SKY HSC

Module 7 IQ4: Fossil Fuels vs Biofuels

A comprehensive guide to understanding chemical energy, combustion, and sustainable alternatives in HSC Chemistry.

📚 Estimated reading time: 45 minutes
🔬 Includes 10 practice MCQs
📊 Interactive calculators & diagrams

Part 1: Types of Fuels

A fuel is a substance that burns to produce energy (typically heat and light). However, not all combustible substances are considered fuels in the chemical sense.

Fuel vs Non-Fuel

Fuels

  • Readily available
  • Cheap and economical
  • Easy to transport and store
  • Release significant energy on combustion
  • Examples: Coal, petroleum, natural gas, ethanol

Non-Fuels

  • Combustible but impractical
  • Released energy is low
  • Difficult or dangerous to handle
  • Examples: Paper, wood pulp, rubber

Classification of Fuels

Classification Examples Characteristics
Fossil Fuels Coal, Oil, Natural Gas, LPG Non-renewable, formed over millions of years
Biofuels Ethanol, Biodiesel, Biogas Renewable, produced from biomass
Gaseous Fuels Natural Gas, LPG, Biogas Easy to transport via pipelines
Liquid Fuels Petrol, Diesel, Ethanol Energy-dense, portable
Solid Fuels Coal, Wood, Coke Lower energy density, more polluting

Part 2: Fossil Fuels

Fossil fuels are non-renewable energy sources formed from ancient organic matter. They take millions of years to form and are finite resources.

Coal

Formation: Compressed remains of ancient plants and trees over 300+ million years.

Composition: Primarily carbon with hydrogen and small amounts of oxygen, nitrogen, and sulfur.

Types: Anthracite (hardest, most carbon), Bituminous, Lignite (softest, most volatile)

Uses: Electricity generation, heating, industrial processes

Disadvantages: Highest CO₂ emissions, air pollution (SO₂, NOₓ), mining hazards

Petroleum (Crude Oil)

Formation: Dead marine organisms compressed under heat and pressure over 150+ million years.

Composition: Complex mixture of hydrocarbons (alkanes, alkenes, aromatics)

Uses: Fuels (petrol, diesel, kerosene), lubricants, petrochemicals

Separation: Fractional distillation based on boiling point differences

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.

LPG (C₃-C₄) Temp: ~25°C, BP: <40°C Petrol (C₅-C₁₀) Temp: ~150°C, BP: 40-200°C Kerosene (C₁₁-C₁₅) Temp: ~250°C, BP: 200-300°C Diesel (C₁₆-C₂₀) Temp: ~280°C, BP: 250-350°C Fuel Oil (C₂₁-C₅₀) Temp: ~350°C, BP: 350-500°C Bitumen (>C₅₀) Temp: ~350°C, BP: >500°C °C 25°C 150°C 250°C 280°C 350°C

Natural Gas & LPG

Composition: Primarily CH₄ (methane) with ethane, propane, and butane.

Advantages: Cleaner than coal, lower CO₂ than coal, odorless (mercaptan added for safety)

Uses: Heating, electricity generation, vehicle fuel (LPG)

Source: Often found with crude oil or from coal seams

Part 3: Biofuels Overview

Biofuels are renewable fuels produced from biomass (plant or animal material). They can be replenished relatively quickly and are considered part of a sustainable energy future.

Biogas

Composition: Mixture of CH₄ (~60%), CO₂ (~40%), and traces of H₂S

Production: Anaerobic digestion of organic waste (animal manure, food scraps, sewage)

Advantages: Renewable, reduces waste, produces fertilizer as byproduct

Disadvantages: Lower energy density than natural gas, requires purification

Biodiesel

Composition: Fatty acid methyl esters (FAME) produced from oils

Production: Transesterification of vegetable oils (rapeseed, soybean) or animal fats

Uses: Blended with diesel (B20 = 20% biodiesel) or used pure (B100)

Advantages: Renewable, biodegradable, reduces particulate emissions

Disadvantages: Can clog fuel filters at low temperatures, more expensive

Bioethanol

Chemical Formula: C₂H₅OH (same as ethanol from other sources)

Production: Fermentation of carbohydrates (sugars, starch, cellulose)

Uses: Mixed with petrol (E10 = 10% ethanol), or used pure (E85)

Renewable: Carbon cycle - CO₂ from combustion reabsorbed in growing crops

Note: While renewable, production still requires energy for cultivation, harvesting, and distillation

Part 4: Ethanol Deep Dive

The Ethanol Cycle (Renewable Route)

This diagram illustrates the complete cycle of bioethanol production and combustion, emphasizing the renewable nature of the process.

Solar Energy Photosynthesis 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ Glucose C₆H₁₂O₆ (in plants/crops) Fermentation C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ Ethanol C₂H₅OH (Bioethanol fuel) Combustion 2C₂H₅OH + 6O₂ → 4CO₂ + 6H₂O, ΔH = -2727kJ/mol CO₂ CO₂ CO₂ CO₂ recycled ⚠ NOT truly greenhouse neutral! Energy required for: cultivation, harvesting, transportation, fermentation, distillation BUT still lower net emissions than fossil fuels

Ethanol Production Methods Comparison

NON-RENEWABLE (Synthetic Ethanol) Petroleum Refining Fractions Cracking Ethylene (C₂H₄) Hydration (+ H₂O) Ethanol (C₂H₅OH) RENEWABLE (Bioethanol) Biomass Acid Digestion Carbs (Glucose/Sugar) Fermentation Ethanol(aq) (aqueous) Fractional Distillation Pure Ethanol

Heat of Combustion Comparison

ΔHc (kJ/g) 0 10 20 30 50 Methanol 22.7 kJ/g Ethanol 29.6 kJ/g Propanol 33.6 kJ/g Octane 47.8 kJ/g Heat of Combustion of Fuels Alcohol-based Hydrocarbon

Ethanol Combustion Reaction

C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g)
ΔHc = -1367 kJ/mol (for ethanol)
or -29.6 kJ/g

Part 5: Advantages & Disadvantages

Comparison Table: Fossil Fuels vs Biofuels

Property Fossil Fuels Biofuels
Renewable No - millions of years to form Yes - can regrow crops annually
CO₂ Emissions High - adds CO₂ to atmosphere Lower - CO₂ recycled via photosynthesis
Energy Density Very high (petrol: 47.8 kJ/g) Lower (ethanol: 29.6 kJ/g)
Production Cost Established, cheaper Still expensive, developing
Transport Easy - pipelines, tankers Similar infrastructure needed
Vehicle Modification None required E10 compatible, E85/100 needs mods
Pollutants SO₂, NOₓ, particulates Lower emissions of pollutants
Land Use Mining/drilling areas Large agricultural land needed

Fuel Advantages & Disadvantages Explorer

Natural Gas (CH₄)

Advantages
  • Lower CO₂ emissions than coal or oil
  • High energy content (~55 MJ/kg)
  • Easy to transport via pipelines
  • Clean combustion, less air pollution
  • Cost-effective for large-scale use
Disadvantages
  • Non-renewable resource
  • Still releases CO₂ (greenhouse gas)
  • Methane leaks contribute to climate change
  • Extraction causes environmental damage
  • Requires infrastructure for delivery

Biogas (CH₄ + CO₂)

Advantages
  • Renewable - produced from waste
  • Reduces methane from landfills
  • Produces fertilizer as byproduct
  • Local production possible
  • Reduces organic waste
Disadvantages
  • Lower energy density than natural gas
  • Requires purification (remove CO₂, H₂S)
  • Inconsistent supply/production rate
  • Requires specialized equipment
  • Scaling up is challenging

Petrol (Hydrocarbons)

Advantages
  • High energy density (~46 MJ/kg)
  • Established infrastructure
  • Easy to transport and store
  • Well-developed engines
  • Relatively cheap (historically)
Disadvantages
  • Non-renewable - limited supply
  • High CO₂ emissions
  • Air pollution (NOₓ, particulates)
  • Price volatility/geopolitical issues
  • Oil spills damage ecosystems

LPG (Propane/Butane)

Advantages
  • Cleaner than petrol or coal
  • Can be stored in tanks
  • Portable - good for remote areas
  • Used in vehicles (LPG cars)
  • Lower emissions than petrol
Disadvantages
  • Non-renewable fossil fuel
  • Still produces CO₂ and pollutants
  • Requires specialized storage tanks
  • Safety concerns (flammable gas)
  • Lower energy density than petrol

Bioethanol (C₂H₅OH)

Advantages
  • Renewable - produced annually
  • Lower net CO₂ (recycled via photosynthesis)
  • Compatible with existing vehicles (E10)
  • Reduces petroleum dependence
  • Supports agriculture industry
  • Biodegradable
Disadvantages
  • Lower energy density (29.6 kJ/g vs 47.8) — ~34% less energy per volume
  • Production requires significant energy input — distillation is energy-intensive
  • Large land use for crops — competes with food production, drives food price increases
  • Competition with food production — can cause conflict and price instability
  • Still requires fossil fuels for processing — NOT truly greenhouse neutral
  • Higher cost than petrol currently — commercially not viable for mass production
  • Cellulose → glucose conversion is commercially infeasible — no efficient industrial method exists
  • Fermentation and acid hydrolysis processes are time-consuming and inefficient

⚠ Exam Alert: Critical Distinctions

1. NOT Greenhouse Neutral: Although the ethanol cycle appears carbon-neutral (CO₂ absorbed = CO₂ released), extra CO₂ is produced during farming, transportation, and distillation. This makes ethanol NOT greenhouse neutral — it just has lower greenhouse impact than fossil fuels (20-50% less CO₂).

2. Commercially Not Viable: Mass production of pure plant-derived ethanol from cellulose is economically not feasible with current technology. The acid hydrolysis process to convert cellulose to glucose is too slow and inefficient for industrial scale.

3. Lower Energy Density: Ethanol has ~34% less energy per volume than petrol. Cars travel further on the same amount of octane (petrol) than ethanol, making ethanol more expensive per km.

4. Non-renewable vs Renewable Ethanol: Ethanol from petroleum (via cracking + hydration of ethylene) is NOT bioethanol — it's non-renewable. Only ethanol from biomass (fermentation) is bioethanol.

Part 6: Calorimetry - Determining Heat of Combustion

Calorimetry Apparatus

Thermometer (±0.05°C) Water 100g (known mass) Heat shield (reduces heat loss) Spirit Burner (ethanol, methanol, etc.) Equipment Details Thermometer: • Range: -10°C to 110°C • Precision: ±0.05°C Conical Flask: • Volume: 250 mL • Contains water (known mass) Spirit Burner: • Contains fuel to test • Initial mass recorded Heat Shield: • Reduces heat loss to surroundings Procedure 1. Measure initial water temp (T₁) 2. Weigh spirit burner with fuel 3. Heat water ~20°C, measure final temp (T₂) 4. Weigh burner again - find mass burned

Heat of Combustion Calculator

Calculate ΔHc from Experimental Data

15%

Important Notes on Calorimetry

Heat Loss: Real experiments lose heat to the surroundings. Theoretical values are always higher than experimental values.

q = mcΔT Formula: Used to calculate heat absorbed by water. Specific heat capacity of water = 4.18 J/g°C

ΔHc Calculation: q (heat) is multiplied by molar mass and divided by mass of fuel burned to get kJ/mol

Improvements: Use lid on flask, stir water, repeat multiple times, use larger fuel mass, better insulation

Part 7: Key Equations Reference

Combustion Reactions

Ethanol Combustion:
C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g)
ΔHc = -1367 kJ/mol = -29.6 kJ/g
Octane (Petrol) Combustion:
2C8H18(l) + 25O2(g) → 16CO2(g) + 18H2O(g)
ΔHc = -5470 kJ/mol per octane molecule = -47.8 kJ/g
Methane (Natural Gas):
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
ΔHc = -890 kJ/mol = -55.5 kJ/g

Fermentation & Photosynthesis

Glucose Fermentation to Ethanol:
C6H12O6(aq) → 2C2H5OH(aq) + 2CO2(g)
Photosynthesis:
6CO2(g) + 6H2O(l) → C6H12O6(aq) + 6O2(g)

Calorimetry

Heat absorbed by water:
q = mcΔT
where: q = heat (J), m = mass of water (g), c = specific heat capacity of water (4.18 J/g°C), ΔT = temperature change (°C)
Molar heat of combustion:
ΔHc = (q × Molar Mass) / mass of fuel burned
Units: kJ/mol (note: divide q in J by 1000 to get kJ)

Alcohol Chemistry

Combustion of alcohol:
CnH2n+2O + (3n+1)/2 O2 → nCO2 + (n+1)H2O
(Primary alcohols from alkanes)
Hydration of alkene (industrial ethanol):
C2H4 + H2O ⇌ C2H5OH (with acid catalyst, ~300°C, high pressure)

Part 8: Flashcard Review

Click cards to flip. Test your knowledge of key Module 7 IQ4 terms.

Part 9: Practice Quiz - 10 MCQs from Past Papers

These questions are based on actual HSC Chemistry trials. Select your answer and check immediately.

Part 10: Exam Strategy & Cheat Sheet

Common Question Types

1. Combustion Reactions & Equations

What to expect: Write balanced equations, calculate ΔH, predict products

Tips:

  • Always balance carbon first, then hydrogen, then oxygen
  • Remember: combustion always produces CO₂ and H₂O
  • Know standard ΔH values for key fuels
  • Use subscipts for molecular formulas

2. Comparing Fuels

What to expect: Advantages/disadvantages, environmental impact, energy density

Tips:

  • Have a comparison table memorized (renewable, CO₂, energy density, cost)
  • Fossil fuels: non-renewable, high energy, lots of CO₂
  • Biofuels: renewable, lower energy, less CO₂ (still not zero!)
  • Know E10, E85, B20 blends

3. Calorimetry Calculations

What to expect: Calculate ΔH from experimental data, identify errors, improve method

Tips:

  • Always use q = mcΔT first (heat absorbed by water)
  • Then: ΔH = (q × M) / m_fuel (in kJ/mol)
  • Don't forget: experimental < theoretical (heat loss)
  • Common error: forgetting to convert J to kJ

4. Biofuel Production Routes

What to expect: Describe fermentation vs hydration, compare efficiency

Tips:

  • Fermentation: glucose → ethanol (renewable but needs energy)
  • Hydration: ethylene + H₂O → ethanol (fossil fuel base)
  • Know conditions: yeast, no O₂ for fermentation
  • Know conditions: 300°C, acid catalyst, pressure for hydration

Key Facts to Memorize

Substance Formula ΔHc (kJ/mol) Key Property
Methanol CH₃OH -726 22.7 kJ/g
Ethanol C₂H₅OH -1367 29.6 kJ/g, renewable
Propan-1-ol C₃H₇OH -2021 33.6 kJ/g
Octane C₈H₁₈ -5470 47.8 kJ/g, fossil fuel
Methane CH₄ -890 Natural gas, 55.5 kJ/g

Exam Writing Tips

  • Always show working - even if answer is wrong, you may get method marks
  • Use correct units - kJ/mol for ΔH, not just kJ
  • Explain "why" - don't just state facts. Explain the mechanism or chemistry
  • Define terms precisely - "combustion" is burning in oxygen
  • Compare, don't just list - "ethanol has lower energy than petrol because..." not just "ethanol: 29.6, petrol: 47.8"
  • Use correct notation - ΔH (not H), subscript c for combustion, state symbols (g), (l), (aq), (s)
  • Know your audience - assume reader has no background knowledge

Ready to Excel in Your HSC Chemistry?

Join thousands of students using SKY HSC materials to ace their exams.

Start Your Free Trial

Comprehensive notes, practice papers, and expert guidance