Study Notes
Alkanes are saturated hydrocarbons with single bonds, known for their low reactivity and use as fuels. They can be produced through hydrogenation of alkenes or by cracking longer chain hydrocarbons. Alkanes undergo combustion, which can be complete or incomplete, and can participate in free radical substitution reactions.
- Hydrogenation — a reaction where hydrogen gas is added to an alkene to form an alkane. Example: Ethene + H₂ → Ethane
- Cracking — breaking down long-chain hydrocarbons into shorter ones using heat and a catalyst. Example: C₈H₁₈ → C₃H₈ + C₄H₁₀
- Complete Combustion — a reaction where an alkane burns in oxygen to produce carbon dioxide and water. Example: CH₄ + 2O₂ → CO₂ + 2H₂O
- Incomplete Combustion — occurs when there is insufficient oxygen, producing carbon monoxide and/or carbon. Example: CH₄ + O₂ → CO + 2H₂O + C
- Free Radical Substitution — a reaction where a free radical replaces a hydrogen atom in an alkane. Example: C₂H₆ + Cl₂ → C₂H₅Cl + HCl
Exam Tips
Key Definitions to Remember
- Hydrogenation
- Cracking
- Complete Combustion
- Incomplete Combustion
- Free Radical Substitution
Common Confusions
- Confusing complete and incomplete combustion products
- Misunderstanding the role of catalysts in cracking
Typical Exam Questions
- What is the product of hydrogenation of ethene? Ethane
- How does cracking differ from hydrogenation? Cracking breaks down larger molecules; hydrogenation adds hydrogen to unsaturated molecules.
- What are the products of complete combustion of methane? Carbon dioxide and water
What Examiners Usually Test
- Understanding of reaction conditions for hydrogenation and cracking
- Ability to write balanced equations for combustion reactions
- Knowledge of the environmental impact of combustion products