IB Chemistry SLTopic 4 — Energy from FuelsPaper 1 & 2Core skill~13 min read
Carbon Dioxide from Burning Fuels
Every carbon atom in a fuel leaves as a molecule of carbon dioxide. That makes the emissions of a fuel something you can calculate exactly, from nothing more than its formula and a balanced equation.
📚 What you need to know
Fossil fuels — coal, oil and natural gas — are hydrocarbons formed from buried organisms, and are finite and non-renewable.
Specific energy is the energy released per unit mass of fuel, in kJ g–1 or MJ kg–1. Energy density is per unit volume.
Moles of CO2 = moles of fuel × the number of carbon atoms in the fuel.
Going up the alkane series, each gram of fuel gives more CO2 and less energy.
Fuels are compared fairly as CO2 released per unit of energy, not per gram.
Methane has the highest hydrogen-to-carbon ratio of any hydrocarbon, so it is the cleanest per joule.
Fossil fuels
Coal, oil and natural gas are all mixtures of hydrocarbons, formed over millions of years from the remains of dead organisms buried in the Earth’s crust. They are non-renewable: they are being consumed vastly faster than geology replaces them, so the supply is finite.
Fuel
In its favour
Against it
Coal
Cheap, abundant, safe to store, long-lasting reserves
Most CO2 per joule; sulfur impurities give SO2 and acid rain; particulates; mining damages habitats and endangers miners
Oil
High energy density, easy to transport by pipeline or tanker, separable by fractional distillation into useful fractions
CO2, SO2 and CO on burning; spills damage ecosystems; unevenly distributed geographically
Natural gas
Highest specific energy, cleanest burning, little sulfur so no significant acid rain
Still releases CO2; low energy density means pressurised storage; leaks release methane, itself a potent greenhouse gas
Specific energy
Specific energy
the energy released per unit mass of fuel, in kJ g–1 or MJ kg–1
Specific energy is simply the enthalpy of combustion divided by the molar mass, which makes it a one-line calculation from data you already have. Keep it separate from energy density, which is energy per unit volume — the distinction matters for hydrogen, which is superb per kilogram and hopeless per litre.
Aircraft care about specific energy, because every kilogram has to be lifted. Car boots and gas cylinders care about energy density, because space is what runs out first. A question that mentions tank size is asking about the second one.
The trend along the alkanes
Both curves flatten out, because as the chain gets longer, adding one more CH2 changes the overall carbon-to-hydrogen ratio less and less.
The chemistry behind both curves is the same single fact: burning hydrogen releases more energy per gram than burning carbon does. So a fuel’s hydrogen-to-carbon ratio decides everything.
Methane, CH4, has four hydrogens per carbon — the highest possible — so it gives the most energy and the least CO2 per gram.
As the chain lengthens the ratio falls towards 2 hydrogens per carbon, so each gram carries relatively more carbon: more CO2, less energy.
Two further consequences of longer chains are worth knowing: stronger London dispersion forces make them less volatile and harder to vaporise, and their higher carbon content makes incomplete combustion more likely.
Calculating the carbon dioxide
🧩 The method, every time
Write and balance the equation for complete combustion.
Find moles of fuel from n = mass ÷ Mr.
Moles of CO2 = moles of fuel × number of carbons, straight from the equation.
Convert back: mass = moles × 44.01.
If you are comparing fuels, divide by the energy released, not by the mass.
WORKED EXAMPLE
Calculate the mass of carbon dioxide produced when 1.00 g of propane, C3H8, burns completely. (Mr: C3H8 = 44.11, CO2 = 44.01)
Step 1 — the equationC₃H₈ + 5O₂ → 3CO₂ + 4H₂OStep 2 — moles of propanen = 1.00 ÷ 44.11 = 0.02267 molStep 3 — moles of CO₂0.02267 × 3 = 0.06801 molStep 4 — mass0.06801 × 44.01 = 2.993mass of CO₂ = 2.99 gThree times the mass of fuel you started with. The extra mass is oxygen taken from the air.
WORKED EXAMPLE
Compare the CO2 released per gram by methane (Mr = 16.05) and octane (Mr = 114.26) burning completely.
Methane, 1 carbonn = 1.00 ÷ 16.05 = 0.0623 mol → 0.0623 mol CO₂0.0623 × 44.01 = 2.74 gOctane, 8 carbonsn = 1.00 ÷ 114.26 = 0.00875 mol → × 8 = 0.0700 mol CO₂0.0700 × 44.01 = 3.08 g2.74 g vs 3.08 g per gram of fuelA 12% difference — real, but smaller than most people expect. The bigger gap appears once you allow for the energy each one gives.
Comparing fuels fairly
Per gram is the wrong comparison, because nobody buys a fuel by the gram — they buy it to deliver a certain amount of energy. Divide the CO2 by the energy released and the honest comparison appears.
The same job done three ways. This is the calculation behind every claim that natural gas is a “cleaner” fossil fuel.
WORKED EXAMPLE
Methane has a specific energy of 55.5 kJ g–1 and octane 47.9 kJ g–1. Using the masses of CO2 per gram from the previous example, compare the CO2 released per megajoule.
Methane2.74 ÷ 55.5 = 0.0494 g per kJ → 49 g per MJOctane3.08 ÷ 47.9 = 0.0643 g per kJ → 64 g per MJmethane emits about 30% less CO₂ per MJPer gram the gap was 12%; per unit of useful energy it is nearly a third. Methane wins twice over — less carbon to start with, and more energy from each gram.
💡 Exam tip
The carbon count is the ratio. Moles of CO2 per mole of fuel is just the number of carbons — no need to think about the oxygen at all.
Use Mr = 44.01 for CO2 from the data booklet, and keep an extra significant figure until the end.
Read whether the question wants per gram, per mole, or per unit of energy. They give different answers and different conclusions.
Explaining the alkane trend? The mark is for higher carbon content per gram, not just “bigger molecule”.
Specific energy is per mass, energy density is per volume. Examiners test the difference.
⚠️ Common mix-up
Forgetting to multiply by the number of carbons. One mole of octane gives eight moles of CO2.
Using the mass of fuel as the mass of CO2. The product is heavier, because oxygen has been added.
Comparing fuels per mole. A mole of octane is seven times heavier than a mole of methane, so the comparison is meaningless.
Assuming the biggest molecule releases least energy overall. Per gram it does; per mole octane releases far more.
Treating natural gas as carbon-free. It is the cleanest fossil fuel, not a clean one.
Up next: CO2 Levels and the Greenhouse Effect — where all that carbon dioxide ends up, and the reason a molecule of it warms the planet while a molecule of nitrogen does not.
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