The big idea: A fuel is a substance that releases useful energy when it is burned.
Combustion is the reaction of a fuel with oxygen, releasing energy as heat (and usually light). Combustion is always exothermic — the products store less chemical energy than the reactants, so energy is released to the surroundings (ΔH is negative).
Most everyday fuels are hydrocarbons (compounds of only carbon and hydrogen, such as methane CH4 and octane C8H18) or alcohols (such as ethanol C2H5OH).
Combustion is exothermic: the products sit below the reactants, so energy is released to the surroundings (ΔH < 0).
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Why energy is released: Burning a fuel breaks the bonds in the fuel and oxygen (this takes in energy) and makes the bonds in CO2 and H2O (this gives out energy).
For combustion, the bonds made release more energy than the bonds broken take in — so overall energy is released and the reaction is exothermic.
How much oxygen is available decides what is produced and how much energy is released.
It depends on the oxygen supply: - Complete combustion (plenty of oxygen): the only products are carbon dioxide and water. - Incomplete combustion (limited oxygen): carbon is not fully oxidised, so you also get carbon monoxide (CO) and/or carbon (soot) alongside water.
Incomplete combustion
- Happens in a limited supply of oxygen.
- Products can include carbon monoxide (CO) and/or carbon (C, soot), plus water.
- Releases less energy per mole of fuel.
- CO is a toxic gas; soot causes smoke and respiratory harm.
Complete combustion
- Happens in a plentiful (excess) supply of oxygen.
- Products are only carbon dioxide (CO_{2}) and water (H_{2}O).
- Releases the maximum energy per mole of fuel.
- No toxic CO or soot is produced.
The same fuel, two equations: Complete combustion of methane:
CH4 + 2O2 → CO2 + 2H2O
Incomplete combustion of methane (limited O2) can instead give carbon monoxide:
2CH4 + 3O2 → 2CO + 4H2O
Why incomplete combustion matters: Incomplete combustion releases less energy per mole of fuel and produces carbon monoxide — a colourless, odourless, toxic gas that binds to haemoglobin — plus soot, which causes smoke and breathing problems. This is why fuel-burning appliances need a good air supply and ventilation.
Learn what examiners really want
See exactly what to write to score full marks. Our AI shows you model answers and the key phrases examiners look for.
Fuels are compared by how much energy they release for a given mass or volume. Two measures are used:
Two ways to compare a fuel's energy
- Specific energy = energy released per unit mass (kJ g⁻¹ or MJ kg⁻¹) — important when weight matters (e.g. rocket and aircraft fuel).
- Energy density = energy released per unit volume (kJ cm⁻³ or MJ dm⁻³) — important when storage space matters (e.g. a car fuel tank).
Renewable vs non-renewable: - Fossil fuels (coal, oil, natural gas) are non-renewable — they formed over millions of years and are being used far faster than they can re-form. - Biofuels (e.g. ethanol from fermented sugar cane, biodiesel from plant oils) come from recently grown biomass, so they are renewable — the crops can be regrown.
| Fossil fuels | Biofuels | |
|---|---|---|
| Source | coal, crude oil, natural gas (formed over millions of years) | recently grown biomass (e.g. ethanol from sugar cane, biodiesel from plant oils) |
| Renewable? | non-renewable — used far faster than they form | renewable — crops can be regrown each season |
| CO2 balance | releases carbon locked away for millions of years → net rise in atmospheric CO2 | the crops absorb CO_{2} as they grow, roughly balancing the CO2 released when burned → near carbon-neutral |
| Climate impact | larger net contribution to the enhanced greenhouse effect | smaller net contribution to climate change |
Why biofuels are nearer carbon-neutral: As the crop grows it absorbs CO_{2} from the air by photosynthesis. When the biofuel is burned, it releases roughly the same CO2 back again, so there is little net increase in atmospheric CO2.
Fossil fuels instead release carbon that was locked away for millions of years, adding new CO2 to the atmosphere — so they contribute more to the enhanced greenhouse effect and climate change.
Worked example — specific energy of a fuel
Burning 1.50 g of ethanol releases 44.7 kJ of energy. Calculate the specific energy of ethanol, in kJ g⁻¹.
Solution
- Formula first — specific energy is energy released per unit mass:
- Substitute the values:
- Work it out — keep the unit:
Final answer
Specific energy = 29.8 kJ g⁻¹.
How this is tested: R1.3 shows up as a short qualitative ask, not a big calculation:
- Paper 1A (MCQ): identify the renewable energy source, or the products of complete vs incomplete combustion. - Paper 2: outline advantages of a biofuel over a fossil fuel, or explain why biofuels contribute less to climate change.
For 'explain' marks you must give the reason (the CO2 balance), not just state that biofuels are 'cleaner'.
Score the marks: For climate questions, the key phrase is the carbon balance: growing crops absorb CO_{2}, so burning them is near carbon-neutral, whereas fossil fuels release carbon stored for millions of years, raising net atmospheric CO2.
IB-style question — ethanol vs gasoline (a)
(a) Outline two advantages of using ethanol from sugar cane, rather than gasoline (a fossil fuel), as a vehicle fuel. [2]
How to score the marks
- Mark 1 — renewable. Ethanol is a biofuel made from a crop that can be regrown, so it is renewable; gasoline comes from crude oil, which is non-renewable and finite.
- Mark 2 — climate. Sugar cane absorbs CO_{2} as it grows, so burning the ethanol releases roughly the same CO2 back — it is near carbon-neutral and contributes less to climate change than gasoline.
Final answer
(1) ethanol is renewable (the crop is regrown) while gasoline is non-renewable; (2) ethanol is near carbon-neutral because the crop absorbs CO2 as it grows, so it adds less net CO2 to the atmosphere.
IB-style question — climate impact (b)
(b) Explain why burning a biofuel contributes less to climate change than burning a fossil fuel. [2]
How to score the marks
- Mark 1 — the crop absorbs CO_{2}. As the biofuel crop grows it takes in CO_{2} from the atmosphere by photosynthesis, which roughly balances the CO2 released when the fuel is burned (near carbon-neutral).
- Mark 2 — fossil carbon is extra. A fossil fuel releases carbon that was locked away for millions of years, adding new (net) CO_{2} to the atmosphere and increasing the enhanced greenhouse effect.
Final answer
A biofuel's crop absorbs CO2 as it grows, roughly balancing the CO2 released on burning (near carbon-neutral); a fossil fuel adds CO2 that was stored for millions of years, so it gives a larger net rise in atmospheric CO2.