C. Chemical Reactions · Chemical ReactionsCombustion: Complete and Incomplete
C2 C3 C1 A1.4About 70 minutes
Learning goals- I can write balanced equations for the complete combustion of hydrocarbons and of fuels that contain oxygen, such as alcohols.
- I can distinguish complete from incomplete combustion by products, flame colour and energy released.
- I can write balanced equations for incomplete combustion that produces carbon monoxide or soot.
- I can explain the health and safety risks of incomplete combustion and how carbon monoxide alarms protect people.
Words to know
combustion
A rapid reaction of a substance with oxygen that releases energy as heat and light.
hydrocarbon
A compound of only hydrogen and carbon, such as methane CH₄, propane C₃H₈ and octane C₈H₁₈.
complete combustion
Combustion with enough oxygen; a hydrocarbon or C, H, O fuel produces only carbon dioxide and water.
incomplete combustion
Combustion with limited oxygen; some carbon ends up as carbon monoxide (CO) or solid carbon (soot).
carbon monoxide
CO, a colourless, odourless, toxic gas produced by incomplete combustion.
fire triangle
The three things a fire needs: fuel, oxygen and heat.
The reaction that heats Canada
On a cold January night, most Canadian homes are warmed by a combustion reaction: natural gas (mostly methane) burning in a furnace, or wood in a stove, or heating oil. Our cars, school buses, barbecues and camp stoves run on combustion too. Combustion is so common that it is easy to forget how carefully it must be controlled.
In Grade 10 you met complete and incomplete combustion. In this lesson you will write and balance both kinds for many fuels, compare them, and connect them to health and safety.
Complete combustion
With plenty of oxygen, a fuel made of carbon and hydrogen (and possibly oxygen) burns completely:
fuel + O₂ → CO₂ + H₂O + energy
- Methane (natural gas): CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)
- Propane (barbecues): C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(g)
- Butane (lighters): 2C₄H₁₀(g) + 13O₂(g) → 8CO₂(g) + 10H₂O(g)
- Ethanol (added to gasoline): C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(g)
Balancing order: C, then H, then O last. Oxygen is last because O₂ appears by itself. If the fuel contains oxygen, subtract the fuel's oxygen atoms before deciding how much O₂ you need. If you get a half, double everything.
A complete combustion flame is blue, hot and clean, like a well-adjusted gas stove.
Incomplete combustion
When oxygen is limited (a blocked furnace vent, a closed garage, a smouldering campfire), there is not enough oxygen to turn all the carbon into CO₂. Some carbon ends up as:
- carbon monoxide, CO: 2CH₄(g) + 3O₂(g) → 2CO(g) + 4H₂O(g)
- solid carbon (soot), C: CH₄(g) + O₂(g) → C(s) + 2H₂O(g)
Real flames with limited oxygen produce a mixture of CO₂, CO, C and H₂O, so incomplete combustion can be written in more than one correct way. In questions, use the products you are told.
An incomplete combustion flame is yellow and smoky. The yellow glow comes from tiny hot particles of soot. Incomplete combustion releases less energy from the same fuel, wastes fuel and makes pollution.
| Complete combustion | Incomplete combustion |
|---|
| Oxygen supply | plenty | limited |
| Carbon products | CO₂ only | CO and/or C (soot), often with some CO₂ |
| Flame | blue, hot | yellow, smoky |
| Energy released | the most possible for the fuel | less |
| Hazards | CO₂ adds to climate change | toxic CO, soot (harms lungs), wasted fuel, plus CO₂ |
Why carbon monoxide is so dangerous
Carbon monoxide is colourless, odourless and tasteless, so you cannot notice it. When breathed in, CO attaches to haemoglobin in red blood cells much more strongly than oxygen does, so the blood carries less oxygen to the body. Early symptoms (headache, dizziness, nausea, tiredness) are easy to mistake for the flu. High levels can cause unconsciousness and death.
Prevention:
- Furnaces, water heaters, fireplaces and wood stoves need yearly inspection and clear vents and chimneys (including clearing snow away from outside vents in winter).
- Never run a car, generator, barbecue or camp stove inside a garage, house or tent.
- Ontario law requires carbon monoxide alarms in homes with a fuel-burning appliance, a fireplace or an attached garage. Test them regularly, like smoke alarms.
- If a CO alarm sounds, get everyone outside into fresh air and call 911 from outside.
Worked exampleComplete combustion of a hydrocarbon
Write the balanced equation for the complete combustion of pentane, C₅H₁₂.
- C: 5 carbon atoms → 5CO₂.
- H: 12 hydrogen atoms → 6H₂O.
- O on the right: 5 × 2 + 6 × 1 = 16 atoms → 8O₂.
- Check: C 5 = 5; H 12 = 12; O 16 = 10 + 6. ✔
C₅H₁₂(l) + 8O₂(g) → 5CO₂(g) + 6H₂O(g)
Worked exampleA fuel that contains oxygen
Write the balanced equation for the complete combustion of methanol, CH₃OH.
- C: 1 → CO₂. H: 4 → 2H₂O.
- O on the right: 2 + 2 = 4. The fuel already supplies 1, so O₂ must supply 3 atoms: 3/2 O₂.
- Double everything: 2CH₃OH + 3O₂ → 2CO₂ + 4H₂O.
- Check: C 2 = 2; H 8 = 8; O 2 + 6 = 8 = 4 + 4. ✔
2CH₃OH(l) + 3O₂(g) → 2CO₂(g) + 4H₂O(g)
Worked exampleIncomplete combustion
Propane burns in a poorly ventilated space, producing carbon monoxide and water. Write the balanced equation.
- C: 3 → 3CO. H: 8 → 4H₂O.
- O on the right: 3 + 4 = 7 → 7/2 O₂.
- Double: 2C₃H₈ + 7O₂ → 6CO + 8H₂O.
- Compare with complete combustion, C₃H₈ + 5O₂ → 3CO₂ + 4H₂O: per propane molecule, incomplete combustion used 3.5 O₂ instead of 5. Less oxygen, less energy, toxic product.
2C₃H₈(g) + 7O₂(g) → 6CO(g) + 8H₂O(g)
Hands-on activityThe chemistry of a candle
You need: a paraffin candle in a stable holder on a metal tray or plate, matches or a lighter (grown-up), a stainless-steel spoon, kitchen tongs, a large clean glass jar, a timer, and a grown-up.
Safety: a grown-up lights the candle and stays the whole time. Tie back long hair and roll up sleeves. Keep paper and fabric away. The spoon and jar get hot: hold the spoon with tongs and let everything cool before touching. Never leave a flame unattended.
Paraffin wax is a mixture of hydrocarbons, so a candle is a small combustion lab.
- Water test: hold the cold spoon (with tongs) a few centimetres above the flame for 2 seconds. Look at the underside: it fogs with tiny droplets of water, a combustion product.
- Soot test: hold the spoon in the yellow part of the flame for 3 seconds. A black layer of soot (carbon) forms. The spoon cooled the flame and blocked oxygen, causing incomplete combustion.
- Oxygen test: turn the jar upside down over the candle and time how long until the flame goes out. Repeat with a smaller jar if you have one.
Questions: Which product of combustion did each test show? Why did the flame go out under the jar (think fire triangle)? Predict how the time would change with a jar twice the volume, and test it if you can. The flame uses up only part of the oxygen in the jar before it goes out, so do not use this to calculate the percentage of oxygen in air.
Just for funThe famous scientist Michael Faraday gave a whole series of Christmas lectures for young people (in 1848, and again in 1860) about just one object: a candle. The 1860 talks were published as The Chemical History of a Candle and are still in print. So if anyone says a candle is boring, you can point out that it once filled an entire lecture series.
Your turn
1Balance the complete combustion of ethane, C₂H₆.
2C₂H₆(g) + 7O₂(g) → 4CO₂(g) + 6H₂O(g)
2Balance the complete combustion of hexane, C₆H₁₄.
2C₆H₁₄(l) + 19O₂(g) → 12CO₂(g) + 14H₂O(g). One hexane: 6CO₂, 7H₂O, 19 O atoms = 19/2 O₂; double.
3Balance the complete combustion of ethanol, C₂H₅OH.
C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(g)
4Balance the incomplete combustion of methane to carbon monoxide and water.
2CH₄(g) + 3O₂(g) → 2CO(g) + 4H₂O(g)
5Give two observations that tell you combustion is incomplete.
A yellow (not blue) flame; soot or smoke; black deposits on cookware; a CO alarm sounding.
6Why must outdoor furnace vents be kept clear of snow?
A blocked vent stops exhaust gases from leaving and limits the air supply, which can cause incomplete combustion and push carbon monoxide into the home.
More practice: warm-up
1What are the products of complete combustion of a hydrocarbon?
Carbon dioxide and water.
2Name the three sides of the fire triangle.
3What colour is a complete combustion flame?
4Which toxic gas is produced by incomplete combustion?
5Why can't people smell carbon monoxide?
It is odourless (and colourless and tasteless).
More practice: on your own
1Balance the complete combustion of glucose, C₆H₁₂O₆.
C₆H₁₂O₆(s) + 6O₂(g) → 6CO₂(g) + 6H₂O(l). C: 6CO₂; H: 6H₂O; O right = 18; glucose gives 6, so 12 more = 6O₂.
2Balance the incomplete combustion of butane, C₄H₁₀, producing solid carbon and water.
2C₄H₁₀(g) + 5O₂(g) → 8C(s) + 10H₂O(g). One butane: 4C and 5H₂O, which has 5 O = 5/2 O₂; double.
3Octane, C₈H₁₈, burns partly incompletely in a car engine. Write a balanced equation in which all the carbon becomes carbon monoxide.
2C₈H₁₈(l) + 17O₂(g) → 16CO(g) + 18H₂O(g). One octane: 8CO and 9H₂O = 17 O atoms = 17/2 O₂; double.
4Compare the oxygen needed for complete vs incomplete (CO) combustion of one methane molecule.
Complete: CH₄ + 2O₂ → CO₂ + 2H₂O, 2 O₂. Incomplete: 2CH₄ + 3O₂ → 2CO + 4H₂O, 1.5 O₂ per methane. Incomplete combustion uses less oxygen, which is why it happens when oxygen is limited.
5A wood stove is burning with a lazy yellow flame and a lot of smoke. Suggest what the owner should change and explain using the fire triangle.
Open the air damper (or check the chimney is clear) to let in more oxygen. With more oxygen, combustion becomes more complete: hotter flame, more heat from the same wood, less soot and less carbon monoxide.
Challenge
1Write a balanced equation for the incomplete combustion of propane that produces carbon dioxide, carbon monoxide and water in a 1 : 2 ratio of CO₂ : CO.
C₃H₈(g) + 4O₂(g) → CO₂(g) + 2CO(g) + 4H₂O(g). C: 1 + 2 = 3; H: 8 = 8; O right: 2 + 2 + 4 = 8 = 4O₂. ✔
2Explain why burning the same mass of fuel releases less energy in incomplete combustion than in complete combustion.
In incomplete combustion some carbon is only partly oxidized (to CO) or not oxidized at all (soot). Forming CO and C releases less energy than forming CO₂, and the CO and soot still contain chemical energy that could have been released. So less of the fuel's chemical energy is turned into heat.
3Ethanol is sometimes called a cleaner fuel than gasoline. Write the complete combustion equations for ethanol and for octane, and use them to give one argument for and one argument against that claim.
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O and 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O. For: ethanol already contains oxygen and burns more cleanly, and when made from plants, the plants absorbed CO₂ as they grew. Against: ethanol still produces CO₂ when burned; it releases less energy per litre, so more must be burned; and growing, fertilizing and processing the crops uses energy that may come from fossil fuels.
Quiz
1. What is the coefficient of O₂ in the balanced complete combustion of CH₄?
2. Which product shows that combustion was incomplete?
4. Why is carbon monoxide dangerous?
5. Which is the balanced complete combustion equation for ethanol?
6. Which action removes the oxygen side of the fire triangle?
This lesson has a real safety message. Use it to check your home: Are your carbon monoxide alarms working and dated? Are outdoor vents clear of snow in winter? Has the furnace been serviced?
Common difficulties:
- Balancing oxygen last and remembering to subtract oxygen already in the fuel (alcohols, glucose).
- Thinking incomplete combustion has only one correct equation. It can produce different mixtures; the question tells you which products to use.
Activity safety: a grown-up lights and supervises the candle the whole time, long hair is tied back, the spoon is held with tongs, and the jar is allowed to cool before it is touched. Keep a lid or damp cloth nearby.