Alkanes
A-level Chemistry (7405) · exam-style practice, examiner-report intelligence and the tools that drill it.
The topic on one screen
- Alkanes are saturated hydrocarbons, general formula CnH(2n+2) — check every formula you write fits this before you commit to it.
- Fractional distillation separates crude oil by boiling point; longer chains have stronger van der Waals forces, so they condense higher up (lower down the column).
- Cracking breaks long alkanes into shorter alkanes + alkenes — supply-and-demand economics, and the alkenes feed polymer manufacture.
- Complete combustion gives CO2 + H2O; incomplete combustion gives CO (toxic) and C (soot/particulates). Sulfur impurities burn to SO2 → acid rain.
- Free-radical substitution (initiation → propagation → termination) needs UV light; the propagation steps regenerate the radical.
- CFCs release Cl radicals in the upper atmosphere; Cl catalyses ozone breakdown and is regenerated, so one atom destroys many O3 molecules.
Where students actually lose marks
Students balanced a combustion equation by counting atoms and wrote a formula that isn't a real alkane (e.g. C6H18). Always sanity-check that a hydrocarbon formula fits CnH(2n+2) before you write it down.
June 2024 Paper 2 examiner report (Q03.1)
The role of chlorine radicals in the decomposition of ozone was reported as 'surprisingly poorly answered'. You need both propagation steps and the point that Cl is regenerated — that is what makes it a catalyst.
June 2022 Paper 2 examiner report (Q02.5)
In free-radical questions, ultraviolet light as a necessary condition is 'often incorrect or omitted'. Conditions score marks — never leave 'UV light' off a radical step.
June 2022 Paper 2 examiner report (Q10.3)
Try it — exam-style
Write the equation for the complete combustion of propane (C3H8).
A car engine burns octane in a limited supply of air. Name two carbon-containing products, other than CO2, and give one problem each causes.
Chlorine reacts with methane in UV light. Write equations for the initiation step and the two propagation steps.
In the upper atmosphere, chlorine radicals break down ozone. Write the two propagation steps and explain why Cl* is acting as a catalyst.
Explain why the boiling point of straight-chain alkanes increases as the number of carbon atoms increases.
Questions are written in the style of past AQA papers (source shown on each) — never copied from them.
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