Representing and recognising alkane molecules Revision Notes | GCSE AQA Higher Chemistry

1. What Are Alkanes?

Picture the simplest possible molecules made from just carbon and hydrogen. These are alkanes - the building blocks of natural gas, petrol, and countless other fuels we use every day.

The key word here is 'saturated' - this means every carbon atom forms four single bonds, with no double or triple bonds anywhere. Think of it like a carbon atom with four hands, each holding onto another atom.

2. The Four Alkanes You Must Know

You need to recognise four specific alkanes by name and formula. Let's build them up step by step, starting with the smallest.

Methane (CH4): One carbon atom surrounded by four hydrogen atoms. This is natural gas - what heats your home and powers gas cookers.

Ethane (C2H6): Two carbon atoms joined together, each with three hydrogen atoms. Found in natural gas alongside methane.

Propane (C3H8): Three carbons in a chain. This is camping gas - the fuel in portable gas canisters.

Butane (C4H10): Four carbons in a chain. Used in cigarette lighters and some camping stoves.

3. Different Ways to Show the Same Molecule

Here's where students often get confused. The same alkane molecule can be written in several different ways. It's like having different photos of the same person - they look different but show the same thing.

Worked example: Representing Ethane in Different Forms

Show ethane (2 carbon atoms, 6 hydrogen atoms) in all possible representations

1. C2H6 — Shows the actual number of each type of atom
2. H-C-C-H with H atoms above and below each C — Shows every single bond between atoms
3. CH3-CH3 or CH3CH3 — Groups the hydrogens with their carbon atoms
4. C-C (line drawing) — Shows just the carbon skeleton - hydrogens are implied

Answer: All four representations show the same molecule: ethane with 2 carbons and 6 hydrogens

4. Recognising Alkanes from Any Formula

You need to spot alkanes even when they're written in unfamiliar ways. Here's your foolproof method - it works every time.

Worked example: The Alkane Recognition Test

Is C5H12 an alkane? Use the general formula to check.

1. n = 5 carbons — This is our 'n' value in the general formula
2. CnH2n+2 becomes C5H(2×5)+2 — Substitute n = 5 into the general formula
3. H = 10 + 2 = 12 — So an alkane with 5 carbons should have 12 hydrogens
4. C5H12 matches C5H12 ✓ — The hydrogen count matches, so this IS an alkane

Answer: Yes, C5H12 is an alkane because it fits the general formula CnH2n+2

This test works for any formula. If the hydrogen count doesn't match CnH2n+2, it's not an alkane - it might be an alkene, because alkenes have the general formula CnH2n. Alkynes are beyond this GCSE treatment and do not have the formula CnHn..

5. Exam Technique - Getting Full Marks

Examiners have favourite ways to test alkanes. Here's how to spot their tricks and avoid the common traps that cost students marks.

Common exam mistake: Students see CH3CH2CH3 and think it's different from C3H8. Count the atoms - it's the same molecule (propane) written structurally.

Exam precision note: In Representing and recognising alkane molecules - Crude oil, hydrocarbons and alkanes - Carbon compounds as fuels and feedstock, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

Students should distinguish between molecular formulae, structural formulae, and displayed formulae. A molecular formula gives only the numbers of atoms, such as C2H6. A structural or condensed formula may show the carbon backbone more simply. A displayed formula shows every atom and every bond. When questions ask for a displayed formula, the answer must include all hydrogen atoms and all single bonds. This explicit distinction helps cover the specification point about representing alkanes in more than one way.