Chemical reactions of the alkenes
2 marks · NormalAlkenes are a family of unsaturated hydrocarbons that undergo two main types of chemical reactions: combustion and addition.
Describe what happens during:
(i) the combustion of an alkene in oxygen
(ii) an addition reaction of an alkene
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(a) The alkene reacts with oxygen to form carbon dioxide and water. · must show alkene burns, produces CO2 and H2O
(b) Part of the double bond in the alkene breaks and new atoms add to the carbon atoms originally joined by the double bond. · must show double bond breaks, new atoms join the carbons
Common mistakes: Confusing addition with substitution; omitting formation of water and CO2 in combustion; only describing physical change
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Marking points
B1 (a) · 1 mark · States that combustion of an alkene produces carbon dioxide and water.
B1 (b) · 1 mark · Describes that in addition reactions the double bond in the alkene breaks and new atoms add to the carbon atoms, or words to this effect.
Worked solution
a) When an alkene undergoes combustion in oxygen, it reacts to form carbon dioxide and water.
b) In an addition reaction, part of the C=C double bond in the alkene breaks, allowing atoms (such as hydrogen or halogens) to add to each of the previously double-bonded carbons.