14.1.2—Describe
- Syllabus
- 9701–2028–2029
- Objective
- 14.1.2
- Level
- AS
Alkanes burn completely or incompletely and undergo UV radical substitution
Complete combustion of an alkane gives CO₂ and H₂O when oxygen is sufficient. Limited oxygen causes incomplete combustion, producing CO and/or carbon. Under UV light, Cl₂ or Br₂ can substitute for H in a free-radical chain.
Balance combustion by carbon, hydrogen, then oxygen. Radical substitution proceeds through initiation, propagation and termination; it is not electrophilic addition.
Ethane burns completely as 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O. Under UV, CH₃CH₃ + Cl₂ → CH₃CH₂Cl + HCl, with further substitution possible.
Incomplete combustion is not a single fixed product, and UV light initiates the radical chain rather than acting as a catalyst.
ConceptA-Level CAIE Chemistry AS