Structure and Reactions of Alkenes
Get to grips with Structure and Reactions of Alkenes for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) chemistry topic: alkenes are unsaturated with a C=C double bond and react by addition, for example with bromine or hydrogen.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Alkenes are hydrocarbons containing a carbon-carbon double bond. This makes them unsaturated, because they do not contain the maximum possible number of hydrogen atoms. This is a Triple-only topic.
The general formula is CₙH₂ₙ, and the first four are ethene C₂H₄, propene C₃H₆, butene C₄H₈ and pentene C₅H₁₀.
The double bond is the functional group, and it makes alkenes far more reactive than alkanes. Alkenes react by addition, in which the double bond opens and atoms are added across it. Reaction with hydrogen gives an alkane, with water gives an alcohol, and with a halogen gives a dihalo compound.
Revision notes
Structure and formula
Alkenes contain a carbon-carbon double bond, which is the functional group.
They are unsaturated because they do not contain the maximum possible number of hydrogen atoms. General formula CₙH₂ₙ.
Addition reactions
The double bond opens and atoms are added across it.
With hydrogen and a nickel catalyst, an alkane is formed. With steam and a catalyst, an alcohol is formed. With a halogen such as bromine, a dihalo compound is formed.
Combustion
Alkenes burn in air, but with a smoky yellow flame.
This is because they have a higher proportion of carbon than alkanes, so combustion is often incomplete and produces sooty particulates.
Key points
- Alkenes contain a carbon-carbon double bond.
- They are unsaturated hydrocarbons.
- The general formula is CₙH₂ₙ.
- The double bond is the functional group.
- Alkenes react by addition.
- Alkenes burn with a smoky flame.
Worked examples
Example 1
Explain why alkenes are described as unsaturated. [2 marks]
Model answer
They contain a carbon-carbon double bond (1 mark), so they do not contain the maximum possible number of hydrogen atoms (1 mark).
Example 2
Name the product formed when ethene reacts with hydrogen. [2 marks]
Model answer
Ethane (1 mark). The double bond opens and a hydrogen atom is added to each carbon, forming a saturated alkane (1 mark).
Example 3
Explain why alkenes burn with a smokier flame than alkanes. [2 marks]
Model answer
Alkenes have a higher proportion of carbon relative to hydrogen (1 mark), so combustion is more often incomplete, producing sooty carbon particulates (1 mark).
Common mistakes
Using the alkane formula for alkenes.
Alkenes are CₙH₂ₙ; alkanes are CₙH₂ₙ₊₂.
Saying alkenes are saturated.
The double bond makes them unsaturated.
Forgetting to name the addition product correctly.
Adding hydrogen to an alkene gives the corresponding alkane.
Saying the double bond breaks completely.
One of the two bonds opens; the carbons remain joined.
Exam tips
- Learn CₙH₂ₙ and the first four alkene names.
- Say the double bond opens in addition reactions.
- Define unsaturated through the double bond.
- Remember this is a Triple-only topic.
Key terms
- Alkene
- An unsaturated hydrocarbon with a carbon-carbon double bond.
- Unsaturated
- Not containing the maximum possible hydrogen atoms.
- Functional group
- The part of a molecule determining its reactions.
- Addition reaction
- A reaction where atoms add across a double bond.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.