Chemistry • Year 12 • Module 7 • Lesson 11
Combustion of Alcohols & Comparison with Fossil Fuels
Apply the three-step calorimetry calculation, interpret energy density data, and analyse the Australian context of biofuels vs fossil fuels.
1. Sequence the steps, spirit burner calorimetry calculation
The six steps of a ΔHc calculation are listed below in the wrong order. Write the correct order (1–6) in the “Order” column. 6 marks (1 each)
| Order | Step |
|---|---|
| Divide q (in kJ) by n to obtain ΔHc in kJ/mol; apply negative sign. | |
| Record the initial and final mass of the spirit burner to determine Δm (mass of fuel burned). | |
| Calculate q = mcΔT where m is the mass of water, c = 4.18 J g−1 °C−1, and ΔT = Tfinal − Tinitial. Result is in joules. | |
| Convert q from joules to kilojoules by dividing by 1000. | |
| Calculate n = Δm / M to find the moles of alcohol burned (M = molar mass of the alcohol). | |
| Record the initial and final temperature of the water to determine ΔT (°C). |
2. Interpret the energy density graph
The bar chart below shows the energy density (kJ/g) for five primary alcohols and two fossil fuels. Use the data to answer the questions. 8 marks
Energy density (kJ/g) = |ΔHc| ÷ molar mass. Values from lesson Card 04. Methane and octane represent natural gas and petrol respectively.
2.1 Describe the trend in energy density across the five alcohols (methanol to pentan-1-ol). 2 marks
2.2 Using bond chemistry, explain why energy density increases as alcohol chain length increases. 3 marks
2.3 Compare the energy density of ethanol with that of octane. What practical consequence does this difference have for a car running on pure ethanol versus petrol? 3 marks
3. Cause-and-effect chain, E10 petrol in Australia
E10 petrol sold across Australia contains 10% bioethanol (produced by fermenting sugarcane) blended with 90% petrol. Complete the cause-and-effect chain below by filling in the empty boxes. 5 marks (1 per box)
4. Case study, Qantas sustainable aviation fuel (SAF)
In 2022 Qantas completed the world’s first passenger flight using 10% sustainable aviation fuel (SAF) blended with conventional jet fuel on a Sydney–Melbourne route. SAF can be produced from ethanol derived from agricultural waste. Conventional aviation jet fuel (Jet-A) has an energy density of approximately 43.2 kJ/g; ethanol has an energy density of 29.7 kJ/g. 5 marks
4.1 Calculate the percentage difference in energy density between Jet-A fuel and ethanol. 2 marks
4.2 Explain one reason why Qantas uses only a 10% SAF blend rather than 100% SAF on commercial flights, using your calculation in 4.1. 2 marks
4.3 Identify one environmental advantage of using a 10% SAF blend over 100% Jet-A fuel on the same route. 1 mark
Q1, Sequence the steps
Correct order: 6, 2, 3, 4, 5, 1
Step 1: Record initial/final temperatures (ΔT). Step 2: Record initial/final spirit-burner masses (Δm). Step 3: Calculate q = mcΔT (joules). Step 4: Convert q to kJ (÷1000). Step 5: Calculate n = Δm/M. Step 6: Calculate ΔHc = −q/n (kJ/mol).
Q2.1, Trend in energy density
Energy density increases steadily from methanol (22.7 kJ/g) to pentan-1-ol (37.8 kJ/g) as chain length increases. The increase is approximately 3–4 kJ/g per additional CH2 unit, but with diminishing increments (the values converge toward the alkane ceiling as chain length grows).
Marking notes: 1 mark for stating an increasing trend with reference to at least one data value; 1 mark for noting it applies across the C1–C5 series or identifying a pattern in the increments.
Q2.2, Bond chemistry explanation (3 marks)
Each additional CH2 unit adds two C–H bonds and one C–C bond [1]. When these bonds are combusted, new C=O bonds (in CO2) and O–H bonds (in H2O) form; the energy released forming these bonds exceeds the energy required to break the C–H and C–C bonds, releasing approximately 650 kJ/mol net per CH2 [1]. The molar mass only increases by 14 g/mol per CH2, so the ratio |ΔHc|/M (energy per gram) also increases with chain length [1].
Q2.3, Ethanol vs octane comparison (3 marks)
Ethanol has an energy density of 29.7 kJ/g while octane has 47.9 kJ/g, octane delivers approximately 61% more energy per gram than ethanol [1]. A car running on pure ethanol would need approximately 50% more fuel by volume to travel the same distance as a car running on petrol [1]. This means larger fuel tanks, more frequent refuelling, and higher mass of fuel carried, offsetting some of the economic and environmental advantages of ethanol [1].
Q3, E10 cause-and-effect chain
Effect 1: solar energy (from photosynthesis / chemical potential energy)
Effect 2: CO2
Effect 3: was originally absorbed from the atmosphere during photosynthesis, so it is “recycled” atmospheric CO2, not a net addition
Effect 4: near-carbon-neutral (or carbon-neutral)
Overall outcome: the CO2 it releases was previously removed from the atmosphere by the growing sugarcane, making it a partial renewable replacement that can reduce net CO2 emissions compared to 100% fossil petrol
Marking notes: 1 mark per box. Accept equivalent phrasings.
Q4, Qantas SAF case study
4.1 Percentage difference = (43.2 − 29.7) / 43.2 × 100 = 13.5/43.2 × 100 = 31.3% lower energy density for ethanol compared to Jet-A. [1 mark calculation, 1 mark correct direction and unit]
4.2 A 100% SAF blend would require approximately 30% more fuel volume per flight to deliver the same total energy as Jet-A [1]. This would increase aircraft weight, reduce range, and require engine modifications designed for a lower-energy-density fuel blend, making it impractical for current commercial long-haul operations [1].
4.3 A 10% SAF blend reduces net CO2 emissions compared to 100% Jet-A because approximately 10% of the fuel’s CO2 output is near-carbon-neutral (recycled from the atmosphere during biomass growth), lowering the net carbon addition per flight. [1 mark]