Reactions of alkenes — AQA GCSE Chemistry
Test yourself on Reactions of alkenes with AQA GCSE practice questions.
7 days Premium · Then free forever · No card, no charge
Reactions of alkenes explained
Alkenes are a homologous series of unsaturated hydrocarbons whose molecules contain at least one carbon-carbon double bond, C=C.
Read the full explanation
This double bond is the functional group: the reactive part that gives the series its characteristic chemistry. Because of the C=C double bond, alkene molecules contain fewer hydrogen atoms than the maximum possible, meaning they are not saturated with hydrogen. Ethene, C₂H₄, is the simplest alkene; propene, C₃H₆, and butene, C₄H₈, follow. The general formula CₙH₂ₙ applies to non-cyclic alkenes with one double bond. Students should draw the displayed formula of ethene, recognise the C=C in a larger molecule, and distinguish alkenes from the saturated alkanes, which contain only single C–C bonds.
Your focus
- Define an alkene as a hydrocarbon containing the C=C functional group.
- Draw and interpret the displayed formula of ethene and other simple alkenes.
- Use the general formula CₙH₂ₙ to deduce the molecular formula of a non-cyclic alkene.
Reactions of alkenes exam tips
Marking Points
- Alkenes are hydrocarbons: their molecules contain carbon and hydrogen only.
- The functional group of an alkene is the carbon-carbon double bond, C=C.
- The C=C makes an alkene unsaturated, because the carbons are bonded to fewer hydrogen atoms than the maximum possible.
- Ethene, C₂H₄, is the simplest alkene and its displayed formula shows a double line between the two carbon atoms.
- The general formula of a non-cyclic alkene with one C=C is CₙH₂ₙ, so ethene is C₂H₄ and propene is C₃H₆.
- Alkanes are saturated and contain only single carbon-carbon bonds, so they are a different homologous series from alkenes.
Examiner Tips
- 💡When asked to identify an alkene, locate the C=C and state that the compound is unsaturated.
- 💡Use the general formula CₙH₂ₙ to deduce a molecular formula, then check that the structure contains one C=C.
- 💡In displayed formulae, draw the double bond as two parallel lines between the carbon atoms and complete the outer shells with hydrogen atoms.
Common Mistakes
- Calling any hydrocarbon with a double bond an alkane; the correct term is alkene, and the C=C must be identified.
- Writing the general formula as CₙH₂ₙ₊₂, which describes alkanes; the correct alkene formula is CₙH₂ₙ.
- Drawing ethene with a single C–C bond; the corrected displayed formula must show C=C.