VCE Chemistry (Year 12) · Unit 3 — Supplying Energy: Fuels, Cells and Electrolysis · 14m read

Fuels, Combustion and Thermochemistry

Ever wonder why petrol and wood release such different amounts of energy when they burn? Understanding fuels means learning which ones are fossil fuels (like coal and oil) versus biofuels (grown from plants), writing equations that show exactly what happens during combustion, and comparing how much energy different fuels pack per kilogram. This matters for VCE because combustion questions appear constantly, and in real life, it shapes energy choices from your car to renewable alternatives.

Learning objective: Classify fuels as fossil or biofuels, write balanced combustion equations, and compare energy densities.

By the end of this lesson you can:

• I classify a fuel as fossil or biofuel based on its source.
• I balance complete and incomplete combustion equations.
• I compare fuels by their energy density (kJ/g) and renewability.

Fuels are energy savings accounts. Chemical bonds store energy; combustion withdraws it as heat and light.

Fossil fuels vs biofuels

A fuel is a substance that releases energy on combustion. Fossil fuels (coal, petroleum, natural gas) formed from ancient biomass over millions of years and are non-renewable. Biofuels (bioethanol, biodiesel, biogas) are derived from recently-living biomass and are considered renewable when replanted at harvest rates.

Combustion is rapid oxidation. Complete combustion in excess O₂ produces CO₂ and H₂O. Incomplete combustion (limited O₂) produces CO and/or C (soot), releasing less energy per mole of fuel.

Complete combustion of a hydrocarbon CₓHᵧ: CₓHᵧ + (x + y/4) O₂ → x CO₂ + (y/2) H₂O.

Energy density (kJ/g) compares fuels by mass. Hydrogen has the highest specific energy (~140 kJ/g) but is hard to store; petrol is ~47 kJ/g; ethanol ~30 kJ/g. Energy per mole differs from energy per gram — know which the question asks.

Combustion rules

• Complete combustion: Hydrocarbon + O₂ (excess) → CO₂ + H₂O.
• Incomplete combustion: Limited O₂ → CO and/or C (soot).
• Balanced check: Equal atoms of each element on both sides.
• Fossil vs bio: Fossil = non-renewable; bio = renewable if replanted.
• Energy density units: Per gram (specific energy) or per mole (molar enthalpy).

Quick reference

| Fuel | Type | Note |
|---|---|---|
| Coal | Fossil | High CO₂ |
| Natural gas (CH₄) | Fossil | Cleaner-burning fossil fuel |
| Biogas | Renewable | From waste |
| Bioethanol | Renewable | Fermentation |
| Biodiesel | Renewable | Transesterification |
| Complete combustion | → CO₂ + H₂O | |

Worked example (Medium)

Write the balanced equation for the complete combustion of octane, C₈H₁₈.

1. Apply hydrocarbon formula. x = 8, y = 18. Coefficient of O₂ = x + y/4 = 8 + 4.5 = 12.5
2. Write unbalanced. Unbalanced with fractional O₂. C₈H₁₈ + 12.5 O₂ → 8 CO₂ + 9 H₂O
3. Double to remove fraction. Multiply everything by 2. 2 C₈H₁₈ + 25 O₂ → 16 CO₂ + 18 H₂O

Answer: 2 C₈H₁₈(l) + 25 O₂(g) → 16 CO₂(g) + 18 H₂O(l).

Worked example (Hard)

Compare the energy density (kJ/g) of ethanol (C₂H₅OH, ΔH = −1367 kJ/mol, M = 46.07) and octane (C₈H₁₈, ΔH = −5471 kJ/mol, M = 114.23). Which is the better fuel by mass?

1. Ethanol. Energy density = |ΔH|/M. 1367/46.07 = 29.7 kJ/g
2. Octane. Energy density = |ΔH|/M. 5471/114.23 = 47.9 kJ/g
3. Compare. Octane releases ~62% more energy per gram. However, ethanol is renewable and produces less CO₂ per kJ.

Answer: Octane (47.9 kJ/g) > ethanol (29.7 kJ/g). Octane is energy-denser but non-renewable; ethanol is renewable with lower carbon intensity.

Common mistakes

• Forgetting state symbols in thermochemical equations (costs marks on VCAA exams) This is a commonly penalised error in VCAA exams. Fix: Review the underlying concept and practise similar problems.
• Confusing energy per mole with energy per gram when comparing fuels Incorrect application leads to wrong answers. Fix: Always return to first principles and verify your working.
• Writing incomplete combustion when the question asks for complete combustion A frequent source of confusion in VCE Chemistry. Fix: Distinguish between related concepts through targeted practice.

Biofuels vs fossil fuels and the carbon cycle

Biofuels are derived from recently-living biomass and are considered renewable when replanted at harvest rates. The carbon neutrality argument: CO₂ absorbed during growth equals CO₂ released during combustion. However, life cycle analysis reveals that production energy (tractors, fertilisers, distillation) and land use change reduce net carbon savings. Sugarcane ethanol (Brazil) achieves ~80% emission reduction vs petrol; corn ethanol (USA) only ~20%. VCAA expects nuanced analysis comparing: renewability, energy density, carbon neutrality (with caveats), and agricultural land competition.

Practice questions

1. Which of these is a biofuel?

   1. Bioethanol
   2. Petrol
   3. Natural gas
   4. Coal

   (2 marks)

2. In incomplete combustion of methane with limited oxygen, the products could include:

   1. CO and H₂O
   2. CO₂ and O₂
   3. H₂ and C
   4. Only CO₂

   (3 marks)

3. Write the balanced equation for the complete combustion of ethanol, C₂H₅OH.

   (3 marks)

Summary

• Classify a fuel as fossil or biofuel based on its source.
• Balance complete and incomplete combustion equations.
• Compare fuels by their energy density (kJ/g) and renewability.