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

1. What Makes Alkenes Special?

You've met alkanes - the saturated hydrocarbons with single bonds everywhere. But what happens when carbon atoms form a double bond? You get a completely different family of compounds with their own personality.

The C=C double bond is what we call a functional group - it's like the 'business end' of the molecule that determines how it reacts. Think of it as the molecule's personality trait.

2. The Functional Group Principle

Here's a powerful idea that unlocks all of organic chemistry: it's the functional group that determines how a molecule reacts, not the rest of the carbon chain.

Whether you have ethene (C2H4) or a massive alkene with 20 carbons, they'll react in similar ways because they both have that C=C double bond. The functional group is like a chemical 'signature' - it tells you what reactions to expect.

This principle saves you memorising thousands of reactions. Learn how the C=C group behaves, and you can predict what any alkene will do.

3. Alkene Structure and Naming

Let's see what alkenes actually look like. The simplest alkene is ethene, with just two carbons joined by a double bond.

Notice how the double bond makes the molecule flat around those two carbons. This is different from alkanes, where the carbons can rotate freely. The double bond locks the shape.

The naming follows a pattern: ethene, propene, butene. The '-ene' ending tells you there's a C=C double bond somewhere in the molecule.

4. Why Alkenes Burn Differently

Here's where alkenes show their true colours. When you burn them, something interesting happens - they produce smoky flames. But why?

Alkenes have a higher carbon-to-hydrogen ratio than alkanes. When they burn, there often isn't enough oxygen to convert all the carbon to CO2. Instead, you get incomplete combustion.

Worked example: Complete vs Incomplete Combustion of Ethene

Compare what happens when ethene burns in plenty of oxygen versus limited oxygen.

1. $C_2H_4 + 3O_2 \rightarrow 2CO_2 + 2H_2O$ — Plenty of oxygen available - clean burn
2. $C_2H_4 + 2O_2 \rightarrow 2CO + 2H_2O$ — Limited oxygen - produces toxic CO
3. $C_2H_4 + O_2 \rightarrow 2C + 2H_2O$ — Very little oxygen - produces carbon soot
4. Smoky flame with black particles — The carbon particles make the flame smoky

Answer: Alkenes burn with smoky flames because incomplete combustion produces carbon particles (soot)

Those black particles in the smoke? That's unburnt carbon - soot. This is why alkenes tend to burn with smokier flames than alkanes.

5. Exam Technique and Common Mistakes

Alkene questions often trip students up in predictable ways. Here's how to avoid the traps and pick up full marks.

Common Exam Mistake: Students often write the products of incomplete combustion as CO2 + H2O. Remember - if it's incomplete combustion, you get CO and C, not CO2.

Mark-winning tip: When explaining why alkenes burn with smoky flames, mention both 'incomplete combustion' and 'carbon particles' - both terms often appear in mark schemes.

Exam precision note: In Introduction to Alkenes and Their Structure - Reactions of alkenes - Introduction to Alkenes and Their Structure, 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 a carbon-carbon double bond. That double bond gives them different chemistry from alkanes and also explains the general formula CnH2n for simple alkenes. Structural formulae are important because they show exactly where the double bond is.