GCSE Chemistry Organic Chemistry Notes
Organic chemistry is the chemistry of carbon compounds. At GCSE the focus is crude oil, hydrocarbons and simple reactions of alkanes and alkenes.
Key concepts
- Hydrocarbon
- A compound containing only hydrogen and carbon.
- Alkane
- Saturated hydrocarbon with only single bonds. General formula CₙH₂ₙ₊₂ (e.g. methane, ethane, propane, butane).
- Alkene
- Unsaturated hydrocarbon with a C=C double bond. General formula CₙH₂ₙ.
- Cracking
- Breaking long-chain alkanes into shorter alkanes and alkenes using heat and a catalyst.
- Crude oil is separated by fractional distillation, exploiting differences in boiling points.
- Shorter fractions are more volatile, more flammable, less viscous and easier to ignite.
- Complete combustion of a hydrocarbon → CO₂ + H₂O; incomplete → CO / C (soot) + H₂O.
- Alkenes decolourise bromine water; alkanes do not.
Worked example
Write a balanced equation for the complete combustion of methane (CH₄).
- 1Products: carbon dioxide + water.
- 2Skeleton: CH₄ + O₂ → CO₂ + H₂O.
- 3Balance H: 2 H₂O.
- 4Balance O: 2 O₂. → CH₄ + 2O₂ → CO₂ + 2H₂O.
Answer: CH₄ + 2O₂ → CO₂ + 2H₂O
Common mistakes
✗ Confusing alkane and alkene formulas.
✓ Alkanes CₙH₂ₙ₊₂ (all single bonds); alkenes CₙH₂ₙ (one double bond).
✗ Writing water as OH₂ or leaving formulas unbalanced.
✓ Use H₂O and balance atoms on each side.
✗ Saying cracking produces only alkanes.
✓ Cracking always produces at least one alkene.
Quick check
Try these, then reveal the answers.
General formula for alkanes?Reveal answer
CₙH₂ₙ₊₂
Test to distinguish an alkene from an alkane?Reveal answer
Bromine water — alkene decolourises it; alkane does not.
Products of complete combustion of a hydrocarbon?Reveal answer
Carbon dioxide and water.
Exam tips
- Learn the first four alkanes and alkenes by name and formula.
- For fractional distillation, link separation to boiling point differences.
- Explain trends (viscosity, flammability, boiling point) using intermolecular forces and chain length.
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Frequently asked questions
Why are alkenes more reactive than alkanes?+
The C=C double bond in alkenes can open up and react (e.g. with bromine water), whereas alkanes only have unreactive single bonds.
Why is cracking useful?+
It converts less-useful long-chain alkanes into shorter, more valuable products such as petrol and ethene for polymers.
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