AQA GCSE Chemistry Foundation (8462), Foundation tier · Organic chemistry › Reactions of alkenes and alcohols
Practise Structure and formulae of alkenes. 6 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
What alkenes are, why they are unsaturated, their general formula, and how to name, recognise and draw the first four. This subtopic is in GCSE Chemistry only, not Combined Science. Expect formula, naming and displayed formula questions, usually worth 1 or 2 marks each.
Key facts
Alkene = hydrocarbon with a C=C double bond
General formula CnH2n
Unsaturated: contains C=C; two fewer H atoms than the alkane with the same number of C atoms
Alkenes are hydrocarbons that contain a carbon–carbon double bond, C=C.
They form a homologous series with the general formula CnH2n.
Alkenes are unsaturated: they have two fewer hydrogen atoms than the alkane with the same number of carbon atoms. Ethene is C2H4; ethane is C2H6.
Alkanes have single bonds only, so they are saturated.
There is no alkene with one carbon atom: a C=C needs at least two carbon atoms.
The first four alkenes
Ethene C2H4, propene C3H6, butene C4H8 and pentene C5H10. These are the only alkene names you need.
The names use the same stems as the alkanes (eth = 2, prop = 3, but = 4, pent = 5 carbon atoms) with the ending -ene.
A formula such as CH2=CHCH3 (propene) shows where the double bond is.
Displayed formulae
diagram
Ethene:
Propene:
Every carbon atom still makes four bonds: the double bond counts as two.
In butene and pentene, the C=C can be between different carbon atoms in the chain. Each position gives a correct structure; the one usually drawn has the double bond at the end of the chain, e.g. CH2=CHCH2CH3.
How to answer each type of question
Name or give the formula of an alkene
1 mark eachGrade 4
Use the stem for the number of carbon atoms (eth 2, prop 3, but 4, pent 5) with the ending '-ene'.
The number of hydrogen atoms is twice the number of carbon atoms.
Check that the name and formula match: an '-ene' has 2n hydrogens.
Example. (a) Give the molecular formula of butene. (b) Name the alkene with the formula C5H10. (c) Which of these compounds is an alkene: C3H8, C3H6 or C3H7OH?
Show the model answerHide the model answer
(a) C4H8 (1) (b) Pentene (1) (c) C3H6, because it is a hydrocarbon that fits CnH2n (1)
Describe the differences between an alkane and an alkene
2 marksGrade 5
Give the bonding difference: the alkene has a C=C double bond; the alkane has single bonds only.
Give the formula difference: the alkene has two fewer hydrogen atoms.
Use 'saturated' and 'unsaturated' correctly if you use them.
Example. Butane and butene both have four carbon atoms in each molecule. Describe two differences between a molecule of butane and a molecule of butene.
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Butene has a C=C double bond, but butane has only single bonds (1). Butene has two fewer hydrogen atoms: C4H8 compared with C4H10 (1).
Don’t lose marks
Drawing a carbon atom with five bonds, usually next to the double bond. Count to four on every carbon.
Calling an alkene saturated. Saturated means single bonds only.
Mixing up propane (C3H8) and propene (C3H6). Check the ending and the hydrogen count.
Trying to name 'methene'. There is no alkene with one carbon atom.
Writing CH3 at the end of the chain when a displayed formula is asked for. Draw all three C–H bonds.
More tips
Memory tricks
'-ane' = single bonds only (saturated); '-ene' = there is a double bond (unsaturated).
Alkene hydrogens: just double the carbons (C4 gives H8). Alkane hydrogens: double and add two.
Carbon stems: meth 1, eth 2, prop 3, but 4, pent 5 (pent as in pentagon, 5 sides).
Bond check on every carbon: four bonds, where the = counts as two.
Unsaturated means there is 'room' for more hydrogen atoms: the double bond could open up to take them.
Exam technique
For 'describe two differences', give one difference in bonding and one in the formula, and compare using 'but' or 'whereas'.
Draw the double bond as two clear parallel lines, so it cannot be mistaken for a single bond.
If a question shows a displayed formula, count the C and H atoms carefully before deciding whether it is an alkane or an alkene.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State what an alkene isA hydrocarbon with a carbon–carbon double bond, C=C.
Grade 4
Name the first four alkenesEthene C2H4, propene C3H6, butene C4H8 and pentene C5H10.
Grade 5
Use the general formula of the alkenesCnH2n: the number of hydrogen atoms is double the number of carbon atoms.
Grade 5
Explain what unsaturated meansAlkenes have two fewer hydrogen atoms than the alkane with the same number of carbon atoms, because they contain a C=C.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
What is the general formula of the alkenes?
CnH2n
Name the functional group that all alkenes contain.
Carbon–carbon double bond (C=C)
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) What is the general formula of the alkenes?[1]
(b) Name the alkene with the formula C3H6.[1]
(c) Alkenes are described as unsaturated. What does this mean?[1]
(d) Draw the displayed formula of ethene, C2H4.[1]
Show the answer and mark scheme
(a)Answer: CnH2n
CnH2n
(b)Answer: Propene
propene
(c)Answer: They contain a carbon–carbon double bond.
they contain a carbon–carbon double bond (C=C) / they have two fewer hydrogen atoms than the alkane with the same number of carbon atoms
(d)Answer: Two carbon atoms joined by a double bond, each bonded to two hydrogen atoms.
two C atoms joined by a double bond, each bonded to two H atoms
Question 2Medium3 marks
A technician has two colourless liquids, one an alkane and one an alkene, but does not know which is which.
Describe a chemical test the technician could use to identify which liquid is the alkene, including the result expected.[3]
Show the answer and mark scheme
Answer: Add a few drops of each liquid to separate samples of bromine water and shake. The liquid that turns the bromine water from orange to colourless is the alkene; the alkane causes no colour change.
add a few drops of each liquid to separate samples of bromine water (and shake/mix)
the alkene turns the bromine water from orange to colourless
the alkane causes no colour change (the bromine water stays orange)