GCSE · Chemistry · AQA · Spec 8462

Crude oil, hydrocarbons and alkanes

Crude oil looks like one black liquid. It's actually a crowd of very many compounds, and two quick questions will tell you which of them are alkanes, from the formula alone.

Crude oil · Spot the alkane

Two tests, one pile of formulae

Tap a formula, then tap the column it belongs in. Sort them all with Test 1, then switch to Test 2 and sort the same formulae again.

Read the symbols. A hydrocarbon is made of hydrogen and carbon atoms only. If there's even one atom of anything else, it's out.

Still to sort

Hydrocarbon (0)

Only H and C atoms in the formula

Where the line is: Having carbon and hydrogen isn't enough. There must be nothing else.

Not a hydrocarbon (0)

At least one atom that isn't H or C

9 of 9 still to sort.

Nothing here needs a name. The formula alone tells you everything.

Reading a drawn alkane

Where does the 2n + 2 come from?

View: Drawn out. Showing 2 layers: Carbon chain, Drawn out

View

Explore

switch views, then tap a part ↓

Here's C₅H₁₂ with every atom and every bond drawn. Drawing a molecule like this is called a displayed formula. Each line is one bond, and the carbons are joined in a chain by single bonds. Count the lines around any carbon: always four.

A five-carbon alkane drawn out in full, with every bond shown as a line

Your turn

From name to formula

The first four alkanes are methane, ethane, propane and butane, in that order. In order, they have 1, 2, 3 and 4 carbon atoms. Use CₙH₂ₙ₊₂ to write the formula of each one.

Methane: 1 carbon, so n = 1. Hydrogens = 2 × 1 + 2 = 4. Formula: CH₄.With one carbon atom there's no number after the C.
Ethane: 2 carbons, so n = 2. Hydrogens = 2 × 2 + 2 = 6. Formula: C₂H₆.
missing step

Which line is step 3?

Propane: 3 carbons, so n = 3. Hydrogens = 2 × 3 = 6. Formula: C₃H₆.Propane: 3 carbons, so n = 3. Hydrogens = 2 × 3 + 2 = 8. Formula: C₃H₈.Propane: 3 carbons, so n = 3. Hydrogens = 3 + 2 = 5. Formula: C₃H₅.

Zoom back out

So what actually is crude oil?

Crude oil comes out of the ground looking like one thick, dark liquid.

Which is closest to what you think crude oil is?

It's one substance, a single compound, like water is.It's a compound of carbon, hydrogen and oxygen.It's a mixture of lots of compounds, and every one of them is an alkane.It's a mixture of a huge number of compounds. Most are hydrocarbons, and most of those are alkanes.

How sure are you?

I'm sureI think soI'm guessing

It's one substance, a single compound, like water is.

This is a common misconception

It looks like one liquid, but it's a mixture of a very large number of different compounds. Think less 'glass of water', more 'giant bag of mixed sweets': lots of different things mixed together, each one still itself.

It's a compound of carbon, hydrogen and oxygen.

This is a common misconception

Two things to fix. It isn't one compound: it's a mixture of very many. And most of those compounds are hydrocarbons, so they contain no oxygen at all.

It's a mixture of lots of compounds, and every one of them is an alkane.

This is a common misconception

Very close, but 'every' is too strong. Most of the compounds are hydrocarbons, and most of those hydrocarbons are alkanes. That's 'most' at both steps, not 'all'.

It's a mixture of a huge number of compounds. Most are hydrocarbons, and most of those are alkanes.

This matches the evidence

Exactly. Picture it nested: all the compounds in crude oil → most of them are hydrocarbons → most of those are alkanes.

Crude oil is a mixture of very many compounds, mostly hydrocarbons, and mostly alkanes among those. Say 'most', not 'all'.

Crude oil's back story

Reason it through

Why is crude oil called a finite resource?

Link 1 of 4

First link · your turn

Start with what it's made from. What was crude oil, originally?

Locked — reveal the link above first

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

CₙH₂ₙ₊₂

Only hydrogen and carbon in the formula? Then count: if hydrogens = 2 × carbons + 2, it's an alkane.

What you need to know

Crude oil is a finite resource found in rocks, formed from the remains of ancient plankton buried in mud.
Crude oil is a mixture of a very large number of compounds.
Most of those compounds are hydrocarbons: molecules made of hydrogen and carbon atoms only.
Most of the hydrocarbons are alkanes, with the general formula CₙH₂ₙ₊₂.
The first four alkanes are methane, ethane, propane and butane.

The big picture

Crude oil is a finite resource found in rocks. It formed from the remains of an ancient biomass, mainly plankton, that was buried in mud. It's a mixture of a very large number of compounds. Most of them are hydrocarbons (hydrogen and carbon atoms only), and most of those are alkanes, which fit the general formula CₙH₂ₙ₊₂.

Key points

1Test 1 (hydrocarbon?): every symbol in the formula must be C or H. One other element, such as O or Cl, rules it out.

2Test 2 (alkane?): count the carbons, n. An alkane has exactly 2n + 2 hydrogens.

3Every alkane is a hydrocarbon, but not every hydrocarbon is an alkane.

4Methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀: 1, 2, 3 and 4 carbons.

5In the displayed formula of an alkane drawn as one chain, each carbon has 2 hydrogens (the 2n) and the two end carbons have one extra each (the +2).

6Crude oil is finite because it formed over a very long time and we use it far faster than new crude oil forms.

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Worked example

Problem

An alkane molecule has 34 hydrogen atoms. What is its molecular formula?

⚠ Watch out

Calling a compound a hydrocarbon just because it contains carbon and hydrogen. C₃H₈O has both, and its 8 hydrogens even match 2 × 3 + 2. But that oxygen atom means it isn't a hydrocarbon, so it can't be an alkane either.

🧠

Memory hook

Monkeys Eat Peanut Butter: Methane, Ethane, Propane, Butane, with 1, 2, 3 and 4 carbons. Then 'double it and add two' gives the hydrogens.

✓

Check yourself

Cover the page and try these. Is C₁₀H₂₂ an alkane? Is C₆H₁₂? For each one, say which test it passes or fails, and why.

Flashcards

(11)

Where is crude oil found, and what is it the remains of?

It's found in rocks. It's the remains of an ancient biomass, mainly plankton, that was buried in mud.

Why is crude oil a finite resource?

It formed over a very long time, and we use it far faster than new crude oil forms, so the supply is limited.

Is crude oil a single substance?

No. It's a mixture of a very large number of compounds.

What is a hydrocarbon?

A molecule made up of hydrogen and carbon atoms only.

What are most of the compounds in crude oil?

Hydrocarbons.

What are most of the hydrocarbons in crude oil?

Alkanes.

General formula of the alkanes?

CₙH₂ₙ₊₂, where n is the number of carbon atoms.

The first four alkanes, in order?

Methane, ethane, propane, butane.

Formulae of the first four alkanes?

CH₄, C₂H₆, C₃H₈, C₄H₁₀.

Can a hydrocarbon fail to be an alkane?

Yes, if its hydrogen count isn't 2n + 2 (for example C₂H₄). But every alkane is a hydrocarbon.

In the displayed formula of an alkane drawn as one chain, where do the 2n + 2 hydrogens come from?

Each carbon has two hydrogens (2n), and each of the two end carbons has one more (+2).

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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Two tests, one pile of formulae

From name to formula

So what actually is crude oil?

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