GCSE · Chemistry · Edexcel · Spec 1CH0

Hydrocarbons defined

Methane has carbon and hydrogen in it. So does an alcohol. Only one of them is a hydrocarbon, and the whole difference comes down to one small word.

Chemistry · Hydrocarbons

Is it a hydrocarbon? Run the test

Which kinds of atom do each substance's molecules contain? Tick every set that fits (a substance can be in more than one), then check. The ones that land in carbon and hydrogen with nothing else are the hydrocarbons.

  • A Contains carbon atoms
  • B Contains hydrogen atoms
  • C Contains an element other than carbon and hydrogen
  1. Methane, CH₄
  2. Cyclohexane, C₆H₁₂
  3. Ethene, C₂H₄
  4. Ethanol (an alcohol), C₂H₅OH
  5. Water, H₂O
  6. Carbon dioxide, CO₂
  7. Hydrogen gas, H₂

Chemistry · Hydrocarbons

Hydrocarbon molecules, drawn out
MethaneCH₄HHHHCEthaneC₂H₆HHHHHHCCPropaneC₃H₈HHHHHHHHCCCButaneC₄H₁₀HHHHHHHHHHCCCCEtheneC₂H₄HHHHCC

View: All five molecules. Showing 5 layers: Methane, Ethane, Propane, Butane, Ethene

View

Layers

Explore

Switch views to compare, tap a hotspot, and count the lines leaving each atom.

Five hydrocarbons, each made of carbon and hydrogen atoms only.

Every one is made of carbon and hydrogen atoms only, joined by covalent bonds. Count the lines leaving each atom.

Chemistry · Hydrocarbons

So what is crude oil?

Crude oil is pumped out of the ground and turned into fuels. Someone in your class calls it 'a hydrocarbon'.

Which idea is closest to what you think crude oil is?
How sure are you?

Chemistry · Hydrocarbons

What job is the hydrocarbon doing?

Hydrocarbons get used in three main ways. Pick a use, then pick the job the hydrocarbon is doing in it.

Still to sort

Fuel (0)

Burned, for example to generate electricity or to power vehicles.

Where the line is: A fuel is burned. Feedstock is a raw material that other substances are made from.

Lubricant (0)

Used to keep moving parts lubricated.

Feedstock (0)

A raw material used in chemical processes to produce other substances.

Where the line is: The hydrocarbon is the raw material, and the substance made from it (a petrochemical) is the product.

7 of 7 still to sort.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Carbon and hydrogen atoms only, and why that one little word matters so much

What you need to know

  • A hydrocarbon is a compound whose molecules contain carbon and hydrogen atoms only.
  • Hydrocarbons are simple covalent (simple molecular) substances.
  • To test a substance, ask of its molecules: carbon? hydrogen? anything else? Anything else rules it out.
  • Have a goSam says: 'CH₃OH has carbon and hydrogen in it, so it's a hydrocarbon.' Sam skipped a step of the test. Which one?

    Sam never asked 'anything else?' CH₃OH also has an oxygen atom.

    The test is carbon and hydrogen only, so an atom of any other element rules the molecule out, even just one.

  • Methane (CH₄) and cyclohexane pass the test. An alcohol fails it, because of the oxygen in its OH group.
  • In the hydrocarbons you'll meet next, every carbon atom forms four covalent bonds and every hydrogen atom forms one.
  • Have a goOne carbon atom is bonded only to hydrogen atoms, all by single bonds. Carbon forms four bonds and hydrogen one. How many hydrogen atoms does it hold?

    Four. That molecule is methane, CH₄.

    Each hydrogen uses up one of the carbon's four bonds, so it takes four hydrogens to fill them.

  • Alkanes are hydrocarbons with only single bonds between carbon atoms: methane, ethane, propane and butane.
  • Alkenes, such as ethene, are hydrocarbons with one or more double covalent bonds between carbon atoms.
  • Have a goA molecule has only carbon and hydrogen atoms, and a double bond between two of its carbon atoms. Alkane or alkene?

    An alkene (and a hydrocarbon).

    Carbon and hydrogen only makes it a hydrocarbon; a double bond between carbon atoms is what makes it an alkene rather than an alkane.

  • Hydrocarbons are found mostly in crude oil and natural gas, and most of the ones we use come from crude oil.
  • Crude oil itself is not one hydrocarbon. It is a complex mixture of hydrocarbons, separated by fractional distillation.
  • Hydrocarbons are used mainly as fuels, and also as lubricants and as feedstock: raw materials for making other substances.

The big picture

A hydrocarbon is a compound whose molecules contain carbon and hydrogen atoms only. Run that test on one compound at a time: methane and cyclohexane pass, an alcohol fails because of its oxygen. Hydrocarbons are found mostly in crude oil and natural gas, and they are used as fuels, lubricants and feedstock.

Key points

1Hydrocarbons contain carbon and hydrogen atoms only: 'only' is the word that decides.
2Alkanes (single bonds between carbon atoms) and alkenes (one or more double bonds) are both hydrocarbons.
3Crude oil is a complex mixture of hydrocarbons, not a single hydrocarbon. Hydrocarbons are found mostly in crude oil and natural gas.
4Hydrocarbons are used as fuels, as lubricants and as feedstock for petrochemicals.

Worked example

Problem

Decide whether each substance is a hydrocarbon: (a) propane, C₃H₈; (b) the alcohol with formula C₃H₇OH.

⚠ Watch out

Thinking 'it contains carbon and hydrogen' is enough. A hydrocarbon has carbon and hydrogen ONLY, so an alcohol, with the oxygen in its OH group, is out. A second slip is calling crude oil 'a hydrocarbon' when it is a complex mixture of hydrocarbons.

🧠

Memory hook

Hydro-carbon: hydrogen plus carbon, and the door policy is strict. Nobody else gets in.

✓

Check yourself

Without looking back: name one hydrocarbon, and one compound with carbon and hydrogen that is not a hydrocarbon. What does the second have that the first doesn't?

Flashcards

(11)
What is a hydrocarbon?
A compound whose molecules contain carbon and hydrogen atoms only.
An alcohol has carbon and hydrogen atoms in it. What stops it being a hydrocarbon?
The oxygen in its OH group: that is a third element in the molecule.
What kind of substances are hydrocarbons?
Simple covalent (simple molecular) substances.
In the alkanes and alkenes, how many covalent bonds does each carbon atom form, and each hydrogen atom?
Each carbon atom forms four; each hydrogen atom forms one.
What is an alkane?
A hydrocarbon with only single bonds between carbon atoms, e.g. methane, ethane, propane, butane.
What is an alkene?
A hydrocarbon with one or more double covalent bonds between carbon atoms, e.g. ethene.
Where are hydrocarbons found mostly?
In crude oil and natural gas. Most of the hydrocarbons we use come from crude oil.
Is crude oil a single hydrocarbon?
No. It is a complex mixture of hydrocarbons, separated by fractional distillation.
What are the main uses of hydrocarbons?
Fuels, lubricants and feedstock.
What is feedstock?
A raw material used in chemical processes to produce other substances.
Name four petrochemicals made from hydrocarbon feedstock.
Polymers, pharmaceutical drugs, solvents and detergents.

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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