HSC Exam Practice

Sample response. Cross-linking refers to the formation of covalent bonds between adjacent polymer chains, creating a permanent three-dimensional network structure.

Marking notes. 1 mark for covalent bonds between chains; 1 mark for resulting 3D network (not just "strong bonds within a chain").

Sample response. A thermoplastic has no covalent cross-links (chains are held by intermolecular forces only), so it softens and melts on heating, can be re-moulded, and is generally recyclable. A thermoset has extensive covalent cross-links between chains forming a permanent 3D network; it does not melt but instead decomposes (chars) on heating, so it cannot be remoulded or conventionally recycled.

Marking notes. 1 mark for cross-linking distinction (none/minimal vs extensive covalent); 1 mark for thermoplastic behaviour on heating (melts/softens); 1 mark for thermoset behaviour on heating (decomposes/chars, NOT "melts at higher temperature").

Sample response. Longer chains have greater contact area between adjacent chains, increasing the total number of intermolecular interactions (van der Waals / dispersion forces, and hydrogen bonds where polar groups are present). More points of contact mean: (1) a greater force is needed to pull chains apart — higher tensile strength; and (2) more thermal energy is needed to overcome these forces and allow chains to flow past each other — higher melting point. Longer chains also entangle more, further resisting deformation.

Marking notes. 1 mark for identifying increase in intermolecular interactions with longer chains; 1 mark for linking to tensile strength (more force needed to pull apart); 1 mark for linking to melting point (more thermal energy needed to overcome IMFs).

PET drink bottle — code 1, PET (polyethylene terephthalate) — Yes. | Expanded polystyrene food container — code 6, PS (polystyrene) — No. | HDPE milk jug — code 2, HDPE (high-density polyethylene) — Yes. | LDPE plastic bag — code 4, LDPE (low-density polyethylene) — No (requires separate drop-off; not accepted in standard kerbside). 1 mark per correctly completed row (code + polymer + Y/N all correct).

Sample response. Most synthetic polymer backbones consist of C–C bonds (and in some cases C–F bonds, e.g. PTFE). These bonds are non-polar and chemically inert. Microorganisms (bacteria and fungi) have not evolved efficient enzymatic pathways to cleave the C–C backbone of synthetic polymers, unlike natural polymers such as cellulose (C–O–C linkages) or proteins (amide bonds) which microorganisms can degrade. Physical degradation by UV and mechanical stress only produces smaller fragments (microplastics) without breaking C–C bonds.

Marking notes. 1 mark for identifying C–C (or C–F) backbone; 1 mark for stating that microorganisms lack enzymes to cleave these bonds; 1 mark for contrast with natural polymers OR for identifying that physical breakdown produces microplastics rather than full decomposition.

Sample response. Both are polyethylene (same monomer: ethylene/ethene). LDPE has branched chains that prevent close packing, resulting in lower density (~0.92 g cm−³) and a softer, more flexible material. HDPE has linear (unbranched) chains that can pack closely and regularly, increasing density (~0.95 g cm−³), raising the melting point (~135 °C vs ~110 °C), and producing a harder, more rigid material.

Marking notes. 1 mark for structural difference (LDPE branched vs HDPE linear); 1 mark for linking to packing/density; 1 mark for consequence for hardness / rigidity (can refer to Tm difference as evidence of tighter packing).

Part (a) — 2 marks. Natural polymers based on cellulose (C–O–C linkages, e.g. cotton, cardboard) degrade within months in the ocean because microorganisms possess cellulase enzymes to cleave these bonds. In contrast, synthetic polymers with C–C backbones (PET, HDPE, PS) persist for hundreds to over a thousand years. PLA, despite having ester linkages in its backbone, also persists in the ocean because hydrolysis of ester bonds requires specific temperature and humidity conditions not present in seawater. Marking: 1 mark for identifying natural polymers (C–O linkages) degrade rapidly; 1 mark for identifying synthetic C–C polymers persist for centuries.

Part (b) — 3 marks. PLA has ester (C–O–C=O) linkages that are theoretically hydrolysable, but the hydrolysis reaction requires elevated temperatures (above ~58–60 °C) and specific humidity conditions found only in industrial compost facilities. Ocean water temperatures are typically 4–25 °C, insufficient to activate hydrolysis at a meaningful rate. Additionally, ocean microorganisms have not evolved efficient PLA-degrading enzymatic pathways at ambient marine temperatures. Therefore, PLA behaves like a conventional plastic in marine environments and does not offer significant biodegradation benefit over PET in an oceanic context. Marking: 1 mark for identifying the ester linkage requires elevated temperature/industrial composting conditions; 1 mark for stating ocean temperature (~4–25 °C) is insufficient to drive meaningful hydrolysis; 1 mark for conclusion (PLA does not degrade in ocean despite marketing as biodegradable).

Part (c) — 3 marks. The claim is not supported by the data. The table shows PLA does not measurably degrade in the ocean — its persistence is comparable to conventional plastics under marine conditions. Replacing PET with PLA would only reduce ocean pollution if robust collection and industrial composting infrastructure prevented any PLA reaching the ocean. Without this infrastructure, PLA bottles would persist as long as PET bottles. Additionally, PLA mixed into PET recycling streams contaminates recycled PET, reducing recycling rates. The data support the conclusion that material substitution alone (PET → PLA) without systemic infrastructure change does not substantially reduce ocean plastic pollution. Marking: 1 mark for identifying data shows PLA has no significant ocean degradation; 1 mark for arguing the claim requires industrial composting infrastructure not present in marine environments; 1 mark for a clear evidence-based evaluative conclusion rejecting the claim.

Sample response. A polymer's molecular structure determines its bulk physical properties and, therefore, the applications for which it is suitable. Three key structural variables are chain length, degree of branching, and presence of cross-linking.

Kevlar (an aromatic polyamide) demonstrates how structural design maximises a specific property. Its para-linked benzene rings create a rigid, rod-like backbone, and the amide (C–N) bonds allow extensive inter-chain hydrogen bonding, producing an exceptionally high tensile strength (~3.6 GPa) that makes Kevlar ideal for ballistic protection and racing tyres. However, Kevlar is a condensation polymer that cannot be mechanically recycled by re-melting, and its production involves toxic solvents, creating a significant environmental cost. At end of life, Kevlar typically goes to landfill.

By contrast, LDPE (low-density polyethylene) has a branched chain structure that prevents close packing, yielding a soft, flexible, low-density material suitable for single-use plastic bags and cling film. LDPE is a thermoplastic — it softens on heating, allowing recycling by re-melting. However, its thin-film form means it jams standard recycling machinery and requires separate collection (e.g. REDcycle). Its high production volume and low recycling rate (~13% in Australia, APCO 2022) mean most LDPE ends up in landfill or the environment, where UV degradation produces microplastics. CSIRO research has documented LDPE microplastics in Australian marine sediments and waterways.

This comparison illustrates the trade-off: high-performance polymers like Kevlar are indispensable for their applications but carry significant end-of-life environmental costs. High-volume, low-performance polymers like LDPE have established recycling pathways in principle, but infrastructure limitations mean most is not recovered. Life-cycle assessment (LCA) is required to properly evaluate total environmental impact, as substituting one polymer for another (e.g. PLA for LDPE) does not automatically reduce environmental harm if the degradation conditions required are not present in the disposal environment.

Marking criteria. (1) Identifies at least two named polymers with contrasting structural features (1 mark). (2) Correctly links molecular structure to at least one physical property for each named polymer with accurate chemical reasoning, e.g. chain length/branching → density, cross-linking → can't re-melt, H-bonding in amide → high tensile strength (2 marks). (3) Links each polymer to a specific application and justifies the link (1 mark). (4) Identifies a trade-off between performance and environmental impact for at least one polymer, using specific evidence (e.g. recycling rate data, microplastics, Australian context) (2 marks). (5) Reaches an evaluative conclusion — acknowledges the context-dependence of the performance vs sustainability trade-off, or uses LCA / systems thinking (1 mark).