Year 9 Science · Unit 2 · Lesson 11

Hydrocarbons and Simple Alkanes

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Learning Goals

Use the general formula CₙH₂ₙ₊₂ in both directions: from carbons to hydrogens, and back.
Explain why boiling point and viscosity rise along the alkane series.
Use data to choose the right alkane for a job.

Warm Up

1. State two features that make the alkanes a homologous series.

Recall 2 marks

Order the steps

These five steps for octane (8 carbons) are jumbled. Number them 1 to 5 in the correct order.

Order	Step
	Work out the hydrogens: 2 × 8 = 16, then 16 + 2 = 18.
	Check: 18 hydrogens is exactly 2 more than twice the 8 carbons.
	Count the carbon atoms: octane has 8, so n = 8.
	Write the molecular formula: C₈H₁₈.
	Substitute n = 8 into CₙH₂ₙ₊₂.

Your Turn

2. Work backwards: one member of the series holds 16 H. What is n? Give the full formula.

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Molecular formula

Because… chain

Fill in the two empty boxes. Each box is caused by the one before it.

Alkane chain length increases from methane (n = 1) to pentane (n = 5)

Larger molecules have more surface area in contact with neighbouring molecules

More energy is needed to pull the molecules apart and turn the liquid into a gas

Boiling point increases along the homologous series

Scenario: bottled gas

LPG for barbecues is mainly propane, which boils at −42 °C. It is sold as a liquid, squeezed under pressure into a steel bottle. Natural gas is mainly methane, which boils at −162 °C. It reaches homes as a gas through pipes.

3. Use CₙH₂ₙ₊₂ to confirm that propane (3 carbon atoms) is C₃H₈. Then use the two boiling points to explain why propane is easier to keep as a liquid in a bottle than methane.

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Go Deeper

Read the data

Viscosity measures how thick a liquid is, or how slowly it flows. Water at 25 °C has a viscosity of about 0.9 mPa·s.

Table 1. Five liquid alkanes at 25 °C

Alkane	Carbon atoms	Boiling point (°C)	Viscosity (mPa·s)
Hexane	6	69	0.30
Octane	8	126	0.51
Decane	10	174	0.84
Dodecane	12	216	1.38
Hexadecane	16	287	3.03

Source: published viscosity measurements, rounded.

4. Write the molecular formula of decane and of hexadecane.

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5. Describe how boiling point and viscosity change as the chain gets longer. Does viscosity rise by the same amount for every 2 extra carbon atoms? Quote two values as evidence.

Analyse 3 marks

6. Which alkane in Table 1 is closest to water in how it flows? Use the data to decide.

Analyse 1 mark

Show What You Know

7. A bike shop needs a light oil for bicycle chains. It must cling to the chain rather than drip off, and it must not evaporate away in a few days. Use Table 1 to explain why hexadecane is a better choice than hexane, and link your reasons to chain length.

Analyse 5 marks