Knowledge Organiser: Combustion of Hydrocarbons
Key Terms
- Complete combustion — excess O₂, produces CO₂ + H₂O
- Incomplete combustion — limited O₂, produces CO and/or C
- Carbon monoxide — toxic, colourless, odourless
Complete vs Incomplete
- Complete: blue flame, CO₂ + H₂O, max energy
- Incomplete: yellow flame, CO/C, less energy
- CO binds haemoglobin → prevents O₂ transport
Balancing Equations
- Balance C first → CO₂ count
- Balance H next → H₂O count
- Balance O last → O₂ count
- Check all atoms on both sides
Exam Checklist
- State symbols: CO₂(g), H₂O(l), C(s), CO(g)
- CO = colourless, odourless, toxic
- Yellow flame = incomplete combustion
- Complete = more energy released
Key Equations
- CH₄ + 2O₂ → CO₂ + 2H₂O (complete combustion of methane)
- 2CH₄ + 3O₂ → 2CO + 4H₂O (incomplete combustion — CO)
- CH₄ + O₂ → C + 2H₂O (incomplete combustion — soot/carbon)
Common Mistakes
- Saying incomplete combustion produces CO₂: Incomplete combustion produces carbon MONOXIDE (CO) and/or carbon soot — NOT CO₂
- Forgetting water is always produced in combustion: Both complete AND incomplete combustion of hydrocarbons always produce water — hydrogen atoms in the fuel always bond with oxygen
- Not knowing CO is toxic: Carbon monoxide binds irreversibly to haemoglobin, preventing oxygen transport — it is colourless and odourless (impossible to detect without equipment)
- Saying complete combustion produces more products: Complete combustion actually produces FEWER types of products (only CO₂ and H₂O) but releases MORE energy than incomplete combustion
Memory Aid
- Blue flame (air hole open) = Complete combustion → CO₂ + H₂O (CLEAN)
- Yellow flame (air hole closed) = Incomplete combustion → CO + C (DIRTY)