HSC Exam Practice

Sample response. Addition polymerisation requires alkene (unsaturated) monomers; they join across the double bond to form the polymer chain with no small-molecule by-product. Condensation polymerisation requires monomers with two functional groups; they react repeatedly, and a small molecule (e.g. water or HCl) is eliminated at each step.

Marking notes. 1 mark for addition (alkene monomers, no by-product); 1 mark for condensation (bifunctional monomers, small molecule released); 1 mark for correctly contrasting the two.

Sample response. HDPE has a predominantly linear chain structure: the straight chains can pack closely together, creating higher crystallinity and a higher density (~0.95 g cm⁻³) than LDPE. The tighter packing also increases van der Waals forces between chains, making HDPE stiffer. LDPE has a branched chain structure: the branches prevent close packing, reducing crystallinity and density (~0.92 g cm⁻³) and decreasing the strength of chain–chain interactions, resulting in a more flexible material.

Marking notes. 1 mark for HDPE linear → close packing → higher crystallinity; 1 mark for LDPE branched → less close packing → lower crystallinity; 1 mark for density comparison; 1 mark for flexibility difference linked to intermolecular forces or crystallinity.

Sample response. Thermoplastics (e.g. HDPE, PET) have chains held together by intermolecular forces. On heating, these forces are overcome and the material softens and flows; on cooling it re-hardens. Thermosetting polymers (e.g. Bakelite, epoxy resins) have permanent covalent cross-links between chains. These bonds cannot be broken by heating; instead, the material decomposes at high temperature and cannot be remoulded.

Marking notes. 1 mark for thermoplastic: intermolecular forces → can be remelted; 1 mark for thermosetting: permanent covalent cross-links; 1 mark for thermoset decomposes rather than melting.

Sample response. Nylon-6,6 contains amide (–CO–NH–) groups in its backbone, which can form hydrogen bonds between adjacent chains. Hydrogen bonds are much stronger than the van der Waals dispersion forces that hold polyethylene chains together. More thermal energy is therefore needed to disrupt the hydrogen bonds in nylon-6,6, resulting in a significantly higher melting point (~265°C vs ~130°C for HDPE).

Marking notes. 1 mark for amide groups enabling hydrogen bonding in nylon; 1 mark for PE having only dispersion forces; 1 mark for linking stronger forces to higher melting point.

Sample response. Microplastics are plastic particles smaller than 5 mm. They form when larger plastic items are broken down by UV radiation, mechanical stress (waves, abrasion) and weathering into progressively smaller fragments. Their environmental consequence is that they are persistent, spreading through food webs and accumulating in sediments and the bodies of marine organisms; they do not biodegrade on useful timescales.

Marking notes. 1 mark for definition (<5 mm, from breakdown of larger items); 1 mark for formation mechanism (UV, wave action, physical fragmentation); 1 mark for a relevant environmental consequence (persistence in food webs, ecosystems, etc.).

Sample response. Both PET and nylon-6,6 are made by condensation polymerisation. PET has ester links (–COO–) in its backbone; nylon-6,6 has amide links (–CO–NH–). PET is a thermoplastic: its chains are held by intermolecular forces and can be remelted when heated, allowing it to be processed and reformed in mechanical recycling. A thermoset has extensive covalent cross-links formed permanently during curing; heating causes decomposition rather than melting, so the material cannot be reshaped and cannot enter a standard mechanical recycling stream.

Marking notes. 1 mark for both polymers formed by condensation polymerisation; 1 mark for naming ester links (PET) and amide links (nylon); 1 mark for PET thermoplastic → can be remelted → mechanically recyclable; 1 mark for thermoset cross-links → decomposition not melting → not recyclable.

Part (a) — 3 marks. HDPE is the most suitable choice. Its softening point of ~130°C exceeds the 70°C requirement, so it will not deform in service. It is kerbside recyclable (Code 2). It is also more rigid than flexible LDPE, appropriate for a water pipe application. Nylon-6,6 would also meet the temperature requirement (melts ~265°C) but is less widely recyclable in kerbside streams; PVC softens at ~80°C which barely clears 70°C and is rarely recyclable; epoxy resin cannot be recycled at all. 1 mark for choosing HDPE; 1 mark for temperature justification using data; 1 mark for recyclability justification.

Part (b) — 2 marks. Epoxy resin is a thermosetting polymer with extensive covalent cross-links between chains, formed permanently during curing. These cross-links cannot be disrupted by heating without decomposing the polymer chemically. Because the material cannot be melted and reshaped, it cannot be processed in standard kerbside mechanical recycling, which relies on thermoplastics that soften on heating.

Part (c) — 3 marks. PVC has a chloro-substituted backbone (vinyl chloride monomer) while HDPE has a pure hydrocarbon backbone (ethylene monomer). They have different softening temperatures (~80°C vs ~130°C) and different chemical compositions. When mixed and melted together, the incompatible chains do not blend uniformly: the resulting material has poor mechanical properties because the two polymers form a heterogeneous mixture rather than a homogeneous alloy. Additionally, PVC can release HCl gas at elevated processing temperatures, which could contaminate or degrade the HDPE component. 1 mark for different backbone chemistry; 1 mark for different softening temperatures causing processing incompatibility; 1 mark for poor product properties or HCl release.

Sample response. The claim that polymer waste is not a serious environmental problem because plastics eventually break down in the ocean is scientifically incorrect. Common addition polymers such as polyethylene (HDPE or LDPE) have a hydrocarbon backbone of C–C and C–H bonds. These bonds are chemically stable and resistant to attack by micro-organisms, which lack the enzymes needed to cleave them. Consequently, polyethylene does not biodegrade on any meaningful human timescale. Instead, when plastic items enter the ocean, UV radiation, wave action and physical abrasion cause them to fragment into progressively smaller pieces. Once fragments fall below 5 mm in size they are classified as microplastics. Fragmentation and biodegradation are fundamentally different processes: fragmentation only changes the size of the plastic, not its chemistry; the polymer chain remains intact. Microplastics persist in the environment, spread through food webs, accumulate in sediments and can be ingested by marine organisms. Waiting for “nature to deal with it” is not a viable strategy because the persistence is a direct consequence of the molecular structure of synthetic addition polymers. The most effective management strategy is source reduction: reducing the production and use of single-use plastics before they enter the environment, combined with improved waste collection and sorting to prevent plastic reaching waterways in the first place.

Marking criteria. 1 mark — Names a specific polymer (e.g. polyethylene/HDPE/LDPE) and correctly identifies it as an addition polymer. 1 mark — Explains structural basis of persistence: C–C backbone, microbial inability to cleave these bonds. 1 mark — Describes microplastic formation: fragmentation by UV and physical forces produces particles <5 mm. 1 mark — Correctly distinguishes fragmentation (size change, chemistry unchanged) from biodegradation (chemical breakdown into harmless molecules). 1 mark — Identifies at least one environmental consequence of microplastic persistence (food-web accumulation, sediment accumulation, ingestion by organisms). 1 mark — Suggests a management strategy that addresses the root cause (e.g. source reduction of single-use plastics; improved waste management; chemical recycling to recover monomers) and reaches an evidence-based evaluative conclusion rejecting the claim.