HSC Exam Practice

Sample response. Complete combustion is the reaction of a hydrocarbon with excess oxygen, in which every carbon atom is fully oxidised to carbon dioxide (CO₂) and every hydrogen atom forms water (H₂O).

Marking notes. 1 mark for excess oxygen condition; 1 mark for both CO₂ and H₂O as the only carbon-containing and hydrogen-containing products.

Sample response. Carbon monoxide (CO) and solid carbon particles / soot (C).

Marking notes. 1 mark per correctly identified product. Accept “particulate carbon,” “soot,” or “solid carbon.” Do not accept CO₂ alone (it is also produced in complete combustion).

Sample response. In a substitution reaction, one atom or group in an organic molecule is replaced by another atom or group, so the number of atoms in the molecule stays the same (e.g. CH₄ + Cl₂ → CH₃Cl + HCl; one H is replaced by Cl). In an addition reaction, atoms add across a multiple bond in an organic molecule, increasing the total number of atoms and converting the multiple bond to a single bond (e.g. C₂H₄ + Br₂ → C₂H₄Br₂; Br₂ adds across C=C).

Marking notes. 1 mark for defining substitution (one atom/group replaced by another); 1 mark for defining addition (atoms add across a multiple bond); 1 mark for giving a relevant example or equation for each that demonstrates the distinction.

Sample response. C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

Marking notes. 1 mark for correct products (3CO₂ and 4H₂O); 1 mark for balanced equation (5O₂ on the left). State symbols not required unless specified.

Sample response. UV light provides the energy needed to break the Cl–Cl bond homolytically, splitting it into two chlorine free radicals (Cl•). Each free radical has an unpaired electron, making it highly reactive. The free radicals then initiate the chain reaction by abstracting a hydrogen atom from methane. Without UV light, the Cl–Cl bond cannot be broken under normal conditions, so the chain reaction cannot begin and no significant reaction occurs.

Marking notes. 1 mark for identifying that UV provides energy to break the Cl–Cl bond; 1 mark for identifying the products as free radicals (or “Cl•”); 1 mark for explaining that without UV light the initiation step cannot occur / the chain reaction cannot start.

Sample response. The monomer is the small alkene molecule that reacts — for ethene, the monomer is CH₂=CH₂. The repeating unit is the structural pattern that is repeated inside the polymer chain once the double bond has opened during polymerisation. For polyethene, the repeating unit is [—CH₂—CH₂—]_n. The key difference is that the monomer contains a C=C double bond, whereas the repeating unit contains only C–C single bonds because the double bond opened during the reaction.

Marking notes. 1 mark for correctly identifying ethene (CH₂=CH₂) as the monomer; 1 mark for correctly stating the repeating unit as [—CH₂—CH₂—]_n (or equivalent notation); 1 mark for explicitly noting that the monomer has a C=C double bond that is absent from the repeating unit.

Part (a) — Alkene/alkane classification (2 marks). Alkenes: Q (C₂H₄), R (C₄H₈), T (C₃H₆) — these all decolourise bromine water rapidly, indicating they contain a C=C double bond that can undergo addition with Br₂. Alkanes: P (C₂H₆), S (C₆H₁₄) — these do not decolourise bromine water, consistent with saturated hydrocarbons that have no C=C bond and cannot undergo addition. 1 mark for correctly classifying all five; 1 mark for linking the observation to the presence/absence of a C=C bond.

Part (b) — Equation and reaction type (3 marks). C₂H₄ + Br₂ → C₂H₄Br₂ (1,2-dibromoethane). Reaction type: addition reaction. 1 mark for balanced equation; 1 mark for correct product (dibromoethane, or equivalent); 1 mark for naming the reaction type as addition.

Part (c) — Account for no decolourisation (3 marks). Samples P and S are alkanes (saturated hydrocarbons) containing only C–C and C–H single bonds [1]. They cannot undergo addition reactions because there is no C=C double bond for Br₂ to add across [1]. While alkanes can react with bromine by substitution (replacing an H with Br), this requires UV light to initiate the free-radical chain, which is not provided under normal laboratory lighting conditions [1].

Part (a) — Trend (2 marks). In all three production runs, polyethylene output is close to but slightly less than the ethene input. Run 2 has both the highest input (240 kt) and the highest output (229 kt), and Run 3 has both the lowest input (200 kt) and lowest output (193 kt) — output scales proportionally with input. 1 mark for identifying that output is always slightly less than input; 1 mark for noting the proportional relationship across runs.

Part (b) — Why output < input (3 marks). In addition polymerisation, in theory no atoms are lost — all atoms from the monomer become part of the polymer chain [1]. In practice, the small mass discrepancy arises from process inefficiencies: unreacted ethene monomer that is not captured and recycled; catalyst residue; and low-molecular-mass oligomers (very short chains) that are removed from the product stream [1]. The key chemistry point is that unlike condensation polymerisation, no small molecule (such as water) is deliberately eliminated — any mass loss is due to physical process factors, not the chemistry of the polymerisation reaction itself [1].

Sample Band 6 response. The claim is partially correct but significantly overstated. Alkanes are indeed less reactive than alkenes, but they participate in at least two important reaction types under readily achievable conditions.

Alkanes are saturated hydrocarbons — they contain only strong C–C and C–H sigma bonds with no electron-rich multiple bonds. This absence of a reactive site means that typical reagents such as bromine water, aqueous acid or bases do not react with alkanes at room temperature, so the description “relatively unreactive” is valid under those specific conditions.

However, alkanes readily undergo combustion in the presence of oxygen, which is arguably the most economically significant organic reaction on Earth. For example, methane from natural gas undergoes complete combustion: CH₄ + 2O₂ → CO₂ + 2H₂O. This reaction is highly exothermic and occurs without any special catalyst under ordinary conditions (simply lighting the gas). Incomplete combustion with limited oxygen gives CO and soot. The claim that alkanes “rarely participate in chemical reactions under normal conditions” is therefore false — burning a gas stovetop burner or a coal-fired power station demonstrates alkane combustion under common conditions.

Alkanes also undergo free-radical substitution with halogens when UV light is present. Methane reacts with chlorine gas under UV to give chloromethane and HCl: CH₄ + Cl₂ → CH₃Cl + HCl. This requires a specific condition (UV light to initiate homolytic Cl–Cl bond cleavage), so it would not occur “under all normal conditions”, but UV from sunlight is present outdoors and this reaction is industrially important in halocarbon synthesis.

In summary: alkanes are less reactive than alkenes because they lack a C=C double bond, but they are not unreactive. Combustion occurs readily and substitution with halogens occurs under UV conditions. The claim should be reformulated as: “Alkanes are less reactive than alkenes but do participate in combustion reactions under normal conditions and in halogenation reactions under UV light.”

Marking criteria.

• 1 mark — States an overall evaluative judgement (e.g. “the claim is partially correct but overstated”) and connects the claim to the structural feature of alkanes (saturated; only C–C and C–H single bonds; no C=C).
• 1 mark — Identifies combustion as a reaction type readily undergone by alkanes and gives a correctly balanced equation (e.g. CH₄ + 2O₂ → CO₂ + 2H₂O or equivalent).
• 1 mark — Correctly concedes that complete combustion occurs under common conditions, effectively countering the “rarely under normal conditions” part of the claim.
• 1 mark — Identifies free-radical substitution (or halogenation) as a second reaction type and states the UV light condition.
• 1 mark — Gives a correct equation for the substitution reaction (e.g. CH₄ + Cl₂ → CH₃Cl + HCl under UV).
• 1 mark — Explains why alkanes are less reactive than alkenes (no C=C double bond / only sigma single bonds / no reactive pi bond site for addition) rather than “unreactive.”
• 1 mark — Reaches a reformulated, defensible statement that accepts the relative unreactivity of alkanes while explicitly identifying the two reaction types and their conditions.