GCSE · Chemistry · AQA · Spec 8462

Properties of hydrocarbons and combustion

Light a gas hob and the fuel seems to vanish into thin air. It doesn't. Not one atom is lost. So where does it all go?

Chemistry · Reaction balancer

Burn it, then balance it

When a hydrocarbon burns completely, its carbon and hydrogen combine with oxygen from the air. They are oxidised, and energy is released. No atom is lost along the way: every carbon ends up in carbon dioxide, every hydrogen in water. Tap + and − until every atom count matches.

Reactants
1
C3H8
1
O2
Products
1
CO2
1
H2O
C3→1✗
H8→2✗
O2→3✗
Conservation check: count atoms on both sides.Not yet.

Same order, bigger fuel: carbon, then hydrogen, then oxygen. Both products contain oxygen, so add up the oxygen in the CO₂ and the H₂O before you touch O₂.

Tap + or − under each molecule to set its coefficient.
Watch out: Only change the big numbers in front of each formula. Change a small number inside a formula and you have made a different substance.

Your turn

Now a fuel you haven't balanced

Write a balanced equation for the complete combustion of butane, C₄H₁₀.

  1. C₄H₁₀ + O₂ → CO₂ + H₂OComplete combustion always gives the same two products: carbon dioxide and water. Write them down first, then balance.
  2. missing step
Which line is step 2?

Chemistry · What counts?

Hydrocarbon or not?

You've been burning hydrocarbons. So what exactly counts as one? Pick a formula, then pick the group it belongs in.

Still to sort

Hydrocarbon (0)

Hydrogen and carbon atoms, and nothing else.

Where the line is: Containing hydrogen and carbon is not enough. The molecule must contain hydrogen and carbon ONLY.

Has C and H, plus another element (0)

Carbon and hydrogen are there, but so is something else.

Where the line is: This is the trap group. These look like hydrocarbons because they contain C and H, but one extra element rules them out.

No carbon, or no hydrogen (0)

One of the two elements is missing altogether.

8 of 8 still to sort.

Size and properties

Bigger molecules: what goes up, and what comes down?

Flammability: how easily each hydrocarbon catches fire. Read the notes first. They show what happens to boiling point and viscosity (how thick and slow-flowing a liquid is) as the molecules get bigger. Then predict.

1 · Catches fire most easily4 · Hardest to set alight

  1. C₈H₁₈ · 8 carbons

    A runny liquid. It boils well above room temperature.

  2. CH₄ · 1 carbon

    The smallest. Its boiling point is so far below room temperature that it is a gas.

  3. C₁₂H₂₆ · 12 carbons

    Boils at a higher temperature still, and pours more slowly than C₈H₁₈: it is more viscous.

  4. C₃H₈ · 3 carbons

    Also a gas at room temperature, but its boiling point is higher than CH₄'s.

Choosing a fuel

?

Reason it through

Why do hydrocarbons with small molecules make such useful fuels?

Link 1 of 4

First link · your turn

A fuel has to get from the tank to the place where it burns. Which property helps with that?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Follow every carbon and every hydrogen, and you can write the equation for burning any hydrocarbon.

What you need to know

  • A hydrocarbon is a molecule made up of hydrogen and carbon atoms only.
  • As hydrocarbon molecules get bigger, boiling point increases, viscosity increases and flammability decreases.
  • Boiling point, viscosity and flammability affect how hydrocarbons are used as fuels.
  • The combustion of hydrocarbon fuels releases energy.
  • During combustion, the carbon and hydrogen in the fuel are oxidised.
  • Complete combustion of a hydrocarbon produces carbon dioxide and water, and you should be able to write the balanced equation for a hydrocarbon of given formula.

The big picture

Hydrocarbons are molecules made of hydrogen and carbon atoms only. As hydrocarbon molecules get bigger, their boiling point and viscosity increase and their flammability decreases. These properties decide how useful a hydrocarbon is as a fuel: small ones flow easily, turn into gases easily and catch fire easily. Burning a hydrocarbon fuel releases energy, and the carbon and hydrogen in it are oxidised. Complete combustion produces carbon dioxide and water, and you can balance the equation for any hydrocarbon by sending its carbon to CO₂, its hydrogen to H₂O, and then balancing the oxygen.

Key points

1The test for a hydrocarbon is the word ONLY: hydrogen and carbon, and no other element. Ethanol and glucose contain C and H but also oxygen, so they are not hydrocarbons.
2Two up, one down: as molecules get bigger, boiling point and viscosity rise, but flammability falls.
3Small hydrocarbons make useful fuels because they flow easily, turn into gases easily and catch fire easily.
4Burning a hydrocarbon releases energy. Its carbon is oxidised to carbon dioxide and its hydrogen to water; no atoms are lost.
5To balance a complete combustion equation: carbon first, hydrogen second, oxygen last. If the oxygen needs a half, double every coefficient.

Worked example

Problem

Write a balanced equation for the complete combustion of pentane, C₅H₁₂.

⚠ Watch out

Counting only the oxygen in the carbon dioxide when working out how much O₂ is needed. The water contains oxygen too, so add both before you balance O₂.

🧠

Memory hook

C, H, then O: carbon to CO₂, hydrogen to H₂O, oxygen last. And for size: bigger means hotter to boil, thicker to pour, harder to light.

✓

Check yourself

Cover the page. Say which two properties rise and which one falls as hydrocarbon molecules get bigger. Then balance the complete combustion of ethane, C₂H₆, from a blank start.

Flashcards

(12)
What is a hydrocarbon?
A molecule made up of hydrogen and carbon atoms only.
Ethanol, C₂H₅OH, contains carbon and hydrogen. Why isn't it a hydrocarbon?
It also contains oxygen. A hydrocarbon has no element other than hydrogen and carbon.
What does viscosity describe?
How thick a liquid is: how slowly it flows. High viscosity means thick and slow-flowing.
What does flammability describe?
How easily a substance catches fire.
As hydrocarbon molecules get bigger, what happens to boiling point, viscosity and flammability?
Boiling point rises, viscosity rises, flammability falls. Two up, one down.
Why are hydrocarbons with small molecules useful as fuels?
They flow easily (low viscosity), turn into gases easily (low boiling point) and catch fire easily (high flammability).
What does the combustion of a hydrocarbon fuel release?
Energy.
What happens to the carbon and hydrogen in a fuel when it burns?
They are oxidised: they combine with oxygen. The carbon becomes carbon dioxide and the hydrogen becomes water.
What are the products of the complete combustion of a hydrocarbon?
Carbon dioxide and water.
In what order do you balance a combustion equation?
Carbon first (into CO₂), hydrogen second (into H₂O), oxygen last (O₂).
Balancing gives a half, such as 3½O₂. What do you do?
Double every coefficient in the equation, so all of them are whole numbers.
Why can't you balance an equation by changing the small numbers inside a formula?
That changes the substance itself. Only the numbers in front of formulas can change.

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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