AQA GCSE Chemistry Foundation (8462), Foundation tier · Organic chemistry › Reactions of alkenes and alcohols
Practise Reactions of alkenes. 9 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How alkenes react because of their C=C functional group: combustion, and addition reactions with hydrogen, water (steam) and the halogens. You need the conditions and must be able to draw the products. GCSE Chemistry only. Expect 'name the product', 'draw the product' and 'give the conditions' questions, and explaining the bromine water test.
Key facts
Functional group of the alkenes: C=C
Addition: atoms add across the C=C, which becomes C–C; one product only
Alkene + steam → alcohol (high temperature, high pressure, catalyst)
Alkene + halogen → compound with two halogen atoms, e.g. dibromoethane (room temperature)
C2H4 + H2O → C2H5OH
Bromine water: orange → colourless, because bromine adds across the C=C
Alkenes burn with smoky flames (incomplete combustion)
C2H4 + 3O2 → 2CO2 + 2H2O
Notes
The C=C functional group
A functional group is the group of atoms that decides how a compound reacts. For the alkenes it is C=C.
All alkenes have this group, so they all react in the same way. Learn the reactions of ethene and apply them to any alkene.
Combustion
Alkenes burn in oxygen like other hydrocarbons. Complete combustion gives carbon dioxide and water: C2H4 + 3O2 → 2CO2 + 2H2O.
In air they tend to burn with smoky flames, because of incomplete combustion.
Addition reactions
diagram
Alkenes react with hydrogen, water and the halogens by addition: atoms add across the C=C, and the double bond becomes a single C–C bond.
Only one product forms, so all the atoms of the reactants end up in it (the atom economy is 100%).
Hydrogen: needs a nickel catalyst and heating. The product is an alkane: ethene + hydrogen → ethane.
Water: the alkene reacts with steam at a high temperature (about 300 °C) and high pressure, with a catalyst (phosphoric acid). The product is an alcohol: ethene + steam → ethanol.
Halogens (chlorine, bromine, iodine): react at room temperature with no catalyst. The product has two halogen atoms: ethene + bromine → dibromoethane.
Addition: the C=C opens up and one Br atom adds to each carbon. Ethene + bromine → dibromoethane.
This is why alkenes decolourise bromine water: the orange bromine adds across the C=C, forming a colourless product.
Drawing the products
diagram
Keep the carbon chain the same, and change the C=C to C–C.
Put one added atom or group on each of the two carbon atoms that were joined by the double bond: H and H, H and OH, or two halogen atoms.
Propene + bromine → dibromopropane:
Propene + steam → propanol: one carbon gets an H and the neighbouring carbon gets an OH.
How to answer each type of question
Name the product and give the conditions
1 to 3 marksGrade 5
Identify what is added: hydrogen, steam or a halogen.
Name the product from the alkene's stem: alkane, alcohol, or a di-halogen compound (e.g. dichloropropane).
Give the conditions: nickel catalyst for hydrogen; high temperature, high pressure and a catalyst for steam; none needed for halogens.
Example. (a) Propene reacts with hydrogen. Name the product and the catalyst used. (b) Propene reacts with steam. Name the product.
Show the model answerHide the model answer
(a) Propane (1); nickel (1) (b) Propanol (1)
Combustion of alkenes
2 to 4 marksGrade 5
For a smoky flame, say 'incomplete combustion' and name the soot (carbon) that forms.
For an equation, balance C, then H, then O, as for alkanes.
Double every number if you end up with half an O2.
Example. (a) Propene burns in air with a smoky flame. Explain why. (b) Write a balanced equation for the complete combustion of propene, C3H6.
Show the model answerHide the model answer
(a) Incomplete combustion takes place (1), producing particles of carbon (soot) (1). (b) Correct formulae: C3H6 + O2 → CO2 + H2O (1). Balanced: 2C3H6 + 9O2 → 6CO2 + 6H2O (1). (C3H6 + 4½O2 → 3CO2 + 3H2O is also correct.)
Don’t lose marks
Putting both new atoms on the same carbon atom. One goes on each carbon of the old double bond.
Leaving the double bond in the product. After addition it is a single bond.
Missing out the nickel catalyst for the reaction with hydrogen.
Writing 'water' without saying steam, or leaving out the catalyst, high temperature and high pressure.
Adding a second product, such as HBr or H2O, to an addition reaction. There is only one product.
Saying that alkanes decolourise bromine water.
More tips
Memory tricks
'Add across the double bond': one new atom goes to each end of the C=C.
Hydrogen makes an alkANE; water makes an alcohOL; a halogen makes a di-halogen compound.
Catalysts: nickel for hydrogen, phosphoric acid for steam, nothing for halogens.
Addition makes one product, so its formula is just the two formulae added together: C3H6 + Br2 → C3H6Br2.
Exam technique
When asked for 'conditions', give the catalyst, and the temperature and pressure where they matter.
Name products using the alkene's stem: propene gives propane, propanol or dibromopropane.
In displayed formulae, draw every bond, including the O–H bond in an alcohol.
Use the phrase 'adds across the double bond', and say that the double bond becomes a single bond.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Name the functional group of the alkenesC=C, the carbon–carbon double bond.
Grade 4
Explain why alkenes burn with smoky flamesIn air they tend to undergo incomplete combustion, which produces soot (carbon).
Grade 5
Name the products of addition reactionsHydrogen gives an alkane, steam gives an alcohol, and a halogen gives a compound with two halogen atoms.
Grade 5
Give the conditions for adding hydrogen and steamHydrogen: a nickel catalyst. Steam: high temperature, high pressure and a catalyst (phosphoric acid).
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Ethene reacts with hydrogen. Name the product.
Ethane
Ethene reacts with steam to produce ethanol. What else is needed for this reaction?
A catalyst.
Write a balanced equation for the complete combustion of ethene.
C2H4 + 3O2 → 2CO2 + 2H2O
Bromine water is added to an alkene and shaken. Give the colour change you would see.
Orange to colourless
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) Ethene reacts with hydrogen. Name the product.[1]
(b) What is needed for an alkene to react with hydrogen?[1]
(c) What type of reaction happens when bromine reacts with ethene? Tick (✓) one box.[1]
Addition
Displacement
Neutralisation
Thermal decomposition
(d) Alkenes usually burn in air with a smoky flame. Give the reason for this.[1]
Show the answer and mark scheme
(a)Answer: Ethane
ethane
(b)Answer: A catalyst (nickel).
a catalyst (nickel)
(c)Answer: Addition
(d)Answer: Incomplete combustion.
incomplete combustion
Question 2Medium7 marks
Ethene, C2H4, is the simplest alkene.
(a) Write a balanced equation for the complete combustion of ethene.[2]
(b) Alkenes often burn in air with a smoky flame. Explain why, and name one product of incomplete combustion.[2]
(c) Write a balanced equation for the reaction of ethene with steam.[1]
(d) Give two conditions used for the reaction of ethene with steam.[2]
Show the answer and mark scheme
(a)Answer: C2H4 + 3O2 → 2CO2 + 2H2O
correct formulae: C2H4 + O2 → CO2 + H2O
balanced: C2H4 + 3O2 → 2CO2 + 2H2O
(b)Answer: There is not enough oxygen for complete combustion; carbon (soot) forms.
incomplete combustion (not enough oxygen for all of the carbon to be oxidised)
carbon (soot) / carbon monoxide
(c)Answer: C2H4 + H2O → C2H5OH
C2H4 + H2O → C2H5OH
(d)Answer: A catalyst (phosphoric acid) and a high temperature (about 300 °C).