of alkenes. Catalytic cracking uses a zeolite catalyst, slight pressure and high temperature and produces motor fuels and aromatic hydrocarbons.
• Combustion Complete combustion gives CO2 and H2O. Incomplete combustion gives CO (toxic) or carbon particulates. Unburnt hydrocarbons and nitrogen oxides also form in engines, where the high temperature allows N2 and O2 from the air to react.
• Reducing pollution Catalytic converters use platinum, palladium or rhodium to remove CO, NO and unburnt hydrocarbons, e.g. 2CO + 2NO → 2CO2 + N2. SO2 from burning sulfur-containing fuels is removed from flue gases with calcium oxide or calcium carbonate.
• Free-radical substitution Initiation: Cl2 → 2Cl• in UV light. Propagation: Cl• + CH4 → •CH3 + HCl, then •CH3 + Cl2 → CH3Cl + Cl•. Termination: two radicals combine, e.g. •CH3 + Cl• → CH3Cl. Further substitution gives a mixture of products.
• Ozone depletion UV light breaks C-Cl bonds in CFCs to form Cl•. Cl• + O3 → ClO• + O2, then ClO• + O3 → 2O2 + Cl•. The chlorine radical is regenerated, so one radical destroys many ozone molecules.

Common mistakes in 3.3.2 Alkanes

• Saying nitrogen oxides come from nitrogen in the fuel. They form when N2 and O2 from the air react at the high temperature in the engine.
• Writing propagation steps that do not regenerate a radical, or including Cl• + CH4 → CH3Cl + H•, which is not the accepted step.
• Forgetting UV light as the condition for initiation.
• Mixing up the conditions and products of thermal and catalytic cracking.
• Writing cracking equations that do not balance for hydrogen. Check atoms on both sides.

Try three 3.3.2 Alkanes exam-style questions