Year 9 Science · Unit 2 · Lesson 11

Hydrocarbons and Simple Alkanes

Apply Worksheet

Name

Date

Class

Learning Goals

Apply the general formula CₙH₂ₙ₊₂ to find the molecular formula of any alkane.
Explain why boiling point increases as alkane chain length increases.
Predict properties of alkanes based on their position in the homologous series.

Warm Up

Order the steps

Number the steps from 1 to 6 to show the correct procedure for using the general formula CₙH₂ₙ₊₂ to find the molecular formula of hexane. Step 1 = what you do first.

Order	Step
	Draw the structural formula as a chain of 6 carbon atoms with hydrogen atoms attached: CH₃–CH₂–CH₂–CH₂–CH₂–CH₃
	Write the final molecular formula: C₆H₁₄
	Substitute n = 6 into the formula: C₆H₂×₆₊₂
	Name the compound: hexane (hex- = 6 carbons, -ane = alkane)
	Identify that hexane has n = 6 carbon atoms
	Calculate the number of hydrogen atoms: 2 × 6 = 12, then 12 + 2 = 14

Your Turn

Because… chain

Fill in the missing effects. Each cause leads to the next step in the chain. Use words and ideas from the lesson.

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

→

Larger molecules have greater surface area for 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

Overall outcome:

Show What You Know

1. Methane (CH₄) is a gas at room temperature but pentane (C₅H₁₂) is a liquid. Use what you know about chain length and intermolecular forces to explain why pentane has a higher boiling point than methane.

Apply 3 marks

2. Australia uses LPG (mainly propane, C₃H₈) in remote areas where natural gas pipelines do not reach. Using the general formula CₙH₂ₙ₊₂, confirm the molecular formula of propane and explain one reason why LPG can be stored in portable tanks but natural gas (methane) cannot easily be stored the same way at room temperature.

Apply 3 marks

Wrap Up

In one sentence, what was the main idea of this lesson?