Introduction to Alkenes and Their General Formula Revision Notes | GCSE AQA Higher Chemistry

1. What Makes Alkenes Different?

You've met alkanes - the saturated hydrocarbons where carbon atoms hold hands with single bonds. But what happens when carbon gets greedy and forms a double bond? You get alkenes, and they behave very differently.

Picture methane ($CH_4$) - it's completely satisfied with four single bonds. Now imagine ethene ($C_2H_4$) - two carbons sharing two pairs of electrons in a double bond. This double bond changes everything about how these molecules react.

The key word here is 'unsaturated'. This doesn't mean they're unhappy - it means they contain fewer hydrogen atoms than they could theoretically hold. That double bond takes up space that hydrogen atoms would normally occupy.

2. The Alkene Formula Pattern

Every homologous series has a pattern, and alkenes follow the rule $C_nH_{2n}$. This means for every carbon atom, there are exactly twice as many hydrogen atoms. No more, no less.

Worked example: Using the Alkene Formula

An alkene contains 5 carbon atoms. What is its molecular formula?

1. $n = 5$ carbon atoms — Count the carbon atoms given in the question
2. $C_nH_{2n} = C_5H_{2 \times 5}$ — Substitute n=5 into the alkene formula
3. $C_5H_{10}$ — 2 × 5 = 10 hydrogen atoms
4. 5 carbons → 10 hydrogens ✓ — Hydrogen count is exactly double the carbon count

Answer: $C_5H_{10}$ (pentene)

Notice how systematic this is. You can work backwards too - if someone gives you $C_4H_8$, you know it's an alkene because 8 = 2 × 4.

3. Why Alkenes Are Called 'Unsaturated'

Compare ethane ($C_2H_6$) with ethene ($C_2H_4$). Both have 2 carbon atoms, but ethene has 2 fewer hydrogen atoms. This isn't an accident - it's because of that double bond.

Think of it like this: each carbon can form 4 bonds total. In alkanes, carbons use single bonds to other carbons and fill the rest with hydrogens. In alkenes, In a C=C double bond, each carbon uses two bonds to the other carbon instead of one, so each carbon has one fewer bond available for hydrogen and the molecule has two fewer H atoms overall..

Worked example: Comparing Alkane and Alkene with Same Carbon Count

Compare the hydrogen content of butane and butene (both have 4 carbons).

1. Alkane: $C_nH_{2n+2} = C_4H_{10}$ — Alkanes are saturated - maximum hydrogen
2. Alkene: $C_nH_{2n} = C_4H_8$ — Alkenes have the double bond
3. $10 - 8 = 2$ fewer hydrogens — Butene has 2 fewer H atoms than butane
4. Double bond replaces 2 C-H bonds — The C=C double bond 'costs' 2 hydrogen atoms

Answer: Butene ($C_4H_8$) has 2 fewer hydrogen atoms than butane ($C_4H_{10}$) due to the C=C double bond.

4. Recognising Alkenes from Their Formulae

You need to spot alkenes quickly in exams. The trick is checking if the hydrogen count fits the $C_nH_{2n}$ pattern. If it does, you've found an alkene.

Worked example: Identifying Alkenes from Molecular Formulae

Which of these are alkenes: $C_3H_8$, $C_5H_{10}$, $C_6H_{12}$, $C_4H_{10}$?

1. If alkene: $C_3H_{2 \times 3} = C_3H_6$ ≠ $C_3H_8$ — Not an alkene - too many hydrogens (it's propane)
2. If alkene: $C_5H_{2 \times 5} = C_5H_{10}$ ✓ — This IS an alkene (pentene)
3. If alkene: $C_6H_{2 \times 6} = C_6H_{12}$ ✓ — This IS an alkene (hexene)
4. If alkene: $C_4H_{2 \times 4} = C_4H_8$ ≠ $C_4H_{10}$ — Not an alkene - too many hydrogens (it's butane)

Answer: $C_5H_{10}$ and $C_6H_{12}$ are alkenes. The others are alkanes.

This pattern recognition is crucial for exam success. Examiners often mix alkanes and alkenes in questions to test if you can tell them apart.

5. Exam Technique and Common Mistakes

Alkene questions appear frequently in GCSE papers, often testing your ability to use the general formula or identify unsaturated compounds. Here's how to avoid the common traps.

**Trap**: Many students think 'unsaturated' means the molecule is unstable or incomplete. Actually, it's a precise term meaning fewer hydrogen atoms than the saturated equivalent due to multiple bonds.

**Mark-winner**: In exam questions asking you to explain why alkenes are unsaturated, always mention both the double bond AND the comparison with the corresponding alkane. For example: 'Ethene ($C_2H_4$) is unsaturated because it contains a C=C double bond and has 2 fewer hydrogen atoms than ethane ($C_2H_6$).'

Exam precision note: In Introduction to Alkenes and Their General Formula - Structure and formulae of alkenes - Reactions of alkenes and alcohols (chemistry only), 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.

Alkenes are unsaturated hydrocarbons because they contain at least one carbon-carbon double bond. Compared with the corresponding alkane, each carbon in the double bond has one fewer bond available for hydrogen, so the general formula becomes CnH2n instead of CnH2n+2. GCSE students only need to think about C=C double bonds here, not triple bonds.