Chemistry • Year 12 • Module 7 • Lesson 11
Combustion of Alcohols & Comparison with Fossil Fuels
Build HSC Band 5–6 extended-response technique on spirit burner investigations, energy density evaluation, and biofuel lifecycle analysis.
1. Data + scenario — compare and evaluate four-alcohol experiment (Band 5–6)
8 marks Band 5–6
Stimulus. A student burns four primary alcohols using a spirit burner calorimeter (200 g water, same apparatus, same duration for each). The results are shown below.
| Alcohol | M (g/mol) | Δm (g) | ΔT (°C) | Exp. ΔHc (kJ/mol) | Theoretical ΔHc (kJ/mol) |
|---|---|---|---|---|---|
| Methanol | 32.04 | 0.62 | 7.1 | −307 | −726 |
| Ethanol | 46.07 | 0.51 | 7.4 | −559 | −1367 |
| Propan-1-ol | 60.09 | 0.44 | 6.6 | −748 | −2021 |
| Butan-1-ol | 74.12 | 0.39 | 6.2 | −983 | −2676 |
Q1. Analyse and evaluate the data from this practical investigation. In your response you must:
- Verify the ethanol result (show the full three-step calculation using the data in the table).
- Identify and explain the trend in the ratio of experimental to theoretical ΔHc across the four alcohols.
- Identify THREE controlled variables in this experiment and explain why each was kept constant.
- Propose and justify TWO specific reliability improvements, each linked to a named source of systematic error.
- Reach an overall evaluation of whether this spirit burner method is fit for purpose as a comparison of relative ΔHc values.
2. Source critique — evaluate this fuel policy argument (Band 5–6)
7 marks Band 5–6
“Australia should replace all petrol (octane) in passenger cars with pure ethanol immediately. Ethanol is better in every way: it is renewable, it is completely carbon-neutral because it comes from plants, and both fuels produce exactly the same combustion products so there is no energy trade-off whatsoever. Switching to 100% ethanol today would eliminate Australia’s transport CO2 emissions overnight.”
Source: fictional social media post, adapted for this exercise.
Q2. Evaluate this claim. For each underlined assertion, identify whether it is correct, partially correct, or incorrect, and justify using precise chemical reasoning. Then reformulate the claim into a scientifically defensible statement, referencing relevant Australian data where appropriate (e.g. E10 petrol, CSIRO sugarcane bioethanol research).
Q1 — Sample Band 6 response (8 marks), annotated
Verification of ethanol result:
q = 200 × 4.18 × 7.4 = 6186.4 J; n = 0.51 / 46.07 = 0.01107 mol; ΔHc = −(6186.4 ÷ 1000) / 0.01107 = −6.186/0.01107 = −559 kJ/mol ✓ [1 — full working with units at each step]
Ratio trend:
Ratios: Methanol 42.3%, Ethanol 40.9%, Propan-1-ol 37.0%, Butan-1-ol 36.7%. The ratio decreases with increasing chain length — the proportional discrepancy grows. [1 — trend identified with values] Explanation: longer-chain alcohols are harder to completely oxidise in an open spirit burner flame; their combustion is more likely to be oxygen-limited, producing more CO and soot. The yellow, sooty flame of butan-1-ol contrasts with the cleaner, bluer flame of methanol. Greater incomplete combustion for longer chains means a larger fraction of theoretical energy is not released, lowering the experimental/theoretical ratio. [1 — mechanistic explanation linked to incomplete combustion]
Three controlled variables:
(1) Mass of water (200 g for all) — ensures the heat capacity of the water system is identical across trials, so any difference in ΔT directly reflects a difference in q from the alcohol, not from different water masses. (2) Distance between flame and calorimeter base — identical heat transfer geometry; prevents one alcohol from being tested with the flame closer, which would reduce relative heat loss. (3) Duration of burning / target ΔT — comparable burn times mean heat loss rates and the degree of incomplete combustion are similar across trials. [1 — three variables named; 1 — at least two explained with mechanism]
Two reliability improvements:
(1) Draught shield (cardboard surround) addresses heat loss to surroundings from air currents — reduces convective heat loss, increasing qmeasured toward qactual, raising experimental |ΔHc| closer to theoretical. (2) Temperature reading every 30 s with cooling-curve extrapolation addresses underestimation of ΔT from reading the temperature before it peaks — the true maximum ΔT is recovered by back-extrapolating the cooling curve to the moment of flame extinction. [1 — improvement 1 with source; 1 — improvement 2 with source]
Overall evaluation:
The spirit burner method is fit for the limited purpose of comparing relative ΔHc values across the series — it correctly shows the increasing trend with chain length — but is not fit for determining absolute ΔHc values, yielding only 37–43% of theoretical values due to systematic heat loss and incomplete combustion. A bomb calorimeter would be required for accurate absolute values. [1 — explicit evaluative judgement distinguishing relative trend (fit) from absolute accuracy (not fit)]
Marking criteria:
- 1 mark — Ethanol verified: correct calculation showing q, n, and ΔHc with units at each step.
- 1 mark — Ratio trend identified: states the ratio decreases with chain length and provides at least two calculated values.
- 1 mark — Trend explanation: links decreasing ratio to greater incomplete combustion for longer-chain alcohols (more oxygen-limited flame, more CO/soot).
- 1 mark — Three controlled variables named (accept water mass, flame distance, burn duration, calorimeter size).
- 1 mark — At least two of the three controlled variables explained with reference to why controlling them enables a valid comparison.
- 1 mark — Reliability improvement 1 with source and mechanism.
- 1 mark — Reliability improvement 2 with source and mechanism.
- 1 mark — Overall evaluative judgement that distinguishes between fitness for relative comparison vs absolute accuracy, and references a specific limitation or improvement needed.
Q2 — Sample Band 6 response (7 marks), annotated
Claim (a) — “renewable”: Correct. Ethanol produced by fermentation of plant sugars (e.g. Australian sugarcane, as studied by CSIRO) is genuinely renewable — the biomass feedstock regrows on a human timescale. This is a valid advantage over petrol (octane), which is derived from non-renewable crude oil extracted over millions of years. [1 — correct evaluation with reasoning]
Claim (b) — “completely carbon-neutral”: Partially correct but overstated. The carbon cycle argument is real: CO2 released when ethanol burns (C2H5OH + 3O2 → 2CO2 + 3H2O) was previously absorbed from the atmosphere as the sugarcane grew. However, ethanol production requires energy-intensive fermentation and distillation steps, which typically consume fossil fuels (grid electricity, heat). CSIRO lifecycle analysis shows Australian sugarcane ethanol reduces net CO2 by approximately 75% compared to petrol — significant but not “complete” carbon neutrality. [1 — identifies the lifecycle limitation that prevents complete neutrality; 1 — references a correct specific mechanism or Australian data point]
Claim (c) — “same combustion products so no energy trade-off”: Incorrect on the second part. It is true that both ethanol and octane produce only CO2 and H2O in complete combustion — the combustion products are the same. However, the claim that this means “no energy trade-off” is scientifically wrong. Ethanol has an energy density of only 29.7 kJ/g, while octane has 47.9 kJ/g — a 61% disadvantage. This arises because the −OH group in ethanol adds mass (oxygen atom, 16 g/mol) without contributing proportional combustion energy; the oxygen in the molecule is already partially oxidised. A switch to pure ethanol would require approximately 50% more fuel by volume to travel the same distance. [1 — correctly identifies same combustion products; 1 — correctly refutes “no energy trade-off” with specific kJ/g values and structural explanation]
Claim (d) — “eliminate transport CO2 overnight”: Incorrect. Even if ethanol were fully carbon-neutral (which it is not), a switch to 100% ethanol could not happen “overnight” — it would require engine modifications (most current Australian petrol engines are designed for no more than E10), a nationwide bioethanol supply chain scaling far beyond current capacity, and infrastructure changes at all petrol stations. Australia’s E10 programme (introduced progressively from 2003) illustrates that even a 10% blend took years to implement. [1 — at least one practical/logistical refutation]
Defensible reformulation: “Replacing a proportion of Australian petrol with bioethanol derived from sugarcane (e.g. E10 blends) offers a genuine, near-carbon-neutral renewable alternative that can reduce net transport CO2 emissions by approximately 75% per litre of ethanol displaced. However, ethanol’s significantly lower energy density (29.7 vs 47.9 kJ/g) means it cannot replace petrol litre-for-litre, full lifecycle emissions are not zero, and a complete transition would require major infrastructure and engine modification investment. The realistic role of ethanol is as a blended supplement, not a total replacement, for Australian transport fuels.” [1 — reformulation that correctly balances the carbon advantage with the energy density limitation and lifecycle caveat]
Marking criteria:
- 1 mark — Correctly evaluates the renewable claim (correct; explains why ethanol is renewable vs fossil).
- 1 mark — Identifies lifecycle emissions (fermentation/distillation energy input) as limiting “complete” carbon neutrality.
- 1 mark — References a correct mechanism or Australian context (CSIRO data, sugarcane lifecycle, E10 programme).
- 1 mark — Correctly affirms that combustion products are the same (CO2 + H2O) for both fuels.
- 1 mark — Correctly refutes the “no energy trade-off” claim with specific energy densities (29.7 vs 47.9 kJ/g) and a structural explanation (−OH oxygen adds mass).
- 1 mark — Refutes the “overnight” elimination claim with at least one logistical/infrastructural reason.
- 1 mark — Provides a scientifically defensible reformulation that balances carbon cycle advantage, energy density limitation, and lifecycle caveat.