IB Chemistry HLTopic 4 — Energy from FuelsPaper 1 & 2Core idea~10 min read
Incomplete Combustion
Starve a flame of oxygen and it does not simply burn less. It burns differently — producing a yellow, sooty flame, less energy, and a gas that has killed people in their sleep. The chemistry is straightforward once you know the pecking order: hydrogen always gets the oxygen first.
📘 What you need to know
Incomplete combustion happens when the oxygen supply is limited.
Carbon is not fully oxidised, so instead of CO2 you get carbon monoxide (CO) or carbon (C, soot), or both.
Water is still always produced. Hydrogen is preferentially oxidised, so hydrogen gas is never a product.
In a Bunsen burner: blue, non-luminous flame means complete combustion; yellow, luminous means incomplete.
Incomplete combustion releases less energy per gram of fuel, because the carbon is only partly oxidised.
Carbon monoxide is colourless, odourless and toxic. It binds irreversibly to haemoglobin, blocking oxygen transport.
The higher the percentage of carbon by mass in a compound, the sootier its flame tends to be.
Common causes: car engines and faulty boilers where the air supply is restricted.
The pecking order for oxygen
Here is the single idea that makes every incomplete combustion equation predictable. When oxygen is scarce, it is not shared out evenly. Hydrogen is oxidised first, every time. Only whatever oxygen is left over goes to the carbon.
That gives a clean three-step ladder as the oxygen supply drops:
In a real faulty boiler you get a mixture of all three carbon products at once. Exam questions usually ask for one specific product and will tell you which.
Incomplete combustion — two versions
fuel + oxygen → carbon monoxide + water
fuel + oxygen → carbon + water
Reading the flame
You already have a piece of laboratory equipment that demonstrates all of this: the Bunsen burner. The air hole controls the oxygen supply, and the flame tells you which reaction is running.
That last point is a nice one to know: a luminous flame glows because it is full of incandescent carbon particles. No soot, no glow — which is why the clean flame is so hard to see.
Feature
Complete combustion
Incomplete combustion
Oxygen supply
Excess / plentiful
Limited or restricted
Carbon-containing product
CO2
CO and / or C
Other product
H2O
H2O, always
Flame appearance
Blue, non-luminous
Yellow, luminous, sooty
Energy released per gram
Maximum
Lower
Hazard
CO2, a greenhouse gas
CO toxicity and soot particulates
Why carbon monoxide is so dangerous
Carbon monoxide is the reason gas appliances need servicing and why you should never run an engine in a closed garage. The danger comes from a combination of properties:
It is colourless and odourless, so there is nothing to notice. You cannot smell or see it, and it cannot be detected without a sensor.
It binds to haemoglobin in the blood, occupying the site where oxygen should sit.
That binding is effectively irreversible, so the affected haemoglobin is out of action permanently — unlike oxygen, which attaches and detaches freely.
With less haemoglobin available, oxygen transport around the body fails, causing headaches, dizziness, loss of consciousness and, untreated, death.
Soot is a hazard too. Carbon particulates cause respiratory problems, blacken buildings, and reflect sunlight back into space — an effect known as global dimming. Soot deposits on the bottom of a calorimeter can also ruin an enthalpy of combustion experiment, since the energy that should have been released was never released at all.
Which fuels soot the most?
The more carbon a molecule contains relative to hydrogen, the harder it is to supply enough oxygen for every carbon atom, so the sootier the flame. A useful quantitative measure is the percentage of carbon by mass:
Percentage carbon by mass
% C = (total mass of carbon ÷ Mr) × 100
Benzene and its relatives burn with famously smoky flames for exactly this reason; alcohols, with an oxygen atom already built in, burn far more cleanly.
WORKED EXAMPLE
Calculate the percentage of carbon by mass in hexane (C6H14, Mr = 86.17), benzene (C6H6, Mr = 78.11) and ethanol (C2H5OH, Mr = 46.07). Predict which burns with the sootiest flame.
Step 1: Mass of carbon in one mole of eachhexane and benzene: 6 × 12.01 = 72.06 g; ethanol: 2 × 12.01 = 24.02 gStep 2: Divide by Mr and multiply by 100hexane: (72.06 ÷ 86.17) × 100 = 83.6%benzene: (72.06 ÷ 78.11) × 100 = 92.3%ethanol: (24.02 ÷ 46.07) × 100 = 52.1%Step 3: PredictBenzene sootiest, ethanol cleanestsame six carbons in hexane and benzene, but benzene has far less hydrogen to dilute them
WORKED EXAMPLE
Write a balanced equation for the incomplete combustion of butane, C4H10, where carbon monoxide is the only carbon-containing product. Use whole-number coefficients.
Step 1: Carbon and hydrogen, exactly as before4 carbons → 4CO; 10 hydrogens → 5H₂OStep 2: Count oxygen on the right(4 × 1) + 5 = 9 oxygen atomsCO has only one oxygen each, which is the whole pointStep 3: Halve to get O₂9 ÷ 2 = 4½, so double everything2C₄H₁₀(g) + 9O₂(g) → 8CO(g) + 10H₂O(l)complete combustion would need 13O₂ for the same 2 butane − noticeably more
WORKED EXAMPLE
Write a balanced equation for the combustion of ethanol, C2H5OH, in a very poor oxygen supply, where soot is the only carbon-containing product.
Step 1: Carbon and hydrogen2 carbons → 2C(s); 6 hydrogens in total → 3H₂OStep 2: Oxygen on the rightsoot has no oxygen, so right-hand total = 3 atomsStep 3: Subtract the alcohol’s own oxygen3 − 1 = 2 atoms from O₂, so just 1O₂C₂H₅OH(l) + O₂(g) → 2C(s) + 3H₂O(l)one third of the oxygen that complete combustion needed, and far less energy out
💡 Exam tip
Read the question for which carbon product is wanted. “To form carbon monoxide” and “to form soot” give different equations.
Water is always a product. Include it even when the question focuses on the carbon.
For the CO danger, the mark-earning words are colourless, odourless, toxic, binds irreversibly to haemoglobin and prevents oxygen transport.
Describing flames, use the full pairs: blue and non-luminous, or yellow and luminous. One word alone often does not score.
Both complete and incomplete combustion are exothermic. Incomplete releases less energy, not none.
If asked to compare sootiness, calculate the % carbon rather than guessing from the formula.
⚠ Common mix-up
Putting H2 on the product side. Hydrogen is oxidised preferentially, so it always leaves as water.
Saying incomplete combustion is endothermic. It is still exothermic, just less so.
Claiming CO binds reversibly. The irreversible binding is what makes it lethal.
Confusing carbon monoxide with carbon dioxide in a hazard question. CO is the toxic one; CO2 is the greenhouse gas.
Forgetting that CO has one oxygen atom when balancing, and counting it as two.
Assuming a yellow flame is hotter because it looks brighter. It is cooler, and the brightness comes from glowing soot.
Blaming a low calorimetry value on heat loss only. If there is soot on the can, incomplete combustion is also to blame.
Up next: Carbon Dioxide from Burning Fuels — how to calculate exactly how much CO2 a fuel produces, and why methane is the cleanest hydrocarbon there is.
Want this explained one-to-one?
Book a free session with an experienced IB Chemistry tutor and get your trickiest topics made simple.
