GCSE Chemistry Revision — Alkanes & Alkenes
Revise Alkanes & Alkenes for GCSE Chemistry with a topic explanation, worked example and common mistakes. Check the board notes for specification differences.
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What is Alkanes & Alkenes?
Alkanes and alkenes are two important homologous series of hydrocarbons. Alkanes are saturated, meaning they only have single C-C bonds, and are generally unreactive. Alkenes are unsaturated, containing at least one C=C double bond, which makes them more reactive than alkanes. They undergo addition reactions.
Board notes: The chemistry of alkanes and alkenes is a major part of the organic chemistry section for all boards. You must know their structures, general formulas, and characteristic reactions, including the bromine water test. Cracking is also a key industrial process to understand.
Step-by-step explanationWorked examples
Worked example
Ethene (an alkene, C₂H₄) reacts with bromine (Br₂) in an addition reaction. The C=C double bond breaks, and a bromine atom adds to each carbon atom, forming dibromoethane (C₂H₄Br₂). The orange bromine water is decolourised.
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Common mistakes
- 1Confusing the reactions of alkanes and alkenes. Alkanes undergo combustion and substitution reactions (e.g., with halogens in UV light), while alkenes undergo addition reactions at the double bond.
- 2Forgetting the test to distinguish between an alkane and an alkene. Alkenes will decolourise bromine water (turn it from orange to colourless), while alkanes will not.
- 3Drawing the structure of an alkene with the wrong number of hydrogen atoms. Remember that each carbon atom can only form a total of four bonds.
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Frequently asked questions
What is cracking?
Cracking is the process of breaking down large, less useful hydrocarbon molecules (long-chain alkanes) into smaller, more useful ones, including alkanes and alkenes. This is done using high temperatures and a catalyst.
Are alkanes or alkenes more useful?
Both are useful. Alkanes are excellent fuels. Alkenes are more reactive and are used to make polymers (plastics) and other important organic chemicals.