Summary
Alkenes are unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon double bond, making them more reactive than alkanes. They belong to a homologous series with the general formula CnH2n and can undergo addition reactions.
- Alkene — a hydrocarbon with at least one carbon-carbon double bond. Example: Ethene (C2H4) is a simple alkene.
- Unsaturated Hydrocarbon — a hydrocarbon containing double or triple bonds. Example: Alkenes and alkynes are unsaturated hydrocarbons.
- Addition Reaction — a chemical reaction where atoms are added to a molecule without taking any atoms away. Example: Ethene reacts with bromine to form dibromoethane.
Exam Tips
Key Definitions to Remember
- Alkenes are unsaturated hydrocarbons with at least one C=C double bond.
- The general formula for alkenes is CnH2n.
Common Confusions
- Confusing alkenes with alkanes; remember alkenes have double bonds.
- Misunderstanding the term 'unsaturated'; it means having double or triple bonds.
Typical Exam Questions
- What is the general formula for alkenes? CnH2n
- How do alkenes react with bromine water? They decolourise bromine water from orange to colourless.
- What type of reaction occurs when alkenes react with hydrogen? Hydrogenation, forming alkanes.
What Examiners Usually Test
- Understanding of the reactivity of alkenes compared to alkanes.
- Ability to identify and describe addition reactions involving alkenes.