IB Chemistry SLTopic 4 — Energy from FuelsPaper 1 & 2Core idea~12 min read
Incomplete Combustion
Restrict the oxygen and the carbon in a fuel cannot be fully oxidised. What comes out instead is carbon monoxide, or soot, or both — less energy, more mess, and a gas that kills people in their sleep.
📚 What you need to know
Incomplete combustion happens when the oxygen supply is limited.
Limited oxygen gives carbon monoxide, CO; very limited oxygen gives carbon, C, as soot.
Water is always produced. Hydrogen is oxidised in preference to carbon, so hydrogen gas is never a product.
A blue, non-luminous flame means complete combustion; a yellow, luminous, sooty flame means incomplete.
Carbon monoxide is colourless and odourless, and binds irreversibly to haemoglobin, blocking oxygen transport.
The higher the percentage of carbon by mass in a fuel, the sootier its flame.
What limited oxygen does
Complete combustion oxidises every carbon atom all the way to CO2. That takes a lot of oxygen. Supply less, and the carbon only makes it part of the way — first to carbon monoxide, and if the supply is very poor, not at all, leaving elemental carbon.
One fuel, three possible outcomes. In a real flame you usually get a mixture of all three.
Notice what does not change: water appears in every row. Hydrogen is oxidised preferentially, so however starved of oxygen a flame is, it never releases hydrogen gas. If you have written H2 as a product, something has gone wrong.
There is an energy consequence too. Carbon monoxide still has an unburnt bond’s worth of energy left in it — that is why it is itself a fuel. So incomplete combustion always releases less energy per mole of fuel than complete combustion, which is why a badly adjusted burner is wasteful as well as dangerous.
Reading the flame
The yellow glow is not the gas. It is solid carbon particles heated until they radiate light — the same effect as a candle.
A Bunsen burner is a controlled demonstration of the whole idea. Close the air hole and the gas mixes with air only at the flame edge, so combustion is incomplete: the flame is yellow, luminous, wavy and relatively cool, and it deposits soot on anything held in it. Open the air hole and the gas premixes with air before it burns: the flame becomes blue, non-luminous and much hotter, with a clear inner cone.
This is why calorimetry with a spirit burner comes out badly. Soot on the bottom of the can is unburnt carbon — energy that was never released at all, which is exactly why measured enthalpies of combustion come out less exothermic than the data booklet value.
Writing the equations
🧩 The method
Decide what the carbon product is: the question will say CO or carbon (soot).
Carbon: one CO or one C per carbon atom in the fuel.
Hydrogen: unchanged — half the H atoms become H2O, exactly as in complete combustion.
Oxygen last: count the O atoms in the products, remembering CO has one and C has none, then halve.
Double through if you need whole numbers.
WORKED EXAMPLE
Write equations for the incomplete combustion of ethane, C2H6, forming (a) carbon monoxide and (b) soot.
(a) carbon monoxide2 C → 2CO 6 H → 3H₂OO atoms in products: 2 from CO + 3 from water = 5, so 2½O₂.C₂H₆ + 2½O₂ → 2CO + 3H₂O2C₂H₆ + 5O₂ → 4CO + 6H₂O(b) sootNow the carbon takes no oxygen at all, so only the water needs any.C₂H₆ + 1½O₂ → 2C + 3H₂OCompare the three oxygen coefficients for ethane: 3½ complete, 2½ to CO, 1½ to soot. Less oxygen, less oxidation.
WORKED EXAMPLE
Calculate the mass of carbon monoxide produced if 1.00 g of methane, CH4, burns with all its carbon converted to CO. (Mr: CH4 = 16.05, CO = 28.01)
Step 1 — moles of methanen = 1.00 ÷ 16.05 = 0.0623 molStep 2 — ratio from the equation2CH₄ + 3O₂ → 2CO + 4H₂O, so CH₄ : CO is 1 : 1.n(CO) = 0.0623 molStep 3 — mass0.0623 × 28.01 = 1.745mass of CO = 1.75 gFrom one gram of gas. In an unventilated room that is a lethal quantity.
Why carbon monoxide is so dangerous
Carbon monoxide is colourless and odourless, so nothing warns you it is there — it takes a detector. Once inhaled it binds to haemoglobin, and it binds far more strongly than oxygen does and effectively irreversibly. Every haemoglobin molecule it occupies is one that can no longer carry oxygen.
The consequences follow from that single fact: the blood’s oxygen-carrying capacity falls, so tissues are starved even though breathing feels normal. Symptoms run from headache and dizziness to loss of consciousness and death. It is a genuine hazard of car engines and faulty boilers, where the oxygen supply is restricted.
Soot causes different problems: it is a particulate, and particulates cause respiratory illness, blacken buildings, and contribute to global dimming.
Which fuels soot the most
Compounds that are richer in carbon have more carbon to dispose of per gram and less hydrogen to help mop up the oxygen, so they are more likely to leave unburnt carbon behind. Aromatic compounds are the classic offenders — benzene burns with a spectacularly smoky flame.
Percentage of carbon by mass is a quick, quantitative way of ranking fuels for sootiness.
WORKED EXAMPLE
Calculate the percentage of carbon by mass in ethanol (C2H5OH, Mr = 46.08), hexane (C6H14, Mr = 86.20) and benzene (C6H6, Mr = 78.12), and predict which burns with the sootiest flame.
The calculation% C = (total mass of carbon ÷ M r) × 100Ethanol(2 × 12.01) ÷ 46.08 × 100 = 52.1%Hexane(6 × 12.01) ÷ 86.20 × 100 = 83.6%Benzene(6 × 12.01) ÷ 78.12 × 100 = 92.2%benzene is sootiest, ethanol cleanestSame six carbons in hexane and benzene, but benzene carries eight fewer hydrogens, so a far greater share of its mass is carbon.
💡 Exam tip
Read which product is wanted. “Incomplete combustion” alone is ambiguous; the question will specify CO or carbon.
Water always appears. Never write hydrogen gas as a product of any combustion.
Asked to explain a sooty flame? Say limited oxygen and high carbon content, and name the products.
For carbon monoxide, the mark is usually for binds to haemoglobin, preventing oxygen transport — not just “it is toxic”.
Percentage-by-mass comparisons need the same units and same Mr values, so use the data booklet masses throughout.
⚠️ Common mix-up
Writing H2 as a product when oxygen is short. Hydrogen is oxidised first, every time.
Giving CO2 and CO together when the question asked for one specific equation.
Forgetting that CO contributes one oxygen atom when balancing.
Saying the yellow flame is hotter because it looks brighter. It is cooler; the light comes from glowing soot.
Confusing carbon monoxide with carbon dioxide in the health answer. CO poisons; CO2 warms the planet.
Up next: Carbon Dioxide from Burning Fuels — how to work out exactly how much CO2 a fuel produces, and why the answer differs so much from one fuel to another.
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