Module 7 IQ4: Fossil Fuels vs Biofuels

Theory · Exam Questions · Model Answers · Marking Criteria

NESA Stage 6 Chemistry — Module 7: Organic Chemistry, Inquiry Question 4
"Compare and contrast fuels from organic sources to biofuels, including ethanol."

Every year, students walk into the HSC Chemistry exam confident they know the difference between fossil fuels and biofuels — and every year, a significant number lose marks. Why? Because they list only differences and forget that "compare and contrast" demands both similarities AND differences.

Estimated reading: 45 min 10 Practice MCQs + 6 Worked Extended Responses Interactive Diagrams & Flashcards

TL;DR — The Core Principle

Fossil fuels are non-renewable hydrocarbonsMolecules containing only carbon and hydrogen atoms (C and H) (C and H only) with high energy density; biofuels are renewable oxygenated organic compoundsOrganic molecules that contain oxygen (O) in addition to carbon and hydrogen derived from biomass with lower energy density but lower net CO₂ emissions. Ethanol is the star of this dot point because it can be produced from both renewable (fermentation) and non-renewable (hydration of ethylene) sources.

Quick Comparison

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 Ranking (kJ/g) — Memorise This Once

How to Use This Guide

Time	Strategy	What to Read
5 min	Last-minute cram	TL;DR + Cheat Sheet (bottom)
20 min	Pre-assessment	Part 3: Ethanol + Part 4: Comparison
Got an hour?	Full guide	Start to finish — every section builds on the last

Exam Verb Strategy — What Markers Actually Want

NESA Verb	What markers want	Similarities?	Judgement?
Compare and contrast	Identify BOTH similarities AND differences using specific data	Yes — this is where most lose marks	No
Assess	Advantages + disadvantages + your judgement with data	Not required but strengthens response	Yes — must include
Explain	Cause → effect chain with chemical reasoning	Only if relevant	No
Discuss	Present multiple viewpoints with evidence	Helpful	Recommended

#1 reason students lose marks on IQ4: They write a list of 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.

Sentence Template Scaffold

Use this structure for any comparison point:

"Both [fossil fuels] and [biofuels] [SIMILARITY], however [fossil fuels] [DIFFERENCE 1] while [biofuels] [DIFFERENCE 2]. For example, [SPECIFIC DATA]."

See worked example of this template

"Both fossil fuels and biofuels undergo complete combustion to produce CO₂ and H₂O, however octane requires 12.5 mol O₂ per mole (from 2C₈H₁₈ + 25O₂) while ethanol requires only 3 mol O₂ per mole, as ethanol already contains an oxygen atom in its molecular structure. This means ethanol is more likely to undergo complete combustion, producing less toxic CO and soot."

Part 1: 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 of fuel took nature ~300 million years to produce — and you'll burn through it in a week. That's the fundamental problem with fossil fuels.

1.1 Definition and Formation

A fossil fuelA fuel formed from anaerobic decomposition of dead organisms over millions of years under heat and pressure 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. Because they form on geological timescales (10⁶–10⁸ years), they are classified as non-renewable.

1.2 Types of Fossil Fuels

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

1.3 Chemical Composition — The Key Detail

All fossil fuels are hydrocarbons — molecules made of carbon and hydrogen only. Because they contain no oxygen in their molecular structure, they require a large supply of external O₂ for complete combustion.

Complete combustion of octane: 2C₈H₁₈(l) + 25O₂(g) → 16CO₂(g) + 18H₂O(l) — ΔH_c = −5470 kJ/mol

Each mole of octane requires 12.5 mol O₂ (from 25/2). This massive oxygen demand means incomplete combustion is common in real engines, producing toxic carbon monoxide (CO) and soot (C).

Fractional Distillation Column

Crude oil is separated into fractions by heating to ~350°C and passing through a fractionating tower where temperature decreases from bottom to top.

Self-Check: Can you write the balanced equation for complete combustion of methane with state symbols?

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) — ΔH_c = −890 kJ/mol

Part 2: 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.

2.1 Definition

A biofuelA fuel derived from biomass — biological material from living or recently living organisms is a fuel derived from biomass — biological material from living or recently living organisms. Because the source organisms can be regrown in months to years, biofuels are classified as renewable.

2.2 Types of Biofuels

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)

2.3 Chemical Composition — The Key Difference

Unlike fossil fuels (C and H only), biofuels are oxygenated organic compounds — they contain oxygen in their molecular structure. This single difference drives several exam-relevant consequences:

Less O₂ needed for complete combustion → cleaner burning
Lower energy density per gram (the C–O bond stores less energy than C–H or C–C bonds)
Different functional groups → different chemical properties

Self-Check: Why does the presence of oxygen in ethanol's structure lead to cleaner combustion?

Ethanol (C₂H₅OH) already contains an oxygen atom, so it requires less external O₂ (only 3 mol vs 12.5 mol for octane). In an engine where air supply is limited, ethanol is more likely to achieve complete combustion, producing less toxic CO and soot.

Part 3: Ethanol — The Star of IQ4

The syllabus dot point specifically names ethanol. Why? Because ethanol is the only common fuel that can be produced from both renewable AND non-renewable sources — making it the perfect molecule for a "compare and contrast" question. If you only write about one production method, you're leaving marks on the table.

3.1 Two Ways to Produce Ethanol

Feature	Fermentation (Renewable)	Hydration of Ethylene (Non-renewable)
Source	Biomass — sugar cane, corn, wheat	Petroleum — ethylene from cracking
Feedstock	Glucose (C₆H₁₂O₆)	Ethylene (CH₂=CH₂)
Catalyst	Yeast (biological enzyme)	H₃PO₄ (phosphoric acid)
Conditions	~37°C, anaerobic, dilute solution	300°C, 70 atm, high pressure
Rate	Slow (batch process, days)	Fast (continuous process)
Yield	Low (~15% before yeast dies)	High (>95% conversion)
Purity	Low — requires distillation	High — relatively pure
Renewability	Renewable	Non-renewable
Scale	Labour-intensive	Few workers, automated

Fermentation: C₆H₁₂O₆(aq) → 2C₂H₅OH(l) + 2CO₂(g) Conditions: yeast, ~37°C, anaerobic

Hydration of ethylene: CH₂=CH₂(g) + H₂O(g) → C₂H₅OH(l) Conditions: H₃PO₄ catalyst, 300°C, 70 atm

HSC MC Trap: "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 — Why It's So Difficult

Understanding the full production chain is critical for explaining why bioethanol is commercially and economically limited:

Cellulose → Glucose

Plant cell walls contain cellulose — a complex polysaccharide made of many glucose units. Breaking cellulose down into usable glucose requires acid digestion/hydrolysis — an extensive chemical treatment that is extremely time-consuming and inefficient. This is the biggest bottleneck: we simply do not have an efficient method to convert cellulose into glucose at scale.

Glucose → Aqueous Ethanol

Yeast converts glucose to ethanol and CO₂. The problem: yeast controls this reaction, not humans. We cannot speed it up or optimise it. Fermentation is a batch process that takes days, produces only ~15% ethanol before yeast dies from alcohol toxicity, and is inherently inefficient.

C₆H₁₂O₆(aq) → 2C₂H₅OH(l) + 2CO₂(g) — yeast, ~37°C, anaerobic

Aqueous Ethanol → Pure Ethanol

The dilute aqueous ethanol must be purified via fractional distillation — a process that demands enormous energy input. The energy required approaches the amount the ethanol can produce when combusted — creating extremely low net energy efficiency.

Why this matters for exams: Inefficiency of Steps 1 & 2 → bioethanol cannot be mass produced → commercially NOT viable. Massive energy cost of Step 3 → production is extremely expensive → economically NOT feasible.

Production Flowcharts

3.3 The Ethanol Carbon Cycle — Theoretically Neutral, Practically NOT

This is the most important concept for this dot point. Three reactions form a theoretical cycle:

The Maths:

Reaction	CO₂
Photosynthesis	−6 mol (absorbed)
Fermentation	+2 mol (released)
Combustion	+4 mol (released)
Net	0 mol (theoretically)

CRITICAL — Do NOT stop here. The production process requires additional energy from fossil fuels: cultivation (tractors), harvesting (machinery), fertiliser production (Haber process), distillation (heating), and transportation. This adds CO₂ not offset by photosynthesis. Net emissions are approximately 20–50% lower than petrol — far from zero.

The exam-safe statement (copy this)

"While the ethanol carbon cycle is theoretically balanced (6 mol CO₂ absorbed = 6 mol CO₂ released), bioethanol is NOT truly carbon neutral because the production process — including cultivation, harvesting, fertiliser production, distillation, and transportation — requires energy from fossil fuels, adding CO₂ that is not offset by photosynthesis. Nevertheless, net emissions are approximately 20–50% lower than those of petrol."

3.4 Combustion — Ethanol vs Octane

Ethanol combustion: C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l) — ΔH_c = −1367 kJ/mol (29.6 kJ/g)

Octane combustion: 2C₈H₁₈(l) + 25O₂(g) → 16CO₂(g) + 18H₂O(l) — ΔH_c = −5470 kJ/mol (47.8 kJ/g)

Property	Ethanol	Octane (Petrol)
Formula	C₂H₅OH	C₈H₁₈
Molar mass	46.07 g/mol	114.23 g/mol
ΔH_c	−1367 kJ/mol	−5470 kJ/mol
Energy density	29.6 kJ/g	47.8 kJ/g
O₂ required per mol	3 mol	12.5 mol
Contains O atom?	Yes	No
Combustion cleanliness	Cleaner — less CO, soot	Dirtier — more incomplete combustion

Why Ethanol Burns More Cleanly

Ethanol (C₂H₅OH) already contains an oxygen atom
Therefore requires less external O₂ (3 mol vs 12.5 mol)
In a car engine where air supply is limited, ethanol is more likely to achieve complete combustion
Octane's massive O₂ demand means it frequently undergoes incomplete combustion → toxic CO and soot

Exam-ready sentence for "Why ethanol burns more cleanly"

"Ethanol burns more cleanly than octane because it contains an oxygen atom in its molecular structure (C₂H₅OH), requiring only 3 mol O₂ for complete combustion compared to 12.5 mol O₂ per mole of octane. This lower oxygen demand means ethanol is less likely to undergo incomplete combustion, producing significantly less toxic carbon monoxide and soot."

Heat of Combustion Comparison

Part 4: The Big Comparison — Fossil Fuels vs Biofuels

If you're writing an extended response, this table is your blueprint. Every row is a potential mark. A 6-mark "compare and contrast" question typically needs 3 similarities + 3 differences, each supported by specific data.

Master Comparison Table

Property	Fossil Fuels	Biofuels (Bioethanol)
Chemical composition	Hydrocarbons (C & H only) — e.g., C₈H₁₈	Oxygenated organics — e.g., C₂H₅OH (contains O)
Source	Mining/drilling (formed over millions of years)	Agricultural crops (grown in months)
Renewability	Non-renewable — will be depleted	Renewable — biomass regrows via photosynthesis
Energy content	Petrol: 47.8, Natural gas: 53.6 kJ/g	Bioethanol: 29.6 kJ/g (~34% less)
CO₂ emissions	High — CO₂ accumulates permanently	Lower — partially offset by photosynthesis (20–50% less)
Combustion quality	Needs more O₂ → prone to incomplete combustion	Contains O atom → cleaner, less CO and soot
Vehicle compatibility	No modification required	E10: no mods. E85+: engine mods needed
Cost / availability	Established infrastructure, volatile pricing	Supplementary only (~10% of fuel mix)
Environmental issues	Oil spills, climate change, air pollution	Deforestation for crops, food vs fuel

4.1 Similarities — Don't Forget These!

This is where most students lose marks. When the question says "compare and contrast," you must include similarities:

Both undergo combustion to release energy as heat — both are exothermic fuels
Both produce CO₂ and H₂O as complete combustion products
Both are used as transport fuels — petrol directly, bioethanol blended as E10
Both are carbon-based organic compounds — molecular substances containing carbon
Both can undergo incomplete combustion when O₂ is limited → CO and soot (biofuels less prone)

4.2 Key Differences

Aspect	Fossil Fuels	Biofuels
Renewability	Non-renewable (finite)	Renewable (regrows)
Time to form	Millions of years	Months (crop cycle)
Molecular oxygen	No O in structure	Contains O (C₂H₅OH)
Energy per gram	47.8 kJ/g (octane)	29.6 kJ/g (ethanol) — ~34% less
Net CO₂ impact	All CO₂ is "new" to atmosphere	Partially offset by photosynthesis
O₂ demand	12.5 mol O₂/mol octane	3 mol O₂/mol ethanol
Production source	Geological extraction	Agricultural cultivation

Part 5: Advantages & Disadvantages of Biofuels

For any question using the verb "assess", you need structured advantages, disadvantages, and a judgement. Each point below includes an equation or data point and a ready-to-use exam sentence.

Advantages

ADV 1 — Renewable Resource

Biofuels are derived from biomass (sugar cane, corn) which can be regrown within months via photosynthesis. Unlike fossil fuels (millions of years), biofuel feedstocks are replenished on human timescales.

"Bioethanol is a renewable fuel because its source — glucose from crops such as sugar cane — is continuously replenished through photosynthesis: 6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g)."

ADV 2 — Lower Net CO₂ Emissions (20–50% less)

The carbon cycle of bioethanol partially offsets combustion emissions. Total CO₂ absorbed = 6 mol, released = 2 + 4 = 6 mol → theoretically balanced. But production energy adds extra CO₂, so net emissions are ~20–50% lower than petrol — not zero.

"The combustion of bioethanol releases CO₂ that was originally absorbed from the atmosphere during photosynthesis of the source crop, resulting in net greenhouse gas emissions approximately 20–50% lower than those of petrol."

ADV 3 — Cleaner Combustion

Ethanol contains O in its structure → needs only 3 mol O₂ vs 12.5 mol for octane → more complete combustion �� less CO and soot.

"Ethanol (C₂H₅OH) contains oxygen in its molecular structure and requires only 3 mol O₂ for complete combustion, compared to 12.5 mol O₂ per mole of octane, resulting in significantly less CO and soot production."

ADV 4 — Reduces Fossil Fuel Dependence

Using biofuels as a supplement reduces reliance on imported fossil fuels, improving national energy security and reducing exposure to volatile oil prices.

ADV 5 — High Octane Rating

Ethanol has a higher octane rating (~108) than regular petrol (~91–98), which reduces engine knock and improves combustion efficiency.

Critical Analysis — The Carbon Cycle is NOT Carbon Neutral

This point doesn't fit neatly into ADV or DIS — it's a nuanced analysis that markers specifically reward.

The three-equation cycle balances to net zero on paper. However, the production chain (cultivation, harvesting, fertiliser, distillation, transport) requires energy from fossil fuels → adds CO₂ not offset by photosynthesis.

Conclusion: Bioethanol is NOT truly carbon neutral. Net emissions are ~20–50% lower than petrol.

Disadvantages

DIS 1 — Lower Energy Density (~34% less)

Ethanol: 29.6 kJ/g vs Octane: 47.8 kJ/g — approximately 34% less energy per gram. Cars travel a shorter distance on the same mass of ethanol.

"Ethanol has an energy density of 29.6 kJ/g compared to 47.8 kJ/g for octane, meaning approximately 34% more ethanol by mass is needed to produce the same amount of energy."

DIS 2 — Food vs Fuel

Crops used for bioethanol (sugar cane, corn, wheat) could be used for food production. Increased demand has been linked to rising food prices and food shortages in developing nations.

DIS 3 — Commercially NOT Viable (Cannot Mass Produce)

Acid hydrolysis (cellulose → glucose) is extremely time-consuming. Fermentation is controlled by yeast — slow, limited to ~15% concentration. Both steps are too inefficient for mass production.

"Bioethanol production is commercially not viable because acid hydrolysis of cellulose into glucose is extremely time-consuming and inefficient, and fermentation is limited by biological constraints of yeast, preventing mass production at the scale needed to replace fossil fuels."

DIS 4 — Economically NOT Feasible (High Production Costs)

Fractional distillation of dilute ethanol (~15%) into pure fuel requires enormous energy input — approaching the energy the ethanol can produce. Extremely low net energy efficiency.

"Fractional distillation of aqueous ethanol into pure fuel-grade ethanol requires almost as much energy as the ethanol itself produces upon combustion, making bioethanol production economically not feasible at current technology levels."

DIS 5 — Land Use and Environmental Damage

Large-scale production requires vast arable land → soil erosion, deforestation, loss of habitats. Paradoxically, clearing forests to plant biofuel crops may increase net CO₂.

DIS 6 — Engine Modification for High Blends

E10 (10% ethanol) needs no modification. E20, E85+ require modified fuel systems — different seals, fuel lines, injectors — because ethanol is corrosive to certain rubber and metal components.

Current Reality vs Future Potential

How you frame your answer depends on the question:

"Assess suitability" = focus on current limitations: cannot mass produce, economically not feasible, 34% less energy. Bioethanol is used only as a supplement (E10), not a replacement.

"Assess potential" = acknowledge limitations but emphasise future: renewability, cleaner combustion, reduced greenhouse impact. Technology improvements (cellulosic ethanol) could overcome current barriers.

Part 6: Band 6 Boosters

These extension points separate Band 5 from Band 6. Use them strategically — one well-placed booster is worth more than three vague points.

6.1 Cellulosic (Second-Generation) Ethanol

First-generation bioethanol uses food crops → food-vs-fuel dilemma. Second-generation bioethanol uses lignocellulosic biomass: agricultural waste (corn stalks, wheat straw), forestry residues, or dedicated energy crops like switchgrass.

"Second-generation cellulosic ethanol, produced from lignocellulosic waste such as corn stalks and wheat straw, addresses the food-vs-fuel limitation of first-generation bioethanol while utilising agricultural waste that would otherwise be discarded."

6.2 Life Cycle Analysis (LCA)

An LCALife Cycle Analysis — evaluates total environmental impact "from cradle to grave" evaluates the total environmental impact from raw material extraction through processing, transport, use, and disposal. For biofuels: the production phase contributes the majority of GHG emissions. Net GHG reduction is only 20–50% when full life cycle is considered.

"A comprehensive life cycle analysis of bioethanol production reveals that while the combustion-phase carbon cycle is theoretically balanced, precombustion activities — particularly fertiliser manufacture, distillation energy, and feedstock transportation — contribute additional greenhouse gas emissions, resulting in a net reduction of only 20–50% compared to petrol."

6.3 E85 and Flex-Fuel Vehicles

E85 = 85% ethanol + 15% petrol. Requires specially designed flex-fuel vehicles (FFVs). Produces significantly less CO and particulates, but ~30% more fuel consumption per km due to lower energy density.

6.4 Biogas — Anaerobic Digestion

Biogas = anaerobic digestion of organic waste by methanogenic bacteria → CH₄(g) + CO₂(g). Simultaneously addresses waste management and energy production — turning waste into fuel while reducing landfill methane emissions.

Exam Q&A Zone — 10 Fully Worked Questions

Attempt each question yourself before revealing the model answer. Every answer includes a mark-by-mark breakdown and a marker insight.

Extended Response Practice (Attempt, then reveal)

Question 5 (3 marks) — Explain two properties of ethanol as a fuel [Girraween 2019]

Q: 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.

Model Answer:

Ethanol can be produced via fermentation of glucose from crops. Since these crops regrow through photosynthesis, ethanol is a renewable fuel:

C₆H₁₂O₆(aq) → 2C₂H₅OH(l) + 2CO₂(g)

Ethanol (C₂H₅OH) contains an oxygen atom, needing only 3 mol O₂ vs 12.5 mol for octane → more complete combustion, less CO and soot:

C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)
2C₈H₁₈(l) + 25O₂(g) → 16CO₂(g) + 18H₂O(l)

Mark 1: Renewable source + fermentation equation
Mark 2: Cleaner combustion (O atom → less O₂)
Mark 3: Both combustion equations with comparison

Question 6 (4 marks) — Compare two methods for producing ethanol

Q: Compare the two industrial methods for producing ethanol. Include chemical equations and evaluate which is more sustainable.

Model Answer:

Fermentation: C₆H₁₂O₆(aq) → 2C₂H₅OH(l) + 2CO₂(g) — yeast, ~37°C, anaerobic. Batch process, slow, dilute product (~15%).

Hydration: CH₂=CH₂(g) + H₂O(g) → C₂H₅OH(l) — H₃PO₄, 300°C, 70 atm. Continuous, fast, high yield (>95%), but non-renewable petroleum feedstock.

Sustainability: Fermentation is more sustainable — biomass feedstock is renewable, and CO₂ from fermentation/combustion is partially offset by photosynthesis during crop growth.

Mark 1: Fermentation equation + conditions
Mark 2: Hydration equation + conditions
Mark 3: Efficiency/rate/yield comparison
Mark 4: Sustainability evaluation (renewable vs non-renewable)

Question 7 (4 marks) — Two advantages and two disadvantages of bioethanol

ADV 1: Renewable + lower net CO₂ — CO₂ partially offset by photosynthesis (20–50% lower than petrol).

ADV 2: Cleaner combustion — O atom in C₂H₅OH, only 3 mol O₂ needed vs 12.5 mol for octane.

DIS 1: Lower energy density — 29.6 kJ/g vs 47.8 kJ/g (~34% less).

DIS 2: Food vs fuel — crops compete with food production, linked to rising food prices.

Each mark requires a mechanism (why) and a number (how much).

Question 8 (5 marks) — Assess the suitability of ethanol as a fuel [Cheltenham Girls 2019]

Model Answer:

Ethanol (C₂H₅OH) is a biofuel produced from fermentation of crops. Currently blended as E10.

Advantages: Renewable (photosynthesis cycle), theoretically carbon neutral (6 mol absorbed = 6 released), burns cleaner (O atom → 3 mol O₂ vs 12.5).

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 promising but currently limited. Best used as a supplement (E10) rather than replacement. Cellulosic ethanol may address limitations in the future.

Mark 1: Define + source
Mark 2: Renewable + carbon cycle with 3 equations
Mark 3: Cleaner combustion
Mark 4: NOT neutral + lower energy + food vs fuel
Mark 5: Balanced judgement

Marker insight: "Assess" requires a judgement. No judgement = lost final mark.

Question 9 (6 marks) — Compare and contrast fuels from organic sources to biofuels (exact dot point)

Similarities: Both undergo combustion producing CO₂ + H₂O (with equations). Both used as transport fuels (E10).

Differences: Non-renewable hydrocarbons vs renewable oxygenated organics. Energy density (47.8 vs 29.6 kJ/g, ~34% more). Fossil CO₂ = permanent increase; bio CO₂ = partially offset (NOT truly neutral, 20–50% lower). Ethanol dual production (fermentation + hydration). Cleaner combustion (3 mol O₂ vs 12.5 mol).

Mark 1: Similarity — both combust → CO₂ + H₂O with equations
Mark 2: Similarity — both transport fuels
Mark 3: Renewability difference
Mark 4: Energy density with data
Mark 5: CO₂ emissions (NOT truly neutral)
Mark 6: Ethanol dual production + cleaner combustion

Question 10 (2 marks) — Calculate energy per tonne of CO₂ from ethanol combustion [PEM 2020]

Q: Octane produces 1.554 × 10⁷ kJ per tonne of CO₂. ΔH_c of ethanol = 1367 kJ/mol. Calculate 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

Surprising result: ethanol produces approximately the same energy per tonne of CO₂ as octane! But ethanol's CO₂ is partially offset by photosynthesis.

Flashcard Review

Click cards to flip. Test your knowledge of key terms.

Final Revision Cheat Sheet

Review this 60 seconds before the exam.

Don't Write This → Write This Instead

Click each "bad" statement to reveal the exam-quality version.

Key Equations — Know All Six

#	Reaction	Equation
1	Photosynthesis	6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g)
2	Fermentation	C₆H₁₂O₆(aq) → 2C₂H₅OH(l) + 2CO₂(g)
3	Hydration of ethylene	CH₂=CH₂(g) + H₂O(g) → C₂H₅OH(l), H₃PO₄, 300°C, 70 atm
4	Combustion of ethanol	C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l), ΔH_c = −1367 kJ/mol
5	Combustion of octane	2C₈H₁₈(l) + 25O₂(g) → 16CO₂(g) + 18H₂O(l), ΔH_c = −5470 kJ/mol
6	Combustion of methane	CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l), ΔH_c = −890 kJ/mol

Master Summary Table

Topic	Key Fact
Fossil fuel composition	Hydrocarbons — C and H only
Biofuel composition	Oxygenated organics — C, H, and O
Ethanol energy density	29.6 kJ/g, 1367 kJ/mol, MM = 46.07
Octane energy density	47.8 kJ/g, 5470 kJ/mol, MM = 114.23
Energy difference	Ethanol ~34% less energy per gram
Why ethanol burns cleaner	Contains O atom → needs less O₂ → less CO and soot
Carbon neutral?	Theoretically yes. In practice NO — production uses fossil fuels
Net CO₂ reduction	~20–50% lower than petrol
E10 fuel	10% ethanol + 90% petrol — no engine mods
Fermentation conditions	Yeast, ~37°C, anaerobic, slow batch
Hydration conditions	H₃PO₄, 300°C, 70 atm, continuous
Key disadvantage 1	Lower energy density — 34% less
Key disadvantage 2	Commercially not viable �� acid hydrolysis too inefficient
Key disadvantage 3	Economically not feasible — distillation costs too high
"Assess" verb	= ADV + DIS + YOUR JUDGEMENT
"Compare and contrast"	= Similarities AND Differences

Quick-Reference Traps

"Ethanol is carbon neutral" — NO. Theoretically balanced, but production uses fossil fuels.
"Compare and contrast = just list differences" — NO. You MUST include similarities.
"Ethanol only comes from fermentation" — NO. Also from hydration of ethylene (non-renewable).
"12.5 mol O₂" — Per mole of octane (from 25/2). Don't mix up per 1 mol vs per 2 mol.
"Biofuels have no disadvantages" — WRONG. Lower energy, food vs fuel, land use, engine mods, distillation cost.

The Universal Sentence Template

"Both [A] and [B] [SIMILARITY]. However, [A] [specific property with data] while [B] [contrasting property with data]. This is because [chemical/structural reasoning]."

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