Study Notes
Alkenes are unsaturated hydrocarbons with a carbon-carbon double bond, making them more reactive than alkanes. They can be produced by cracking larger alkanes and undergo addition reactions where the double bond is broken and new atoms are added.
- Alkenes — unsaturated hydrocarbons with a C=C double bond.
Example: Ethene (C2H4) is an alkene. - Cracking — process of breaking down large alkanes into smaller alkanes and alkenes.
Example: Cracking hexane can produce butene and ethane. - Addition Reaction — a reaction where a double bond is broken and atoms are added.
Example: Ethene reacts with bromine to form 1,2-dibromoethane. - Saturated Hydrocarbons — hydrocarbons with only single bonds.
Example: Methane (CH4) is a saturated hydrocarbon. - Unsaturated Hydrocarbons — hydrocarbons with one or more double or triple bonds.
Example: Ethyne (C2H2) is an unsaturated hydrocarbon.
Exam Tips
Key Definitions to Remember
- Alkenes are unsaturated hydrocarbons with a C=C double bond.
- Cracking is the process of breaking down large hydrocarbons into smaller ones.
- Addition reactions involve breaking a double bond and adding new atoms.
Common Confusions
- Confusing saturated with unsaturated hydrocarbons.
- Misunderstanding the color change in bromine water test.
Typical Exam Questions
- What is the general formula for alkenes? CnH2n
- How can you test for unsaturation in hydrocarbons? Use bromine water; it turns colorless if unsaturated.
- What is produced when ethene reacts with hydrogen? Ethane
What Examiners Usually Test
- Understanding of the properties and reactions of alkenes.
- Ability to distinguish between saturated and unsaturated hydrocarbons.
- Knowledge of the process and purpose of cracking.