GCSE · Chemistry · Edexcel · Spec 1CH0

Homologous series

Methane, ethane, propane, butane: one simple rule makes them a family — and predicts how each longer member behaves.

Chemistry · Is it in the family?

Alkane or not? Test the formula

Pick a formula, then choose where it belongs. First check: is it only carbon and hydrogen? Then check: does the hydrogen count equal 2n + 2?

Still to sort

Fits CₙH₂ₙ₊₂ — an alkane (0)

Only C and H, and hydrogens = 2 × carbons + 2.

Where the line is: Being a hydrocarbon is not enough: the hydrogen count must be exactly 2n + 2.

Hydrocarbon, but doesn't fit (0)

Only C and H, but the hydrogen count is not 2n + 2.

Where the line is: It passes the first check (only C and H) and fails the second (the hydrogen count).

Not a hydrocarbon (0)

Contains at least one atom that is not carbon or hydrogen.

Where the line is: One atom of anything else — oxygen, say — and it is no longer a hydrocarbon, however alkane-like the rest looks.

7 of 7 still to sort.

Alkanes all fit one rule: CₙH₂ₙ₊₂. With n carbon atoms, an alkane has 2n + 2 hydrogen atoms. Use the rule — not your eyes — to sort each formula.

Chemistry · The first four alkanes

Four alkanes, drawn out

Showing 4 layers: Methane, Ethane, Propane, Butane

Explore

Pick a feature to highlight it on the molecules.

Each molecule is the one above it plus one CH₂ unit — the dashed box. Pick a feature below to look closer.

A displayed formula shows every atom and every bond. Read down the page and watch the chain grow.

Chemistry · Using the general formula

Build a formula from the carbon count

An alkane molecule has 7 carbon atoms. What is its molecular formula?

It's an alkane, so it follows CₙH₂ₙ₊₂. The number of carbon atoms is n, so n = 7.
missing step

Which line is step 2?

Number of hydrogen atoms = 2n + 2Number of hydrogen atoms = 2nNumber of hydrogen atoms = n + 2

Chemistry · Trends down the series

Which catches fire most easily?

Flammability — how easily each alkane catches fire

1 · Most flammable4 · Least flammable

Ethane, C₂H₆
2 carbon atoms
Butane, C₄H₁₀
4 carbon atoms
Methane, CH₄
1 carbon atom
Propane, C₃H₈
3 carbon atoms

Watch out: Two trends rise and two fall as the chain gets longer. Boiling point and viscosity go up; volatility and flammability go down.

Chemistry · Explaining the trend

Reason it through

Why does boiling point rise as the carbon chain gets longer?

Link 1 of 4

First link · your turn

Compare a methane molecule with a butane molecule. What is different about them?

Locked — reveal the link above first

Chemistry · What does 'similar' mean?

Same family — but how alike?

Methane (CH₄) and butane (C₄H₁₀) are members of the same homologous series: the alkanes.

Which is closest to what you think right now?

Same family means they behave the same in every way — same reactions, same boiling point.They react in the same kinds of way — both burn, for example — but physical properties like boiling point change gradually along the series.Butane is a bigger molecule, so it must take part in different kinds of reaction from methane.Butane boils at a higher temperature because boiling has to break more bonds inside its bigger molecule.

How sure are you?

I'm sureI think soI'm guessing

Same family means they behave the same in every way — same reactions, same boiling point.

This is a common misconception

Half right. The family link is about chemical properties: they take part in the same types of reaction. Physical properties are different, and they change gradually along the series — butane boils at a higher temperature than methane because its molecules are bigger.

They react in the same kinds of way — both burn, for example — but physical properties like boiling point change gradually along the series.

This matches the evidence

Exactly. Members of a homologous series have similar chemical properties: they take part in the same types of reaction, such as combustion. Their physical properties show a trend: boiling point, for instance, rises as the chain gets longer.

Butane is a bigger molecule, so it must take part in different kinds of reaction from methane.

This is a common misconception

Size changes the physical properties, not the kind of chemistry. Adding CH₂ units makes the molecule bigger, but it is still an alkane with the same type of bonds, so it takes part in the same types of reaction — both methane and butane undergo combustion.

Butane boils at a higher temperature because boiling has to break more bonds inside its bigger molecule.

This is a common misconception

Right trend, wrong reason. When an alkane boils, its molecules move apart from each other; the bonds inside each molecule stay intact. What has to be overcome is the attraction between molecules — and bigger molecules have more of it.

Same series → same types of reaction. Longer chain → physical properties change gradually.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

One general formula, one CH₂ step, and trends you can predict.

What you need to know

A homologous series is a family of compounds with the same general formula; neighbouring members differ by a CH₂ group.
Members of a series have similar chemical properties (the same types of reaction, e.g. combustion) and a gradual trend in physical properties.
Alkanes are saturated hydrocarbons (carbon and hydrogen only, single C–C bonds) with the general formula CₙH₂ₙ₊₂.
The first four alkanes are methane CH₄, ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀.
Longer chains mean higher boiling point and viscosity, but lower volatility and flammability, because larger molecules have more forces of attraction between them.

The big picture

A homologous series is a family of compounds that share the same general formula, with each member one CH₂ group bigger than the one before. The alkanes are a homologous series of saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. Members of a series take part in the same types of chemical reaction, such as combustion, but their physical properties change gradually along the series. As alkane chains get longer, boiling point and viscosity rise while volatility and flammability fall, because bigger molecules have more forces of attraction between them.

Key points

1Same general formula = same homologous series.

2Each member is the one before plus CH₂.

3Alkanes: CₙH₂ₙ₊₂ — double the carbons, add two.

4Same types of reaction; physical properties change gradually.

5Longer chain: boiling point and viscosity up; volatility and flammability down.

6Boiling overcomes forces between molecules, not bonds inside them.

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Worked example

Problem

An alkane molecule contains 22 hydrogen atoms. What is its molecular formula?

⚠ Watch out

Using 2n instead of 2n + 2 — for example, writing propane as C₃H₆ instead of C₃H₈. Double the carbons, then add two.

🧠

Memory hook

Hydrogens? "Double it, add two." Trends? Picture longer molecules as stickier: they boil later and flow more slowly, but evaporate and catch fire less easily. (Sticky is just a picture: the real cause is more attraction between bigger molecules.)

✓

Check yourself

An alkane has 9 carbon atoms. Predict its molecular formula, then say whether its boiling point is higher or lower than butane's — and explain why.

Flashcards

(13)

What is a homologous series?

A family of compounds that have the same general formula.

How do neighbouring members of a homologous series differ?

By a CH₂ group.

What does 'similar chemical properties' mean for members of a series?

They take part in the same types of chemical reaction — for example, combustion.

What happens to physical properties along a homologous series?

They show a trend: a gradual change as the carbon chain gets longer.

What is a hydrocarbon?

A compound made of hydrogen and carbon atoms only.

Why are alkanes described as saturated?

Their carbon atoms are joined only by single bonds.

General formula of the alkanes?

CₙH₂ₙ₊₂ — for n carbon atoms there are 2n + 2 hydrogen atoms.

Name the first four alkanes, in order, with formulae.

Methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀ — all end in -ane.

Longer alkane chain: what happens to boiling point and viscosity?

Both increase.

Longer alkane chain: what happens to volatility and flammability?

Both decrease.

Why does boiling point rise as alkane chains get longer?

Larger molecules have more forces of attraction between them, so more energy is needed to overcome those forces.

When an alkane boils, what is overcome?

The forces of attraction between molecules — the bonds inside each molecule do not break.

How can you tell from a formula alone whether a hydrocarbon is an alkane?

Check the hydrogens equal 2 × the carbons + 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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Alkane or not? Test the formula

Build a formula from the carbon count

Which catches fire most easily?

Same family — but how alike?

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