VCE Chemistry Units 3/4: Fuels and Combustion Explained (with Worked Examples)

Unit 3 Chemistry opens with a deceptively simple question: where does our energy come from, and what does it cost? This guide covers the core ideas you need for the first topic, with worked examples in the style of SAC and exam questions.

Fossil fuels and biofuels

A fuel is a substance that releases useful energy, usually as heat, when it reacts, most often with oxygen.

Fossil fuels (coal, crude oil and natural gas) formed over millions of years from buried organic matter. They are non-renewable because we use them far faster than they form.

Biofuels are made from recently living biomass, so they are renewable if the biomass is regrown:

• Biogas is mainly methane, made by anaerobic digestion of organic waste.
• Bioethanol is made by fermenting sugars: C6H12O6(aq) → 2C2H5OH(aq) + 2CO2(g)
• Biodiesel is made by reacting vegetable oils or animal fats with methanol, using a catalyst.

Comparing fuels: what examiners want

Hydrocarbon fuels release more energy per gram than ethanol or biodiesel, because those biofuels already contain oxygen (they are partly oxidised). Biofuels release carbon that plants absorbed recently, so their net emissions can be lower, but "carbon neutral" is rarely fully true: farming, fertiliser manufacture, processing and transport all use energy.

Exam tip: when evaluating a claim like "bioethanol is carbon neutral", name a specific emission source in production. General statements lose marks.

Combustion is exothermic

More energy is released forming the bonds in carbon dioxide and water than is absorbed breaking the bonds in the fuel and oxygen, so combustion is exothermic and ΔH is negative. The energy drives electricity generation (steam turbines), transport (engines) and heating.

Writing thermochemical equations

A thermochemical equation is a balanced equation with states and a ΔH value that applies to the amounts as written. At standard laboratory conditions, write water as H2O(l) unless told otherwise.

Complete combustion of methane:

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ΔH = −890 kJ mol⁻¹

Complete combustion of octane, balanced with whole numbers:

2C8H18(l) + 25O2(g) → 16CO2(g) + 18H2O(l) ΔH = −10 928 kJ mol⁻¹

Two moles of octane react, so ΔH is double octane's heat of combustion (5464 kJ mol⁻¹). Double the equation, double ΔH; reverse it, flip the sign.

Incomplete combustion happens when oxygen is limited and produces carbon monoxide or soot, releasing less energy:

2CH4(g) + 3O2(g) → 2CO(g) + 4H2O(l)

Energy profile diagrams

For an exothermic reaction, the products sit below the reactants and the drop is ΔH. Activation energy is the climb from the reactants to the top of the curve. Fuels don't ignite at room temperature because their activation energy is large; a spark supplies it, and the energy released keeps the reaction going.

Worked example: heat released

How much heat is released when 10.0 g of ethanol burns completely? (Heat of combustion = 1364 kJ mol⁻¹, M = 46.0 g mol⁻¹)

• n(ethanol) = 10.0 ÷ 46.0 = 0.217 mol
• heat released = 0.217 × 1364 = 296 kJ

Worked example: carbon dioxide produced

What mass of CO2 forms when 1.00 kg of octane burns completely?

• n(octane) = 1000 ÷ 114.0 = 8.77 mol
• From the equation, 2 mol octane gives 16 mol CO2, a 1 : 8 ratio, so n(CO2) = 70.2 mol
• m(CO2) = 70.2 × 44.0 = 3.09 kg

Burning 1 kg of petrol releases about 3 kg of carbon dioxide; the extra mass comes from oxygen in the air.

Where students lose marks

• Missing or wrong states in thermochemical equations
• Forgetting to scale ΔH when the equation is doubled
• Writing a positive ΔH for combustion
• Vague environmental evaluations without a specific example

Next in the course: measuring energy with calorimetry, using the specific heat capacity of water.

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