HSC Exam Practice

Sample response. Addition polymerisation is a chain reaction in which many small monomer molecules, each containing a C=C double bond, join together by opening the pi bond of the C=C to form new C–C single bonds. No atoms are lost and no by-product is produced — every atom in every monomer is incorporated into the polymer chain.

Marking notes. 1 mark for identifying the C=C opening and formation of new C–C bonds between monomers; 1 mark for stating that no by-product is produced (OR: all atoms incorporated into the chain).

(a) PVC: Monomer = chloroethene (vinyl chloride), CH₂=CHCl [1]; Repeat unit = (-CH₂-CHCl-)_n in square brackets with open bonds and subscript n [1].

(b) PTFE: Monomer = tetrafluoroethene, CF₂=CF₂ [1]; Repeat unit = (-CF₂-CF₂-)_n [1].

(c) PP: Monomer = propene, CH₂=CHCH₃ [1]; Repeat unit = (-CH₂-CH(CH₃)-)_n [1].

Marking notes. 1 mark per correct monomer formula; 1 mark per correct repeat unit notation. Repeat unit must have square brackets + open bonds + subscript n to earn full marks. No marks if C=C appears inside the repeat unit.

Sample response. (1) Initiation: An initiator molecule (e.g. an organic peroxide) decomposes to generate a free radical (Rau), which attacks the C=C of the first monomer, opening the pi bond and producing a carbon radical at the chain end. (2) Propagation: The chain radical attacks the C=C of the next monomer, extending the chain by one repeat unit and regenerating a new chain radical. This step repeats thousands of times. (3) Termination: Two chain radicals combine with each other, forming a covalent bond that quenches both radicals and ends the chain growth.

Marking notes. 1 mark per stage (must name the stage AND describe the key chemical event for each mark).

Sample response. Addition polymers such as PE, PP, and PVC are thermoplastics because their chains are held together by intermolecular forces only — London dispersion forces and, in PVC, dipole–dipole interactions from the polar C–Cl bonds [1]. Heating above the melting point weakens these intermolecular forces, allowing the chains to flow and the polymer to be reshaped [1]. On cooling, the forces re-establish and the polymer re-hardens. This is reversible and repeatable. In contrast, thermosets (e.g. epoxy resins) have permanent covalent cross-links between chains that cannot be reversed by heating [1].

Marking notes. 1 mark — chains held by IMF (not covalent cross-links); 1 mark — heating weakens IMF reversibly, allowing flow/remoulding; 1 mark — contrast with thermosets (covalent cross-links, not reversible).

Sample response. Both LDPE and HDPE are made from the same monomer (ethene) and have the same repeat unit (-CH₂-CH₂-)_n. LDPE chains are branched (produced under high pressure, radical conditions), which prevents close chain packing, reduces the contact area between chains, weakens dispersion forces, and gives a lower melting point (~110°C) and lower density. HDPE chains are linear (Ziegler–Natta catalyst), pack closely together, maximise dispersion force contact area, and have a higher melting point (~130°C) and higher density [1 mark for chain architecture difference; 1 mark for linking to packing/dispersion forces; 1 mark for correct melting point values or trend].

Sample response. M_{repeat unit}(C₃H₆) = 3(12) + 6(1) = 36 + 6 = 42 g mol⁻¹ [1]. n = M_polymer / M_{repeat unit} = 840 000 / 42 = 20 000 [1].

Marking notes. 1 mark for correct M_{repeat unit} = 42 g mol⁻¹ (or equivalent working); 1 mark for n = 20 000 (accept 20 000 or 2 × 10⁴).

(a) Trend description [2 marks]. The melting points increase across the four polymers from LDPE (~110°C) to HDPE (~130°C) to PP (~165°C) to PTFE (~327°C) [1]. PTFE has a markedly higher melting point than the other three; the increase from LDPE to PP is more gradual [1]. Award 1 mark if only one polymer is correctly identified in context.

(b) LDPE vs HDPE [2 marks]. Both polymers have the same C–C backbone chemistry and the same repeat unit. HDPE chains are linear and pack closely together, maximising the surface area of contact between adjacent chains and therefore maximising cumulative London (dispersion) forces [1]. More energy (higher temperature) is required to overcome these stronger dispersion forces and allow chains to flow — hence the higher melting point of HDPE (~130°C vs ~110°C). LDPE branches prevent close chain packing, reducing dispersion force contact and lowering the melting point [1].

(c) PTFE melting point [3 marks]. PTFE’s repeat unit is (-CF₂-CF₂-)_n: every hydrogen atom is replaced by a much larger, heavier fluorine atom [1]. The greater atomic mass and size of fluorine atoms increases the electron cloud volume and polarisability of each chain segment significantly, producing stronger London dispersion forces between PTFE chains than between the analogous polyethylene chains [1]. Additionally, PTFE chains pack efficiently because fluorine atoms, while larger than H, create a smooth cylindrical chain profile that allows close interchain contact. The combination of very strong dispersion forces (from large, polarisable F atoms) requires a very large thermal input to separate the chains — hence the exceptionally high melting point of ~327°C [1].

(a) Comparison [3 marks — any 2 criteria, 1.5 marks each, rounded].

Hot water resistance: HDPE is better. HDPE softening point ~130°C is well above 60°C, so it remains dimensionally stable. uPVC softens at ~80°C, which means a 60°C burst is only 20°C below the softening point — thermal safety margin is narrow [1 + 0.5 identify better].

Tensile strength / rigid pressure containment: uPVC is better (40–60 MPa vs 20–30 MPa). uPVC can withstand higher internal pressures without deformation [1 + 0.5].

Impact resistance at low temperature: HDPE is better. HDPE remains flexible and absorbs mechanical impact; uPVC is brittle below 5°C and risks cracking during installation in cold Australian winters [1 + 0.5].

UV resistance: HDPE is better. uPVC degrades rapidly under UV (without stabiliser additives) while HDPE has moderate UV resistance — relevant for roof-space installation [1 + 0.5]. Award marks for any two criteria discussed with specific data citation.

(b) PVC higher tensile strength [3 marks]. The PVC repeat unit (-CH₂-CHCl-)_n contains a C–Cl bond on every second backbone carbon [1]. The C–Cl bond is polar (electronegativity difference Δχ = 3.16 − 2.55 = 0.61), making each repeat unit a permanent dipole. Adjacent chains experience dipole–dipole interactions in addition to London dispersion forces [1]. The combined dispersion + dipole–dipole forces are stronger than the dispersion-only forces between HDPE chains, making PVC chains harder to separate and giving the material greater tensile strength and rigidity [1]. (No covalent cross-links — PVC is still a thermoplastic.)

Sample response. The claim is substantially correct, but requires qualification: the C–C backbone is a primary but not exclusive reason for both commercial value and environmental persistence, and other structural features (C–F bonds in PTFE, C–Cl bonds in PVC) also contribute.

Addition polymers are synthesised by the opening of the C=C double bond in alkene monomers, forming long chains of C–C single bonds. The C–C backbone makes these materials commercially valuable in several ways. The bond is thermally stable (dissociation energy ~347 kJ mol⁻¹), so addition polymers such as HDPE (Qenos, Altona VIC; used in milk crates and piping) do not degrade at normal-use temperatures. The all-carbon backbone is also non-polar, creating polymers with low reactivity toward water, dilute acids, and bases — extending product lifetime. Physical properties (stiffness, flexibility, optical clarity) are tunable by controlling chain architecture (e.g. LDPE branched chains for flexible packaging vs HDPE linear chains for rigid containers), without changing the backbone chemistry. In PTFE, the additional contribution of C–F bonds (~485 kJ mol⁻¹) further enhances chemical resistance beyond what the C–C backbone alone provides — PTFE withstands concentrated acid exposure that HDPE cannot. This shows the C–C backbone is not the only structural contributor to commercial utility.

Environmental persistence also stems primarily from the C–C backbone. Microbial hydrolase enzymes can cleave ester bonds (in polyesters), amide bonds (in proteins), and glycosidic bonds (in cellulose), but no known common soil microbe produces an enzyme capable of hydrolysing a C–C single bond. Without enzymatic attack, addition polymers like HDPE and PVC persist in landfill and ocean environments for hundreds to thousands of years. UV radiation and mechanical weathering physically fragment these materials into microplastics (<5 mm), which enter food chains and are now detected in the blood of mammals. Again, PVC introduces an additional complexity: C–Cl bonds are susceptible to UV-catalysed degradation, releasing HCl and potentially forming toxic dioxins upon incineration — an environmental harm that cannot be attributed solely to the C–C backbone.

In summary, the C–C backbone is the primary structural reason for both the commercial utility and the environmental persistence of addition polymers, but in specific cases (PTFE, PVC) additional substituent bonds (C–F, C–Cl) are co-equal contributors to these properties. The claim is largely correct with this qualification.

Marking notes. 1 mark — correctly identifies C=C opening to form C–C backbone (addition polymerisation definition). 1 mark — links C–C backbone to commercial utility with a named Australian polymer example. 1 mark — second named polymer with distinct property linked to structure. 1 mark — correctly explains why the C–C backbone is not biodegradable (no hydrolysable bonds; contrast with ester/amide bonds). 1 mark — links persistence to microplastic formation with specific size threshold or food-chain impact. 1 mark — evaluates whether C–C backbone is the only relevant structural feature (e.g. C–F in PTFE or C–Cl in PVC also contribute). 1 mark — reaches an explicit evaluative judgement about the claim (“substantially correct but requires qualification”, “correct for most addition polymers”, etc.). 1 mark — quality of chemical reasoning throughout: correct terminology (dispersion forces, bond dissociation energy, hydrolysable vs non-hydrolysable backbone), coherent structure, precision of language.