Stage 2 Chemistry – Materials Quick Review

Polymers

  • Polymer = macromolecule built from repeating monomer units; made via polymerisation.
  • Two polymerisation types:
    • Addition: monomers with C=C double bonds join; no atoms lost; carbon backbone only.
    • Condensation: monomers join with loss of small molecule (often H_2O); forms esters (polyesters) or amides (polyamides).
  • Identify from structure:
    • Continuous C-backbone → addition polymer.
    • Repeating \text{-COO-} or \text{-CONH-} links → condensation polymer.
    • Repeating unit = section in brackets with subscript n.

Polymer Properties & Classes

  • Intermolecular forces control melting point, elasticity, rigidity:
    • Dispersion only → low T_m, flexible.
    • Dipole / H-bonding → ↑T_m, more rigid.
    • Covalent cross-links → very high T_m, rigid, elastic.
  • Cross-linking level defines two classes:
    • Thermoplastics: no cross-links; soften & remould on heating; recyclable.
    • Thermosets: extensive cross-links; char rather than melt; hard to recycle.
  • Synthetic polymer pros: versatile, tunable, cheap. Cons: fossil feedstock, non-biodegradable.

Polymer Feedstocks & Biodegradability

  • Fossil sources: abundant, existing infrastructure; non-renewable, landfill waste.
  • Renewable (biopolymer) sources: abundant, biodegradable, lower toxicity; issues—land use, limited types, higher price.
  • Biodegradability:
    • Polymers with hydrolysable \text{-COO-} or \text{-CONH-} groups → enzymes catalyse hydrolysis → biodegradable.
    • Addition polymers (C–C backbone) lack such bonds → non-biodegradable.
  • Biodegradable polymers: cut landfill time, produce useful compost; need O2/H2O, limited range.

Metals: Ores & Production Stages

  • Ore = rock mined for profitable metal extraction.
  • General stages: Extraction → Concentration → Conversion (if needed) → Reduction → (Refining).
  • Reactivity dictates occurrence & reduction method (see activity series).

Example: Aluminium (from Bauxite)

  1. Extraction: mine & crush bauxite (Al2O3·2H_2O).
  2. Concentration: dissolve in NaOH → NaAlO_2; remove "red mud".
  3. Conversion: precipitate Al(OH)3 with CO2; dehydrate → Al2O3.
  4. Reduction: molten electrolysis; dissolve Al2O3 in cryolite Na3AlF6 (+CaF2) to lower Tm.
    • Cathode: Al^{3+}+3e^-\rightarrow Al(l)
    • Anode: 2O^{2-}\rightarrow O_2+4e^-
  5. Metal collected molten; purity high—no refining needed.

Example: Zinc (from Sphalerite ZnS)

  1. Extraction: mine & crush ore.
  2. Concentration: froth flotation with xanthate collectors → ZnS froth.
  3. Conversion:
    • Roast: 2ZnS+3O2→2ZnO+2SO2
    • Leach: ZnO+H2SO4→ZnSO4+H2O
  4. Reduction: aqueous electrolysis of ZnSO_4.
    • Cathode: Zn^{2+}+2e^-→Zn(s)
    • Anode: 2H2O→O2+4H^++4e^-

Methods of Metal Reduction (linked to Activity Series)

  • Very reactive (K → Al): electrolysis of molten salts/oxides.
  • Mid-reactive (Al, Zn): electrolysis of aq. salts OR carbon reduction of oxides.
  • Less reactive (Fe → Cu): carbon/CO reduction of oxides; roasting sulfides.
  • Least reactive (Ag, Au, Pt): exist native; minimal processing.
  • Electrolysis: reliable but energy-intensive (high T, electricity).
  • Carbon reduction: cheap carbon; emits CO_2, needs high T, pollutants.

Electrolytic Cells – Key Points

  • Non-spontaneous redox driven by electricity.
  • Cathode = negative; reduction of cations.
  • Anode = positive; oxidation of anions/water.
  • Electrons flow external circuit anode → cathode; ions move through electrolyte to opposite charges.

Recycling of Materials

  • Saves finite ores & fossil fuels, cuts mining, energy, pollutants, landfill.
  • Main hurdles: collection, sorting, contamination.

Recycling of Polymers

  • Mechanical: melt/shred; suits thermoplastics.
  • Feedstock: depolymerise chemically to monomers/fuels.
  • Thermoplastics: 100\% recyclable, low T_m.
  • Thermosets: cross-linked; cannot melt—shredded for filler or energy recovery.
  • Resin identification codes aid sorting.

Composite Materials

  • Composite = matrix (binder) + reinforcement (fibres/particles).
  • Properties superior to individual components (strength, weight, corrosion, design flexibility).
  • Examples: fiberglass (glass fibres + polymer), concrete, wood.
  • Recycling issues: thermoset matrices, difficult separation, high cost, degraded quality.