Alkenes
7 marks · HardThis question is about alkenes and their addition reactions.
Explain what is meant by an 'unsaturated hydrocarbon'. Refer to the bonding in your answer.
Ethene reacts with hydrogen gas in the presence of a catalyst. Write a balanced chemical equation for this reaction using molecular formulae.
Hydration is another important addition reaction of ethene. State the name and molecular formula of the organic product formed when ethene reacts with steam. Write a balanced chemical equation for this reaction.
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(a) An unsaturated hydrocarbon contains at least one carbon–carbon double (C=C) or triple bond, not just single bonds.
(b)
(c) Product: ethanol; Molecular formula: . Balanced equation:
Common mistakes: Confusing single and double bonds; Incorrect product formula
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Unsaturated: hydrocarbon contains at least one double (C=C) or triple bond.
B2 (a) · 1 mark · Mentions there are not only single bonds between carbon atoms.
M1 (b) · 1 mark · Sets up: ethene + hydrogen → alkane (ethane).
A1 (b) · 1 mark · Correct balanced equation: .
M2 (c) · 1 mark · Identifies product as ethanol (name or formula).
M3 (c) · 1 mark · Gives correct molecular formula () for ethanol.
A2 (c) · 1 mark · Correct balanced chemical equation: .
Worked solution
a) An 'unsaturated hydrocarbon' is a compound made up of carbon and hydrogen that contains at least one carbon–carbon double (C=C) or triple bond, meaning that not all the carbon–carbon bonds are single bonds.
b) Hydrogenation of ethene: the balanced chemical equation is:
c) Hydration of ethene produces ethanol (molecular formula: ). The balanced chemical equation is: