HydrocarbonsEdexcel GCSE Chemistry: Revision notes
Section 1
What are alkanes and why are they saturated?
Alkanes are hydrocarbons — compounds of carbon and hydrogen only. The four required members are:
| Alkane | Formula |
|---|---|
| Methane | CH₄ |
| Ethane | C₂H₆ |
| Propane | C₃H₈ |
| Butane | C₄H₁₀ |
Alkanes are saturated hydrocarbons because every carbon-carbon bond is a single bond — there are no C=C double bonds, so the carbon atoms hold the maximum possible number of hydrogen atoms. You should be able to draw each molecule showing every covalent bond.
Section 2
What are alkenes and why are they unsaturated?
Alkenes are hydrocarbons containing at least one C=C double bond. The required members are ethene, propene, and but-1-ene/but-2-ene:
| Alkene | Formula |
|---|---|
| Ethene | C₂H₄ |
| Propene | C₃H₆ |
| But-1-ene | C₄H₈ |
| But-2-ene | C₄H₈ |
Alkenes are unsaturated hydrocarbons because they contain the functional group C=C, meaning not every carbon holds the maximum possible number of hydrogens — more hydrogen atoms could be added across the double bond.
Remember but-1-ene and but-2-ene have the same molecular formula (C4H8) but the double bond is in a different position — you must be able to identify which is which from the structure.
Section 3
How does ethene react with bromine, and how does bromine water test for unsaturation?
Alkenes undergo an addition reaction with bromine: the C=C double bond opens up and a bromine atom adds to each carbon, forming a saturated dibromo-compound.
Ethene + bromine → 1,2-dibromoethane (C₂H₄ + Br₂ → C₂H₄Br₂)
This same addition reaction pattern extends to other alkenes reacting with bromine.
Bromine water (orange/brown solution) is used to distinguish between alkanes and alkenes:
- Alkenes decolourise bromine water (orange/brown → colourless) because the addition reaction removes the bromine from solution
- Alkanes do not decolourise bromine water, because they are saturated and cannot undergo an addition reaction with it
When describing the bromine water test, state both outcomes: alkene decolourises it, alkane does not — a mark scheme usually awards a point for each half of the comparison.
Section 4
How do alkanes and alkenes combust?
Complete combustion of both alkanes and alkenes is an oxidation reaction: the hydrocarbon reacts with sufficient oxygen to produce carbon dioxide and water only, releasing energy.
General word equation: hydrocarbon + oxygen → carbon dioxide + water
This applies to both saturated (alkane) and unsaturated (alkene) hydrocarbons — the presence of a C=C bond does not change the products of complete combustion, only the products of addition reactions like the bromine water test.
Complete combustion of ethene: C2H4 + 3O2 → 2CO2 + 2H2O
Must Know
- Alkanes (methane, ethane, propane, butane) are saturated — only C-C single bonds
- Alkenes (ethene, propene, but-1-ene, but-2-ene) are unsaturated — contain the C=C functional group
- Ethene + bromine undergoes an addition reaction to form 1,2-dibromoethane; this pattern extends to other alkenes
- Bromine water distinguishes alkenes (decolourises it) from alkanes (no change) — the standard test for unsaturation
- Complete combustion of both alkanes and alkenes produces carbon dioxide and water, releasing energy
- Be able to draw full structures of methane, ethane, propane, butane, ethene, propene, but-1-ene, but-2-ene showing every covalent bond
That's the notes covered.
Carry on to the next subtopic.