Homologous Series and Polymers
Homologous Series and Polymers tests the pattern of organic families, general formulas, and the formation, properties and uses of natural and synthetic polymers. Expect both naming and classification questions.
Core rules
- A homologous series is a family with the same functional group and similar chemical properties. Successive members differ by CH2, so their molecular masses differ by 14 u.
- General formulas: alkanes CnH2n+2, alkenes CnH2n, alkynes CnH2n-2. Methane, ethane and propane form a homologous series.
- Polymers are large molecules made from many small monomer units. Addition polymerisation joins unsaturated monomers without losing a small molecule; condensation polymerisation releases water or another small molecule.
- Natural polymers include cellulose, proteins and natural rubber. Synthetic polymers include polythene, PVC, nylon, terylene and bakelite.
- Polymer properties depend on chain length, branching and cross-linking. Some polymers are recyclable; others are not easily degraded.
Worked example
An alkane has four carbon atoms. Using CnH2n+2 gives C4H10, butane, with molecular mass 4 x 12 + 10 x 1 = 58 u. The next member has five carbons: C5H12, mass 72 u. The difference is 72 - 58 = 14 u, matching one CH2 unit. The reasoning confirms both the general formula and the homologous-series interval.
Traps
Do not use the same general formula for alkanes, alkenes and alkynes. A polymer is not the same as its monomer. Not all polymerisation is addition; condensation releases a small molecule. Natural rubber is a polymer, not a monomer. Do not assume every plastic is biodegradable.