HSC Chemistry Module 7: Fossil Fuels vs Biofuels
"Compare and contrast fuels from organic sources to biofuels, including ethanol."
| Time | Strategy | What to Read |
|---|---|---|
| 5 min | Last-minute cram | TL;DR + Cheat Sheet |
| 20 min | Pre-assessment | Part 3: Ethanol + Part 4: Comparison |
| Got an hour? | Full guide | Start to finish — every section builds |
TL;DR — The Core Principle
Fossil fuels are non-renewable hydrocarbons (C and H only) with high energy density; biofuels are renewable oxygenated organic compounds derived from biomass with lower energy density but lower net CO₂ emissions. Ethanol is the star because it can be produced from both renewable (fermentation) and non-renewable (hydration of ethylene) sources.
| Property | Fossil Fuels | Biofuels |
|---|---|---|
| Source | Earth's crust (millions of years) | Biomass (grown in months) |
| Renewability | Non-renewable | Renewable |
| Composition | Hydrocarbons (C, H only) | Oxygenated organics (C, H, O) |
| Energy density | Higher (octane: 47.8 kJ/g) | Lower (ethanol: 29.6 kJ/g) |
| Net CO₂ | High — accumulates permanently | Lower — partially offset by photosynthesis |
Energy density (kJ/g), highest to lowest:
Natural gas (53.6) > Octane/petrol (47.8) > Diesel (42.6) > Biodiesel (37.2) > Propanol (33.6) > Ethanol (29.6) > Methanol (22.7)
Exam Verb Strategy
| NESA Verb | What markers want | Similarities? | Judgement? |
|---|---|---|---|
| Compare and contrast | BOTH similarities AND differences with specific data | Yes — most students lose marks here | No |
| Assess | Advantages + disadvantages + your judgement | Not required | Yes — must include |
| Explain | Cause → effect chain with chemical reasoning | Only if relevant | No |
| Discuss | Present multiple viewpoints with evidence | Helpful | Recommended |
They list differences only and forget that "compare" means you must also discuss similarities — e.g., both undergo combustion, both produce CO₂ and H₂O, both are transport fuels.
What Are Fossil Fuels?
Every time you fill up a car with petrol, you're burning the remains of organisms that died hundreds of millions of years ago. That tank took nature ~300 million years to produce — and you'll burn through it in a week.
A fossil fuel is a fuel formed from the anaerobic decomposition of dead organisms over millions of years under heat and pressure deep within the Earth's crust. They are classified as non-renewable — we consume them far faster than they can be replaced.
| Fuel | Main Component | Formula | State | Energy (kJ/g) |
|---|---|---|---|---|
| Natural gas | Methane | CH₄ | Gas | 53.6 |
| LPG | Propane/Butane | C₃H₈ / C₄H₁₀ | Liquefied gas | ~49.5 |
| Petrol | Octane (representative) | C₈H₁₈ | Liquid | 47.8 |
| Diesel | Long-chain alkanes | ~C₁₂H₂₆ | Liquid | 42.6 |
| Coal | Complex C structures | Variable | Solid | 9.8–27.9 |
All fossil fuels are hydrocarbons — molecules made of carbon and hydrogen only. Because they contain no oxygen, they require a large supply of external O₂ for complete combustion.
Each mole of octane requires 12.5 mol O₂ (from 25/2). This massive O₂ demand means incomplete combustion is common, producing toxic CO and soot (C).
What Are Biofuels?
What if instead of digging up ancient carbon, we could grow our fuel in a field and harvest it every season? That's the promise of biofuels — but it comes with trade-offs that markers love to test.
A biofuel is a fuel derived from biomass — biological material from living or recently living organisms. Because the source organisms can be regrown in months, biofuels are classified as renewable.
| Biofuel | Source | Production Method | Formula |
|---|---|---|---|
| Bioethanol | Sugar cane, corn, wheat | Fermentation of glucose | C₂H₅OH |
| Biodiesel | Vegetable oils, animal fats | Transesterification with methanol | Long-chain esters |
| Biogas | Organic waste, manure | Anaerobic digestion | CH₄ (+ CO₂) |
Unlike fossil fuels (C and H only), biofuels are oxygenated organic compounds — they contain oxygen in their molecular structure. This drives several consequences:
- Less O₂ needed for complete combustion → cleaner burning
- Lower energy density per gram (C–O bond stores less energy than C–H or C–C bonds)
- Different functional groups → different chemical properties
Ethanol — The Star of IQ4
The syllabus specifically names ethanol because it's the only common fuel that can be produced from both renewable AND non-renewable sources — making it the perfect "compare and contrast" molecule.
3.1 Two Ways to Produce Ethanol
Source: Biomass — sugar cane, corn, wheat
Feedstock: Glucose (C₆H₁₂O₆)
Catalyst: Yeast (biological enzyme)
Conditions: ~37°C, anaerobic, dilute solution
Rate: Slow (batch process, takes days)
Yield: Low (~15% before yeast dies)
Purity: Low — requires fractional distillation
Source: Petroleum — ethylene from cracking
Feedstock: Ethylene (CH₂=CH₂)
Catalyst: H₃PO₄ (phosphoric acid)
Conditions: 300°C, 70 atm, high pressure
Rate: Fast (continuous process)
Yield: High (>95% conversion)
Purity: High — relatively pure product
| Feature | Fermentation | Hydration |
|---|---|---|
| Source | Biomass (renewable) | Petroleum (non-renewable) |
| Catalyst | Yeast | H₃PO₄ |
| Conditions | ~37°C, anaerobic | 300°C, 70 atm |
| Process | Batch (slow, days) | Continuous (fast) |
| Yield | Low (~15%) | High (>95%) |
| Purity | Low — needs distillation | High — relatively pure |
| Renewability | ✓ Renewable | ✗ Non-renewable |
"Which compound can be derived both from fossil fuels and from biomass materials?" Answer: Ethanol. This catches students who only associate ethanol with fermentation.
3.2 The Full Bioethanol Production Process
Understanding the full production chain is critical for explaining why bioethanol is commercially and economically limited.
Plant cell walls contain cellulose — a complex polysaccharide of many glucose units. Breaking it down requires acid digestion/hydrolysis — extremely time-consuming and inefficient. This is the biggest bottleneck.
The problem: yeast controls this reaction, not humans. We cannot speed it up or optimise the rate. It takes days and produces only ~15% ethanol before yeast dies.
The dilute ethanol must be purified via fractional distillation — demanding enormous energy input. The energy required approaches the amount the resulting ethanol produces when combusted → extremely low net energy efficiency.
Steps 1 & 2 are inefficient → cannot be mass produced → commercially NOT viable.
Step 3 costs too much energy → economically NOT feasible.
These are the two core reasons bioethanol hasn't replaced fossil fuels.
3.3 The Ethanol Carbon Cycle
This is the most important concept for IQ4, and where students most commonly lose marks by oversimplifying.
- CO₂ absorbed: 6 mol (photosynthesis)
- CO₂ released: 2 mol (fermentation) + 4 mol (combustion) = 6 mol
- Net CO₂ = 0 → Theoretically carbon neutral
Many students conclude ethanol is "carbon neutral." This is incorrect in practice.
The production process (cultivation, harvesting, fertiliser, distillation, transport) requires fossil fuel energy, adding CO₂ that is NOT offset by photosynthesis.
Result: net emissions are 20–50% lower than petrol — significant, but far from zero.
3.4 Combustion — Ethanol vs Octane
Both undergo complete combustion to produce CO₂ and H₂O (this is a similarity). But there are critical quantitative differences:
Ethanol (C₂H₅OH)
ΔHc = −1367 kJ/mol | 29.6 kJ/g
3 mol O₂ per mol
Contains O atom ✓
Octane (C₈H₁₈)
ΔHc = −5470 kJ/mol | 47.8 kJ/g
12.5 mol O₂ per mol
No O atom ✗
- Ethanol already contains an oxygen atom in its structure
- Requires less external O₂ (3 mol vs 12.5 mol per mol)
- In a car engine, ethanol is more likely to achieve complete combustion
- Octane's massive O₂ demand means frequent incomplete combustion → toxic CO and soot
The Big Comparison
If you're writing an extended response, this is your blueprint. Every row is a potential mark.
| Property | Fossil Fuels | Biofuels (Bioethanol) |
|---|---|---|
| Composition | Hydrocarbons (C & H only) | Oxygenated organics (contains O) |
| Source | Mining/drilling (millions of years) | Agricultural crops (months) |
| Renewability | Non-renewable | Renewable — regrows via photosynthesis |
| Energy (kJ/g) | Petrol: 47.8, Natural gas: 53.6 | Bioethanol: 29.6, Biodiesel: 37.2 |
| CO₂ emissions | High — accumulates permanently | Lower — 20–50% less than petrol |
| Combustion | Needs more O₂ → more incomplete | Contains O → cleaner combustion |
| Vehicle | No modification | E10: no mod, E85+: modification needed |
- Both undergo combustion to release energy as heat — both are exothermic fuels
- Both produce CO₂ and H₂O as complete combustion products
- Both used as transport fuels — petrol directly, ethanol blended as E10
- Both are carbon-based organic compounds
- Both can undergo incomplete combustion producing CO and soot
| Aspect | Fossil Fuels | Biofuels |
|---|---|---|
| Renewability | Non-renewable | Renewable |
| Time to form | Millions of years | Months |
| Molecular O | No O in structure | Contains O |
| Energy/gram | 47.8 kJ/g (octane) | 29.6 kJ/g (ethanol) — 34% less |
| Net CO₂ | "New" CO₂ to atmosphere | Partially offset by photosynthesis |
| O₂ demand | 12.5 mol O₂/mol octane | 3 mol O₂/mol ethanol |
Advantages & Disadvantages
For any "assess" question you need advantages, disadvantages, and a judgement.
Biofuels from biomass can be regrown within months via photosynthesis, unlike fossil fuels which take millions of years.
The three-step carbon cycle (photosynthesis → fermentation → combustion) is theoretically balanced. In practice, net emissions are 20–50% lower than petrol.
Ethanol needs only 3 mol O₂ vs octane's 12.5 mol O₂. Less O₂ demand → more complete combustion → less CO and soot.
Biofuels as a petrol supplement improve national energy security and reduce exposure to volatile oil prices.
Ethanol's octane rating (~108) exceeds regular petrol (~91–98), reducing engine knock.
This nuanced point is specifically rewarded by markers.
| Reaction | CO₂ |
|---|---|
| Photosynthesis | −6 mol (absorbed) |
| Fermentation | +2 mol (released) |
| Combustion | +4 mol (released) |
| Net (theoretical) | 0 mol |
However, the production chain (cultivation, harvesting, fertiliser, distillation, transport) adds CO₂ from fossil fuels that is not offset.
Conclusion: Bioethanol is NOT truly carbon neutral. Net emissions are ~20–50% lower than petrol — significant, but far from zero.
Ethanol: 29.6 kJ/g vs Octane: 47.8 kJ/g — approximately 34% less energy per gram.
Crops for bioethanol compete with food production, linked to rising food prices and shortages in developing nations.
Acid hydrolysis and fermentation are both fundamentally inefficient. Bioethanol cannot be mass produced with current technology.
Fractional distillation requires enormous energy input approaching the energy the ethanol produces → low net energy efficiency.
Requires vast arable land, causing soil erosion, deforestation, and loss of habitats.
E10 is fine, but E20/E85 requires engine modifications (seals, fuel lines, injectors).
Current Reality
Bioethanol's limitations far outweigh its advantages right now. It cannot be mass produced (commercially not viable), production costs are prohibitive (economically not feasible), and it has 34% less energy per gram. This is why it's used only as a supplement (E10), not a replacement.
Future Potential
Bioethanol shows strong potential. Its advantages — renewability, cleaner combustion, reduced greenhouse impact — would outweigh the disadvantages if research develops more efficient cellulose-to-glucose conversion and reduces distillation costs.
"Assess the suitability of ethanol as a fuel" = focus on current limitations.
"Assess the potential of biofuels as an alternative" = acknowledge limitations but emphasise future potential.
Getting this framing right = Band 4 → Band 6.
Band 6 Boosters BAND 6
Drop one of these into an extended response to separate a Band 5 from Band 6.
Second-generation bioethanol uses lignocellulosic biomass (corn stalks, wheat straw, switchgrass) instead of food crops — solving the food-vs-fuel dilemma.
An LCA evaluates total environmental impact "from cradle to grave." For biofuels it reveals that the production phase (farming, fertiliser, distillation) contributes the majority of greenhouse gas emissions. Net reduction is 20–50%.
E85 = 85% ethanol + 15% petrol. Requires flex-fuel vehicles (FFVs) with modified fuel systems. Lower CO/particulate emissions but ~30% more fuel consumption per km.
Biogas = anaerobic digestion of organic waste by methanogenic bacteria. Primary component: methane (CH₄). Addresses waste management and energy production simultaneously.
Interactive Quiz Zone
Test yourself — attempt each question before revealing the answer. Your score is tracked!
Extended Response Questions
Attempt each question on paper before revealing the model answer.
The use of ethanol as an alternative fuel has been proposed because it can be obtained from renewable resources by fermentation and it also burns more cleanly than petrol. With the aid of chemical equations, explain these two properties of ethanol.
Renewable: Ethanol is produced via fermentation of glucose from crops (regrown via photosynthesis):
Cleaner combustion: Ethanol contains an O atom, requiring only 3 mol O₂ vs 12.5 mol for octane:
Mark 1: Renewable source + fermentation equation | Mark 2: Cleaner combustion (O atom → less O₂) | Mark 3: Both combustion equations compared
Compare the two industrial methods for producing ethanol. Include relevant chemical equations and evaluate which method is more sustainable.
Fermentation (Renewable): Glucose from biomass → ethanol by yeast at ~37°C, anaerobic:
Batch process, slow, ~15% yield, needs distillation.
Hydration of Ethylene (Non-renewable): Ethylene + steam with H₃PO₄ at 300°C, 70 atm:
Continuous process, >95% yield, relatively pure, but non-renewable feedstock.
Evaluation: Fermentation is more sustainable — biomass is renewable and CO₂ is partially offset by photosynthesis.
Mark 1: Fermentation equation + conditions | Mark 2: Hydration equation + conditions | Mark 3: Comparison (rate, yield, efficiency) | Mark 4: Sustainability evaluation
Explain TWO advantages and TWO disadvantages of using bioethanol as an alternative to a fossil fuel.
ADV 1 — Renewable & lower CO₂: Crops regrow via photosynthesis, absorbing CO₂. Net emissions 20–50% lower than petrol.
ADV 2 — Cleaner combustion: O atom in C₂H₅OH → only 3 mol O₂ needed → less CO and soot.
DIS 1 — Lower energy density: 29.6 kJ/g vs 47.8 kJ/g → 34% less energy per gram.
DIS 2 — Food vs fuel: Sugar cane, corn compete with food production → rising food prices.
1 mark per point. Each needs a mechanism (why) and a number (how much).
Assess the suitability of ethanol as a fuel.
Ethanol (C₂H₅OH) is a biofuel from fermentation of sugar cane/corn, blended as E10.
Advantages: Renewable (crops regrow via photosynthesis). Three-step carbon cycle is theoretically balanced (6 CO₂ in = 6 out). Burns cleaner due to O atom.
Disadvantages: NOT truly carbon neutral (production uses fossil fuels, net 20–50% lower). Lower energy density (29.6 vs 47.8 kJ/g). Food vs fuel conflict.
Judgement: Ethanol is a promising but currently limited alternative fuel — best used as a supplement (E10) rather than replacement. Future cellulosic ethanol may overcome limitations.
Mark 1: Define | Mark 2: ADV (renewable + 3 equations) | Mark 3: ADV (cleaner combustion) | Mark 4: DIS (not neutral + energy + food) | Mark 5: Judgement
Compare and contrast fuels from organic sources to biofuels, including ethanol.
Similarities: Both are carbon-based organics that undergo combustion producing CO₂ + H₂O. Both used as transport fuels (petrol directly, ethanol as E10).
Differences: Fossil fuels = non-renewable hydrocarbons (C & H only). Biofuels = renewable oxygenated organics (contain O). Energy: octane 47.8 kJ/g vs ethanol 29.6 kJ/g (34% less). Fossil CO₂ accumulates permanently; bioethanol CO₂ partially offset (but NOT neutral, 20–50% lower).
Ethanol: Unique — produced from both fermentation (renewable) and hydration of ethylene (non-renewable). Burns more cleanly (3 mol O₂ vs 12.5 mol).
Mark 1: Similarity (combustion + equations) | Mark 2: Similarity (transport fuels) | Mark 3: Difference (renewability) | Mark 4: Difference (energy density) | Mark 5: Difference (CO₂) | Mark 6: Ethanol dual-source + cleaner combustion
Octane produces 1.554 × 10⁷ kJ per tonne of CO₂. ΔHc of ethanol = 1367 kJ/mol. Calculate the energy per tonne of CO₂ from ethanol.
Step 1: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O → 1 mol ethanol : 2 mol CO₂
Step 2: n(CO₂) in 1 tonne = 1,000,000 ÷ 44.01 = 22,722 mol
Step 3: n(ethanol) = 22,722 ÷ 2 = 11,361 mol
Step 4: Energy = 11,361 × 1367 = 1.553 × 10⁷ kJ per tonne CO₂
Insight: Almost identical to octane (1.553 vs 1.554 × 10⁷ kJ)! While ethanol has less energy per gram, per tonne of CO₂ they're remarkably similar. The difference: ethanol's CO₂ is partially offset by photosynthesis.
Final Revision Cheat Sheet
Review this 60 seconds before the exam.
Don't Write This / Write This Instead
Key Equations — Know All Six
Quick-Reference Traps
- "Ethanol is carbon neutral" — NO. Theoretically balanced, but production uses fossil fuels.
- "Compare = just list differences" — NO. Must include similarities.
- "Ethanol only from fermentation" — NO. Also from hydration of ethylene.
- "12.5 mol O₂" — This is per mole of octane (from 25/2). Don't confuse per mol vs per 2 mol.
- "Biofuels have no disadvantages" — WRONG. Lower energy density, food vs fuel, land use, engine modification, distillation costs.
Ready to take your HSC Chemistry to the next level?
At SKY HSC College, we don't just teach chemistry — we teach you how to earn marks. Our exam-strategy approach has helped hundreds of students achieve Band 6 results.
Book a Free Trial Lesson