GCSE · Chemistry · Edexcel · Spec 1CH0

Complete combustion of hydrocarbons

Burn a fuel completely and nothing vanishes. Every atom just moves house. Can you track where each one goes?

Chemistry · Reaction balancer

Burn methane: where do the atoms go?

Methane is burning in a plentiful supply of oxygen. Tap + and − under each molecule until the atom counts match on both sides. You can't touch the formulae, only the numbers in front.

Reactants

CH₄

O₂

Products

CO₂

H₂O

C1→1✓

H4→2✗

O2→3✗

Conservation check: count atoms on both sides.Not yet.

CH₄ + 2O₂ → CO₂ + 2H₂O. Every carbon atom from the fuel leaves in carbon dioxide and every hydrogen atom leaves in water. That's why it takes two oxygen molecules to burn one methane molecule completely.

Tap + or − under each molecule to set its coefficient.

Exam line: Count one element at a time on each side. Fix the mismatch with a number in front of a formula, then recount.

Watch out: Don't change a small number inside a formula (turning H₂O into H₄O, say). That changes the substance. Only the big numbers in front can move.

hydrocarbon + oxygen → carbon dioxide + water

Combustion is a substance combining with oxygen and giving out heat and light, so it is exothermic. Tap a molecule to see where the fuel's atoms end up. The numbers in front are the balancer's job, so they aren't shown here.

Tap a molecule to see what it does in the reaction.

What do you think?

Where does the methane go?

Methane burns completely in plentiful oxygen on a Bunsen burner.

Which is closest to what you think has happened to the methane?

It's destroyed. Burning uses fuel up, so it's just gone.It turns into heat and light.Its atoms are rearranged: the carbon ends up in carbon dioxide and the hydrogen ends up in water.It just disappears as smoke.

How sure are you?

I'm sureI think soI'm guessing

It's destroyed. Burning uses fuel up, so it's just gone.

This is a common misconception

It feels that way because the gas and the flame seem to vanish. But the carbon and hydrogen atoms that were in the methane are all still there. They've combined with oxygen to make carbon dioxide and water. The atoms are rearranged, not destroyed, which is exactly why the equation balances.

It turns into heat and light.

This is a common misconception

Heat and light are real, but they're the energy given out (that's what exothermic means). They aren't what the fuel is made into. Fuel plus oxygen makes carbon dioxide and water, and the heat and light come out alongside.

Its atoms are rearranged: the carbon ends up in carbon dioxide and the hydrogen ends up in water.

This matches the evidence

Yes. Every carbon atom leaves as carbon dioxide and every hydrogen atom leaves as water. That's why the atoms on each side of the equation have to match.

It just disappears as smoke.

This is a common misconception

'Smoke' is a vague answer for something that has a precise one. In complete combustion the fuel's carbon ends up in carbon dioxide and its hydrogen ends up in water. Two named products, nothing left unexplained.

Burning rearranges atoms. It doesn't destroy them.

The Bunsen burner test

Air hole openvsAir hole closed

Same burner, same methane. The only thing that changes is the air hole.

Focus

Oxygen available

Air hole open

More oxygen gets in

Air hole closed

Limited oxygen

The insight

This is the one real difference. Everything else in the table follows from it.

Type of combustion

Air hole open

Complete: the methane burns completely

Air hole closed

Incomplete

Flame colour

Air hole open

Blue

Air hole closed

Orange

Energy released

Air hole open

Significant amounts of heat and light

Air hole closed

Less than with the air hole open

Check the method

Which line breaks the rules?

Balance the equation for burning methane completely: CH₄ + O₂ → CO₂ + H₂O

A student's attempt — which line goes wrong?

Carbon: 1 on the left, 1 on the right. Fine, no change needed.
Hydrogen: 4 on the left, 2 on the right. Put a 2 in front of H₂O to get 4 on the right.
Oxygen: 2 on the left. On the right there are 2 in CO₂ plus 2 in 2H₂O, which is 4. So I'll change O₂ to O₄ to get 4 on the left.
Now every element matches: CH₄ + O₄ → CO₂ + 2H₂O.

Edexcel Chemistry · Write it out

Explain the products

Explain why the products of the complete combustion of a hydrocarbon fuel are carbon dioxide and water. [3 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

There is a plentiful supply of oxygen, so the fuel burns completely. (1 mark)
The carbon in the fuel reacts with oxygen to produce carbon dioxide. (1 mark)
The hydrogen in the fuel reacts with oxygen to produce water. (1 mark)

Model answer

When a hydrocarbon burns in a plentiful supply of oxygen, there is enough oxygen to convert all of the fuel. The carbon in the fuel reacts with oxygen to produce carbon dioxide, and the hydrogen in the fuel reacts with oxygen to produce water.

Say the condition once, then give each element its own sentence: element, reacts with oxygen, product.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Burn a fuel in plenty of oxygen and every atom ends up in carbon dioxide or water.

What you need to know

Combustion is a substance combining with oxygen and giving out heat and light. That makes it exothermic: thermal energy goes out.
Hydrocarbons are compounds with hydrogen and carbon atoms only: methane CH₄, ethane C₂H₆, propane C₃H₈.
Have a goPropane is C₃H₈. Which two elements are in it, and could a hydrocarbon also have oxygen atoms in its molecules?
Carbon and hydrogen. No: a hydrocarbon has hydrogen and carbon atoms only.
That word 'only' is the test. Add oxygen atoms to the molecule and it stops being a hydrocarbon.
Fossil fuels contain hydrocarbons, and we burn them for energy: petrol and diesel in cars, cooking, making electricity.
Complete combustion means a plentiful supply of oxygen, enough to turn all of the fuel into carbon dioxide and water.
Follow the atoms: the fuel's carbon reacts with oxygen to make carbon dioxide, and its hydrogen reacts with oxygen to make water.
Have a goYour mate Sam says: 'When methane burns completely, the hydrogen just burns away to nothing.' Where does the hydrogen actually go?
Into water. The hydrogen reacts with oxygen to make water.
Sam has confused 'can't see it' with 'gone'. In complete combustion each element of the fuel turns up in a product, so nothing burns away to nothing.
In words: hydrocarbon + oxygen → carbon dioxide + water. Petrol or diesel fit the same pattern.
Mass is conserved, so a balanced equation has the same number of atoms of each element on both sides.
Formulae never change. You balance with numbers in front, like 2H₂O, which means two whole molecules of water.
Have a goHow many hydrogen atoms are in 2H₂O? Work it out before you look.
4 hydrogen atoms.
The big 2 in front multiplies the whole formula: two molecules, each with 2 hydrogen atoms. The small 2 inside H₂O stays put.
Bunsen burner with the air hole open: more oxygen, blue flame, complete combustion, and more energy than incomplete combustion.

The big picture

Combustion is a substance combining with oxygen and giving out heat and light. When a hydrocarbon burns completely, in a plentiful supply of oxygen, its carbon ends up in carbon dioxide and its hydrogen ends up in water. Because mass is conserved, the symbol equation must have the same number of atoms of each element on both sides, and you get there by changing the numbers in front of the formulae, never the formulae themselves.

Key points

1Combustion is a substance combining with oxygen; it gives out heat and light and is exothermic.

2A hydrocarbon's molecules contain hydrogen and carbon atoms only.

3Complete combustion needs a plentiful supply of oxygen. Its products are carbon dioxide and water.

4The fuel's carbon ends up in carbon dioxide and its hydrogen ends up in water.

5Word equation: hydrocarbon + oxygen → carbon dioxide + water.

6Balanced symbol equation for methane: CH₄ + 2O₂ → CO₂ + 2H₂O.

7Balance by putting numbers in front of formulae. Never change the formulae.

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Worked example

Problem

Balance the equation for the complete combustion of propane: C₃H₈ + O₂ → CO₂ + H₂O

⚠ Watch out

Making the equation balance by changing a formula (H₂O into H₄O, say). Only the big numbers in front are allowed to move.

🧠

Memory hook

C goes to CO₂, H goes to water. And numbers go in front of formulae, never inside them.

✓

Check yourself

A hydrocarbon burns completely. Without looking back, say where its carbon ends up, where its hydrogen ends up, and what the oxygen supply has to be like.

Flashcards

(13)

What is combustion?

The chemical combination of a substance with oxygen, producing heat and light.

What does exothermic mean?

A reaction in which thermal energy is given out.

What is a hydrocarbon?

A compound whose molecules contain hydrogen and carbon atoms only.

Name three hydrocarbons and give their formulae.

Methane CH₄, ethane C₂H₆, propane C₃H₈.

Where do hydrocarbons come from, and what do we burn them for?

Fossil fuels contain hydrocarbons. They are burned to provide energy, for example petrol and diesel in cars, cooking food and generating electricity.

When does complete combustion happen?

When a hydrocarbon burns in a plentiful supply of oxygen, so there is enough oxygen to convert all of the fuel into carbon dioxide and water.

A hydrocarbon burns completely. What does its carbon turn into, and what does its hydrogen turn into?

The carbon reacts with oxygen to make carbon dioxide. The hydrogen reacts with oxygen to make water.

Finish the word equation: hydrocarbon + oxygen → ?

carbon dioxide + water, for complete combustion.

Why must a symbol equation be balanced?

Mass is conserved, so the same number of atoms of each element must appear in the reactants and the products.

How do you balance an equation, and what must you never do?

Place numbers in front of the formulae. The formulae themselves (such as CH₄ and H₂O) cannot be changed.

Balanced symbol equation for the complete combustion of methane?

CH₄ + 2O₂ → CO₂ + 2H₂O

Bunsen burner, air hole open: what are the oxygen supply, flame and energy release?

More oxygen, so methane burns completely with a blue flame, releasing significant amounts of heat and light. Complete combustion releases more energy than incomplete combustion.

Bunsen burner, air hole closed: what happens?

Limited oxygen, so combustion is incomplete, and the flame is orange.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Burn methane: where do the atoms go?

Where does the methane go?

Which line breaks the rules?

Explain the products

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