Study Notes
Alkenes are unsaturated hydrocarbons with one double covalent bond, making them more reactive than alkanes. They have the general formula CnH2n and are produced through cracking, which involves heating hydrocarbons to create smaller molecules like alkenes and hydrogen.
- Alkenes — unsaturated hydrocarbons with a carbon-carbon double bond (C=C).
Example: Ethene (C2H4) is an alkene with a double bond between carbon atoms. - Cracking — a process that breaks down large hydrocarbons into smaller ones, including alkenes.
Example: Heating long-chain alkanes to produce ethene and other smaller alkenes. - Unsaturated Hydrocarbons — contain one or more C=C double bonds and react with bromine water.
Example: Ethene reacts with bromine water, turning it from orange to colorless. - Saturated Hydrocarbons — contain no C=C double bonds and do not react with bromine water.
Example: Methane is a saturated hydrocarbon that does not change bromine water's color. - Addition Reactions — involve the removal of a C=C double bond to form a single bond.
Example: Ethene reacts with bromine to form 1,2-dibromoethane.
Exam Tips
Key Definitions to Remember
- Alkenes are unsaturated hydrocarbons with a C=C double bond.
- Cracking is a process that produces smaller hydrocarbons like alkenes.
Common Confusions
- Confusing saturated and unsaturated hydrocarbons.
- Misunderstanding the reactivity of alkenes compared to alkanes.
Typical Exam Questions
- What is the general formula for alkenes?
CnH2n - How do alkenes react with bromine water?
They decolorize bromine water from orange to colorless. - What is produced when ethene reacts with steam?
Ethanol
What Examiners Usually Test
- Understanding of the general formula and properties of alkenes.
- Ability to describe and predict the products of addition reactions involving alkenes.