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What are the current and future options for supplying energy?
the definition of a fuel, the distinction between fossil fuels (coal, crude oil, natural gas) and biofuels (bioethanol, biodiesel, biogas), and the comparison of fuels with reference to energy content per unit mass (in kJ g^-1) and energy density per unit volume (in kJ L^-1) and renewability
A focused VCE Chemistry Unit 3 answer on fuels. Covers the definition of a fuel, the fossil fuel vs biofuel distinction, energy content (kJ g^-1) vs energy density (kJ L^-1), and how to compare fuels on energy values and renewability.
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What this dot point is asking
VCAA wants the definition of a fuel, the distinction between fossil fuels and biofuels, and a quantitative comparison of fuels using energy content (kJ per gram) and energy density (kJ per litre), with a comment on renewability.
The answer
A fuel is a substance that releases useful energy when it undergoes combustion (an exothermic reaction with oxygen). The most common fuels in human use are the hydrocarbon fossil fuels and the carbon-based biofuels.
Fossil fuels vs biofuels
| Type | Examples | Source | Renewable? |
|---|---|---|---|
| Fossil fuel | Coal, crude oil (petrol, diesel, kerosene), natural gas (methane) | Geological remains of organisms compressed over millions of years | No. Finite. |
| Biofuel | Bioethanol, biodiesel, biogas | Living biomass: sugar/starch crops, vegetable oils, anaerobic digestion of organic waste | Yes. Crops regrow each season. |
Fossil fuels release CO2 that was locked underground for millions of years, adding net carbon to the atmosphere. Biofuels release CO2 that the source crop absorbed during growth in the same season, so the net carbon contribution is close to zero (ignoring transport and processing emissions). This is the "carbon neutral" argument for biofuels.
Energy content vs energy density
Two different quantities, both used to compare fuels.
- Energy content (also called specific energy) is the energy released per unit mass, usually in kJ g^-1 or MJ kg^-1. Best for comparing fuels by weight, for example in aviation.
- Energy density is the energy released per unit volume, usually in kJ L^-1 or MJ L^-1. Best for comparing fuels by tank size, for example in cars.
Typical values (approximate, for the VCE data book era):
| Fuel | Energy content (kJ g^-1) | Energy density (kJ L^-1) |
|---|---|---|
| Hydrogen (gas) | 142 | 13 (low because gas is not dense) |
| Methane (natural gas) | 56 | 38 (compressed) |
| Petrol (octane) | 48 | 34,200 |
| Diesel | 45 | 38,600 |
| Bioethanol | 30 | 23,500 |
| Biodiesel | 38 | 33,300 |
| Coal (black) | 24 | (varies, solid) |
| Wood (dry) | 16 | (varies, solid) |
Two patterns to lock in:
- More C-H bonds, more energy. Hydrocarbons with little to no oxygen pack more chemical energy per gram than oxygenated fuels (alcohols, biodiesel). Hydrogen tops the list on a per-gram basis.
- Liquid fuels usually win on energy density. A gas spreads out; a liquid is hundreds of times denser. Hydrogen has the highest energy content per gram but the worst energy density unless it is compressed, liquefied or stored chemically.
Comparing fuels: the trade-off
The "best" fuel depends on the application.
- Aviation: mass matters most. Aviation kerosene wins (high kJ g^-1, liquid).
- Cars: energy density matters more (limited tank volume). Petrol and diesel win.
- Heating, electricity: cost and infrastructure dominate. Coal, natural gas and (increasingly) biogas compete.
- Carbon footprint: biofuels win in principle, but their land use and processing energy reduce the advantage.
- Sustainability: only biofuels are renewable on a human timescale.
Worked example
A car holds 60 L of fuel. Compare the energy available from a full tank of petrol (energy density 34,200 kJ L^-1) versus a full tank of bioethanol (energy density 23,500 kJ L^-1).
- Petrol: 60 × 34,200 = 2,052,000 kJ = 2,052 MJ
- Bioethanol: 60 × 23,500 = 1,410,000 kJ = 1,410 MJ
The petrol tank delivers about 1.46 times more energy. A car running on E100 (pure bioethanol) needs to refuel about 1.5 times as often as the same car running on petrol, all else equal. This is why most "E10" and "E85" fuels blend ethanol into petrol rather than replacing it entirely.
Common traps
Confusing energy content with energy density. "kJ per gram" and "kJ per litre" are different. A high-energy-density fuel may have a low energy content (e.g. diesel is denser than petrol so it wins on per-litre but is similar per gram).
Calling biofuels "zero carbon". They are close to carbon neutral in the combustion-vs-photosynthesis balance, but the processing, fertilising and transport stages still emit CO2.
Saying coal is the highest-energy fuel. Coal is dense and cheap, but per gram it releases less energy than petrol or methane because it has fewer C-H bonds (more C and impurities).
Forgetting hydrogen's storage problem. Hydrogen has the highest kJ g^-1 but the lowest kJ L^-1 as a gas. Markers reward the explicit storage-volume point.
In one sentence
A fuel releases energy on combustion, fossil fuels (coal, petrol, natural gas) deliver high energy content (kJ g^-1) and energy density (kJ L^-1) but are non-renewable, while biofuels (bioethanol, biodiesel, biogas) typically have lower energy values but are renewable and close to carbon neutral over a growing season.
Past exam questions, worked
Real questions from past VCAA papers on this dot point, with our answer explainer.
2024 VCE4 marksCompare petrol and bioethanol as transport fuels with reference to energy content, energy density and renewability.Show worked answer →
A 4-mark answer needs the two energy comparisons, the renewability point, and a clear data direction.
- Energy content (per gram). Petrol releases about 48 kJ g^-1; bioethanol releases about 30 kJ g^-1. Petrol releases more energy per gram because every carbon in petrol is bonded to hydrogen, while bioethanol's carbon already carries an oxygen (in the OH group) that has been partly oxidised.
- Energy density (per litre). Petrol releases about 34,200 kJ L^-1; bioethanol about 23,500 kJ L^-1. Petrol stores more energy per litre, so a tank of petrol takes a car further than the same tank of bioethanol.
- Renewability. Bioethanol is renewable: it is fermented from sugar or starch crops (sugar cane, corn) that regrow each season and absorb CO2 as they grow. Petrol is non-renewable: it is distilled from crude oil, which formed over millions of years and is being depleted faster than it forms.
- Synthesis. Petrol wins on energy content and energy density; bioethanol wins on renewability and carbon balance.
Markers reward both numerical comparisons (with the correct direction) and the renewability point made explicitly.
2025 VCE2 marksExplain why methane (CH4) has a higher energy content per gram than ethanol (C2H5OH).Show worked answer →
A 2-mark answer needs the oxidation-state link plus the molecular comparison.
Methane's carbon is fully reduced (bonded only to H), so combustion releases the maximum energy as each C-H bond is replaced by C=O and O-H bonds. Ethanol already contains an O atom (in the OH group), so its carbon framework is partially oxidised before combustion begins. Less energy is released per gram of ethanol because some of the chemical energy has already been "spent" in forming the C-O and O-H bonds.
Roughly, methane releases about 56 kJ g^-1 while ethanol releases about 30 kJ g^-1.
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