HSC Exam Practice

Sample response. The molar enthalpy of combustion (ΔH_c) is the heat energy released when one mole of a substance undergoes complete combustion in excess oxygen under standard conditions. All combustion reactions are exothermic, so ΔH_c is always negative.

Marking notes. 1 mark for the definition (one mole, complete combustion, excess oxygen); 1 mark for the sign convention (negative / exothermic).

Sample response. C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(g)

Marking notes. 1 mark for correct products (CO₂ and H₂O only); 1 mark for correctly balanced equation with coefficients 1:3:2:3. Accept state symbols in parentheses. Award 1 mark only if equation is unbalanced but products are correct.

Sample response. In q = mcΔT, m is the mass of water in the calorimeter (in grams). It is used because the thermometer measures the temperature rise of the water — the water is the substance that absorbs the heat released by combustion, and q = mcΔT calculates how much heat the water absorbed. The mass of alcohol burned is used separately in Step 2 (n = Δm/M) to calculate the moles of fuel combusted. Using the mass of alcohol in q = mcΔT would give a physically meaningless quantity because it is not the substance whose temperature is being measured.

Marking notes. 1 mark for identifying m as the mass of water; 1 mark for explaining that q = mcΔT captures heat absorbed by water (the thermometer measures water temperature); 1 mark for explaining that alcohol mass is used in a separate step (n = Δm/M) for a different purpose.

Sample response. Source 1 — Heat loss to surroundings: The copper calorimeter walls, thermometer, bench, and surrounding air absorb some combustion heat. Only the heat that raised the water temperature is captured in q = mcΔT, so q_measured < q_actual. This makes the calculated |ΔH_c| lower than the theoretical value. Source 2 — Incomplete combustion: The spirit burner flame is oxygen-limited; some fuel oxidises only to CO or soot (C) rather than fully to CO₂. These intermediate products retain chemical energy not released as heat, reducing q. This also makes |ΔH_c| lower than theoretical. Both errors are systematic — they consistently push |ΔH_c| in one direction and do not average out.

Marking notes. 2 marks per source: 1 mark for identifying the physical event/mechanism; 1 mark for the direction of effect on q or n and therefore on |ΔH_c|. Other valid sources: alcohol evaporation without combustion (n too high → |ΔH_c| too low); calorimeter heat capacity ignored (q too low); early temperature reading (q too low). Award 4 marks for any two sources correctly explained.

Sample response. Butan-1-ol (C4) has two more CH₂ units than ethanol (C2). Each additional CH₂ unit adds two C–H bonds and one C–C bond. During complete combustion, breaking these bonds and forming additional C=O bonds (in CO₂) and O–H bonds (in H₂O) releases approximately 650 kJ/mol net per CH₂ unit. With two extra CH₂ units, butan-1-ol releases approximately 1300 kJ/mol more than ethanol, consistent with the difference between their theoretical values (2676 − 1367 = 1309 kJ/mol).

Marking notes. 1 mark for identifying that butan-1-ol has more C–H and C–C bonds (or more CH₂ units); 1 mark for explaining that combustion of these extra bonds forms extra CO₂ and H₂O, releasing more energy; 1 mark for quantifying (approximately 650 kJ/mol per CH₂) or citing specific values.

Sample response. Both bioethanol and petrol (octane) produce CO₂ and H₂O in complete combustion — the products are identical. The difference lies in the origin of the carbon. Bioethanol is produced by fermentation of plant biomass; the CO₂ released on combustion was recently absorbed from the atmosphere during photosynthesis as the plant grew. The net addition to atmospheric CO₂ is approximately zero, making bioethanol near-carbon-neutral. Petrol’s carbon was sequestered underground millions of years ago; burning it releases “new” CO₂ to the atmosphere, representing a net addition to the carbon cycle with no corresponding photosynthetic uptake in the same timeframe.

Marking notes. 1 mark for stating both fuels produce CO₂ and H₂O (same products); 1 mark for explaining the carbon cycle argument for bioethanol (CO₂ recycled from atmosphere via photosynthesis); 1 mark for contrasting fossil fuel carbon (sequestered millions of years ago → net addition).

Part (a) — calculation (5 marks).
Step 1: ΔT = 30.8 − 19.4 = 11.4 °C [½ mark]
q = 250 g × 4.18 J g⁻¹ °C⁻¹ × 11.4 °C = 11 913 J [1 mark, accept 11 910–11 915 J]
Step 2: Δm = 204.82 − 204.27 = 0.55 g [½ mark]
n = 0.55 / 60.09 = 0.009153 mol [1 mark]
Step 3: ΔH_c = −(11 913 ÷ 1000) / 0.009153 = −11.913 / 0.009153 = −1301 kJ/mol [1 mark for method; 1 mark for correct answer with units and negative sign]

Part (b) — percentage and discrepancy (2 marks).
% of theoretical = 1301/2021 × 100 = 64.4% [1 mark for correct calculation]
Discrepancy: the experimental value is only 64% of theoretical because of systematic errors including heat loss to surroundings (not captured by water thermometry) and incomplete combustion (some propan-1-ol oxidised only to CO or soot, retaining chemical energy). Both errors reduce q_measured below q_actual, making |ΔH_cexp| < |ΔH_ctheo|. [1 mark for identifying at least one specific source with correct direction of effect]

Part (a) — 2 marks. Octane has the highest energy density at 47.9 kJ/g. Percentage increase from methanol to pentan-1-ol: (37.8 − 22.7) / 22.7 × 100 = 15.1/22.7 × 100 = 66.5% increase. [1 mark for identifying octane; 1 mark for correct percentage ± 3%]

Part (b) — 3 marks. All alcohols contain a hydroxyl group (−OH), which introduces an oxygen atom into the molecule. This oxygen adds 16 g/mol to the molar mass without contributing proportional combustion energy, because the oxygen in −OH is already partially oxidised compared to the carbon atoms [1]. As a result, the ratio |ΔH_c|/M (energy per gram) is lower for alcohols than for the corresponding alkanes [1]. Octane (C₈H₁₈) contains only C and H — no oxygen — so all of its molecular mass contributes maximally to combustion energy release, giving a significantly higher energy density than any alcohol of similar chain length [1].

Sample response. Bioethanol produced from Australian sugarcane offers a partially renewable, near-carbon-neutral alternative to petrol for passenger vehicles, but a full transition to 100% ethanol faces significant energy, infrastructure, and economic barriers that make partial blending (such as Australia’s E10 programme) a more feasible near-term strategy.

Combustion chemistry: Both ethanol (C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O) and petrol (octane: C₈H₁₈ + 12.5O₂ → 8CO₂ + 9H₂O) produce only CO₂ and H₂O in complete combustion, so the combustion products are identical. The difference is not in the chemistry of burning but in the origin and lifecycle of the carbon.

Carbon cycle: Bioethanol produced by fermentation of sugarcane is near-carbon-neutral: the CO₂ released during combustion was recently removed from the atmosphere by photosynthesis as the cane grew. Petrol releases fossil carbon — sequestered for millions of years — as a net addition to atmospheric CO₂. CSIRO research suggests Australian sugarcane bioethanol reduces lifecycle CO₂ by approximately 75% per litre replaced, but not to zero, because fermentation, distillation, and transport consume energy from fossil sources.

Energy density: Ethanol’s energy density (29.7 kJ/g) is significantly lower than octane’s (47.9 kJ/g) — a 61% gap. This is because the −OH group adds an oxygen atom (16 g/mol) that is already partially oxidised and contributes less combustion energy per gram than pure hydrocarbon. A car running on 100% ethanol would need approximately 50% more fuel by volume to achieve the same range, requiring larger fuel tanks and more frequent refuelling.

Practical limitations of a full transition: (1) Engine modification: current Australian standard-grade petrol engines are approved for no more than E10; higher ethanol blends (E85 or E100) require flex-fuel engine conversion, a costly infrastructure challenge. (2) Supply: sufficient sugarcane bioethanol to replace 100% of Australia’s petrol would require a massive expansion of cane farming and processing, with land and water resource pressures. (3) Emissions: NOx emissions and non-combustion volatiles may change with higher ethanol blends, requiring further regulatory analysis.

Evaluation: Bioethanol is a genuine partial solution — E10 blends reduce lifecycle CO₂ without engine modification — but the energy density gap and infrastructure requirement mean it cannot replace petrol fully in the near term. A balanced policy uses ethanol as a blending agent, not a wholesale replacement, while parallel investment in electric vehicles and renewable hydrogen addresses the deeper energy transition.

Marking criteria:

• 1 mark — States or implies both fuels produce CO₂ + H₂O on complete combustion (same products); writes or correctly describes at least one balanced equation.
• 1 mark — Correctly explains the carbon cycle argument for bioethanol (photosynthesis absorbs CO₂ that combustion re-releases → near-neutral net).
• 1 mark — Identifies the lifecycle limitation on carbon neutrality (fermentation/distillation energy use) or a relevant Australian datum (CSIRO ~75% reduction).
• 1 mark — Correctly compares energy densities (ethanol 29.7 vs octane 47.9 kJ/g) and states the practical consequence (more fuel volume required per km).
• 1 mark — Provides a structural explanation for why all alcohols have lower energy density than comparable hydrocarbons (−OH oxygen adds mass; already partially oxidised).
• 1 mark — Identifies at least one named practical limitation of a full transition to 100% ethanol in the Australian context (engine modification for E85/E100; supply chain scale; land/water resources; infrastructure cost).
• 1 mark — Reaches an explicit evaluative judgement that is neither “ethanol is perfect” nor “ethanol is useless” — identifies a realistic role (partial blending, bridging fuel, E10) and uses precise terminology throughout.