HSC Exam Practice

Sample response. Addition polymerisation occurs when monomers containing a C=C double bond react; the double bond opens and new C–C single bonds form between monomers, producing only the polymer (no byproduct). Condensation polymerisation occurs when bifunctional monomers (each carrying two reactive functional groups) react repeatedly, releasing a small molecule (usually water) at each step and forming ester or amide linkages. Distinguishing structural feature: addition monomers must have a C=C double bond; condensation monomers must have two different functional groups (e.g. —OH and —COOH).

Marking notes. 1 mark for correct definition of addition polymerisation (C=C opens, no byproduct); 1 mark for correct definition of condensation polymerisation (bifunctional monomers, small molecule released); 1 mark for C=C double bond as distinguishing feature of addition monomers; 1 mark for bifunctionality (two reactive groups) as distinguishing feature of condensation monomers.

Sample response. (a) PE: monomer is ethene (CH₂=CH₂); addition polymerisation. (b) PVC: monomer is vinyl chloride (CH₂=CHCl / chloroethene); addition polymerisation. (c) Nylon-6,6: monomers are hexamethylenediamine (H₂N–(CH₂)₆–NH₂) and adipic acid (HOOC–(CH₂)₄–COOH); condensation polymerisation.

Marking notes. 1 mark per row: correct monomer(s) identified AND correct polymerisation type stated. For nylon, accept either monomer name or both; accept “diamine + dicarboxylic acid” as monomer description.

Sample response. Atom economy is the proportion of the mass of reactants that is incorporated into the desired product, expressed as a percentage. Addition polymerisation has 100% atom economy because every atom from the monomers becomes part of the polymer — no atoms are lost as byproduct. Condensation polymerisation has less than 100% atom economy because a small molecule (water, H₂O) is released at every step when the diol’s —OH group reacts with the dicarboxylic acid’s —COOH group; these atoms do not end up in the polymer chain.

Marking notes. 1 mark for defining atom economy (proportion of mass of reactants in desired product); 1 mark for explaining 100% in addition (no atoms lost, no byproduct); 1 mark for identifying water as the byproduct of diol + dicarboxylic acid condensation, and linking this to reduced atom economy.

Sample response. A thermoplastic polymer has chains held together by intermolecular forces only (dispersion, dipole–dipole, or hydrogen bonds); no covalent bonds link adjacent chains. When heated, these IMFs are overcome and chains slide past each other — the polymer softens and flows — then re-solidifies on cooling. Because no bonds are broken, the process is reversible and thermoplastics can be remelted and remoulded (recyclable). A thermosetting polymer has covalent cross-links between chains. These covalent bonds cannot be broken by heating; applying heat causes decomposition or charring, not softening. Thermosetting polymers are therefore not recyclable by remelting. Examples: thermoplastic — polyethylene; thermosetting — epoxy resin / bakelite.

Marking notes. 1 mark for thermoplastic: IMFs only between chains; 1 mark for consequence — softens on heating, reversible, recyclable; 1 mark for thermosetting: covalent cross-links between chains; 1 mark for consequence — does not soften/melt, not recyclable. Examples not required but can substitute for a missing explanation point.

Sample response. The student is incorrect because both LDPE and HDPE are made from the same monomer — ethene (CH₂=CH₂) — by addition polymerisation. The difference in properties arises from the chain structure formed during polymerisation: LDPE has many side-chain branches, which prevent the chains from packing closely together, giving lower density and greater flexibility. HDPE has predominantly linear (straight) chains with minimal branching, allowing the chains to pack closely in an ordered arrangement, giving higher density, stronger total intermolecular forces, and greater rigidity.

Marking notes. 1 mark for correctly identifying the flaw (both made from ethene / same monomer); 1 mark for LDPE’s branched chains preventing close packing, giving lower density and flexibility; 1 mark for HDPE’s linear chains enabling close packing, giving higher density and rigidity. Must reference chain structure for full credit.

Sample response. Chlorine is electronegative (χ = 3.0) and forms a polar C–Cl bond with the carbon in the PVC repeat unit (δ+ on C, δ− on Cl). The chain therefore has permanent dipoles along its length. Adjacent PVC chains experience dipole–dipole attractive forces between their C–Cl groups. Polyethylene, by contrast, has only non-polar C–H bonds; the only intermolecular forces between PE chains are (weaker) dispersion (London) forces. Because dipole–dipole forces are stronger than dispersion forces, more energy is required to slide PVC chains past each other when a deforming force is applied, making PVC stiffer than PE at room temperature.

Marking notes. 1 mark for identifying the polar C–Cl bond (electronegative Cl) creating permanent dipoles on PVC chains; 1 mark for stating dipole–dipole forces act between adjacent PVC chains (vs dispersion only for PE); 1 mark for explaining that stronger dipole–dipole forces require more energy to overcome → greater resistance to deformation → stiffer material.

Sample response. P2 has higher density (0.96 vs 0.92 g cm⁻³) and higher tensile strength (30 vs 12 MPa) than P1. P1 has highly branched chains; the side branches prevent adjacent chains from packing closely together, leaving space between them → lower density. Less contact area between chains means fewer total dispersion force interactions → weaker IMFs → lower tensile strength. P2’s linear chains can pack closely in a regular arrangement, maximising chain–chain contact area → greater total dispersion forces → higher tensile strength. The ordered packing also accounts for the higher density.

Marking notes. 1 mark for comparing density values from the table (0.92 vs 0.96) with correct attribution; 1 mark for explaining P1’s lower density via branching preventing close packing; 1 mark for comparing tensile strength with data (12 vs 30 MPa); 1 mark for explaining P2’s higher tensile strength via close packing → greater total dispersion forces.

Sample response. P3’s exceptional tensile strength (145 MPa) despite moderate density (0.94) is due to its very long chain length (n ≈ 150,000). Very long chains have enormous total surface area for dispersion force contact, and they become highly entangled with each other, making it extremely difficult to pull chains apart. P3 cannot be processed by injection moulding because its very long chains create an extremely high melt viscosity when heated — the molten polymer is so viscous it barely flows under typical injection pressures, making it impractical to force into a mould cavity.

Marking notes. 1 mark for identifying very long chain length (n ≈ 150,000) as the structural feature; 1 mark for explaining how very long chains produce greater total dispersion force / entanglement → higher tensile strength; 1 mark for explaining injection moulding difficulty via extremely high melt viscosity / insufficient flow.

Sample response. P1 (highly branched) would dissolve most readily. All three samples are thermoplastics held together only by dispersion forces (non-polar C–H bonds only). P1’s branches prevent chains from packing closely, so the total dispersion force contact between chains is lower than in P2 or P3. Hot organic solvent molecules can most easily displace these weaker chain–chain interactions in P1, allowing dissolution. P3 would dissolve least readily because its very long chains have vastly more total dispersion force contact area and extreme chain entanglement to overcome.

Marking notes. 1 mark for correctly identifying P1 as most readily dissolved; 1 mark for justification referring to weaker/fewer interchain dispersion force contacts due to branching (accept: branches reduce packing → less contact → weaker total IMF → easier to dissolve).

Band 5–6 response should include all of the following (award 1–2 marks per section up to 9):

Chain length (2 marks): Longer chains → greater surface area → stronger total dispersion/IMF forces → higher melting point, higher tensile strength, higher viscosity when melted. Example: wax (short PE chains, MP ~50°C, brittle/soft) vs HDPE (long chains, MP ~135°C, rigid). If chain length increases alone: wax transitions progressively to hard polyethylene.

Branching (2 marks): Branching prevents close packing → lower density, lower MP, more flexible (fewer/weaker total IMF contacts). Example: LDPE (highly branched, 0.91–0.94 g/cm³, flexible → shopping bags) vs HDPE (linear, 0.94–0.97 g/cm³, rigid → pipes, containers). If branching increases in a polyethylene sample, density and tensile strength decrease.

Cross-linking (2 marks): Cross-links are covalent bonds between chains → rigid, insoluble, thermosetting (cannot be remelted or recycled). Example: vulcanised rubber (S–S cross-links), epoxy resin, bakelite. Heating does not soften but eventually chars. If cross-link density increases, stiffness and insolubility increase; recyclability is lost entirely.

Polarity of substituent (2 marks): Polar substituents (e.g. —Cl in PVC, —NH in nylon, —OH) create permanent dipoles or hydrogen-bonding capability between chains → stronger interchain IMFs → higher MP and stiffness than non-polar equivalent. Example: PVC (—Cl, dipole–dipole) stiffer than PE; nylon (H-bonding via —CONH—, MP 265°C) vs PE (dispersion only, MP 130°C). If a non-polar substituent is replaced with a polar one, MP and stiffness increase.

Integration / synthesis (1 mark): Properties are determined by the interplay of all structural features together; e.g. UHMWPE achieves extreme strength through long chains despite having no polar groups or cross-links — it outperforms shorter-chain cross-linked polymers in tensile strength; conversely, cross-linked polymers achieve insolubility that no amount of chain length can replicate.

Marking notes. Award marks for clear, accurate structural reasoning linked to named polymers and observable properties. Deduct for vague statements not grounded in bonding theory. Band 5 responses discuss at least three features with specific examples. Band 6 responses integrate all four features with explicit “if X changes, then Y” reasoning and named real-world applications.