Unsaturated hydrocarbons
4 marks · NormalAlkenes are described as unsaturated because of a feature of their structure.
Describe the bonding in an unsaturated hydrocarbon.
Explain how a C=C bond affects the reactivity of alkenes compared to alkanes.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) — · must show Mentions double bond (C=C) between carbons, Mentions presence of single bonds elsewhere or not all single bonds
(b) — · must show Mentions C=C bond is more reactive, Mentions addition reactions or easier to break than single bonds
Common mistakes: Describing single bonds only; Not linking double bond to reactivity
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Marking points
M1 (a) · 1 mark · States presence of at least one carbon–carbon double bond (C=C) in unsaturated hydrocarbon.
B1 (a) · 1 mark · Mentions the presence of single C–C and C–H bonds as well.
M2 (b) · 1 mark · Recognises C=C bonds are more reactive than single bonds.
B2 (b) · 1 mark · Explains that the double bond allows addition reactions or makes the compound more reactive than alkanes.
Worked solution
a) Unsaturated hydrocarbons like alkenes have at least one carbon–carbon double bond. The rest of the bonds are single C–C or C–H bonds.
b) The C=C double bond is more reactive than single bonds, so alkenes react more easily than alkanes and can undergo addition reactions.