Knowledge Organiser: Alkanes
Key Terms
- Alkane — saturated hydrocarbon, CₙH₂ₙ₊₂
- Saturated — single C-C bonds only
- Homologous series — family with same general formula
- General formula — CₙH₂ₙ₊₂
First Four Alkanes (MEPB)
- Methane — CH₄ (gas)
- Ethane — C₂H₆ (gas)
- Propane — C₃H₈ (gas)
- Butane — C₄H₁₀ (gas)
Properties
- Longer chain = higher bp
- Immiscible with water
- Good fuels (combust in O₂)
- Relatively unreactive
Exam Checklist
- Saturated = single bonds, NOT "full of H"
- Use CₙH₂ₙ₊₂ for formulas
- Carbon always makes 4 bonds
- Alkanes vs alkenes: sat vs unsat
Key Equations
- General formula: CₙH₂ₙ₊₂ (n = number of carbons)
- CH₄ + 2O₂ → CO₂ + 2H₂O (complete combustion of methane)
- C₄H₁₀ + 6½O₂ → 4CO₂ + 5H₂O (complete combustion of butane)
Common Mistakes
- Saying saturated means "full of hydrogen": Saturated means the molecule contains ONLY single C-C bonds — no double bonds
- Using the wrong general formula: Alkanes = CₙH₂ₙ₊₂; alkenes = CₙH₂ₙ — confusing these loses formula marks
- Forgetting carbon makes 4 bonds: Every carbon atom in a structural formula must have exactly 4 bonds — check this before drawing any structure
- Saying alkanes decolourise bromine water: Alkanes do NOT decolourise bromine water — only alkenes do (they have a C=C double bond for addition reactions)
Memory Aid
- Methane — CH₄ (1 carbon)
- Ethane — C₂H₆ (2 carbons)
- Propane — C₃H₈ (3 carbons)
- Butane — C₄H₁₀ (4 carbons)