This guide covers IGCSE Chemistry topic 14 (part 1): organic chemistry – hydrocarbons (alkanes and alkenes). You will revise alkanes (CnH2n+2; saturated; combustion), alkenes (CnH2n; unsaturated; double bond; bromine test), cracking (long chain → short chain + alkene), and crude oil (fractional distillation).

Key concepts to revise

• Alkanes – Saturated; single bonds only; general formula CnH2n+2; burn in excess O2 to CO2 + H2O; used as fuels.
• Alkenes – Unsaturated; at least one C=C double bond; general formula CnH2n; decolourise bromine water (orange to colourless); used to make polymers and other chemicals.
• Cracking – Breaking long-chain alkanes into shorter alkanes + alkenes; catalyst or heat; supplies demand for fuels and alkenes.
• Crude oil – Mixture of hydrocarbons; fractional distillation separates into fractions (refinery gas, petrol, kerosene, diesel, etc.) by boiling point.

Exam tips and command words

• State – The general formula of alkanes and alkenes; what “saturated” and “unsaturated” mean (single bonds only vs double bond).
• Describe – The bromine test (alkene decolourises bromine water; alkane does not); cracking (long chain → shorter alkane + alkene).
• Write – Equation for complete combustion of an alkane (e.g. CH4 + 2O2 → CO2 + 2H2O); equation for cracking (e.g. C10H22 → C8H18 + C2H4).
• Explain – Why cracking is used (demand for shorter-chain fuels and alkenes).

Common mistakes

• Writing alkene formula as CnH2n+2 (that is alkane; alkene is CnH2n).
• Saying alkanes decolourise bromine (alkenes decolourise bromine; alkanes do not).
• Not stating that cracking produces an alkene (needed for polymers) as well as a shorter alkane.

Revision checklist

• State the general formula of alkanes and alkenes and the meaning of saturated/unsaturated.
• Describe the bromine test and which type of hydrocarbon decolourises it.
• Write equations for complete combustion of methane and ethene.
• Describe cracking (reactant, conditions, products) and why it is used.

Next steps

Book a free trial with an IGCSE Chemistry tutor to practise organic hydrocarbons questions, or explore Tutopiya’s learning portal for more revision resources.

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