Unsaturated hydrocarbons
4 marks · NormalAlkenes are described as unsaturated hydrocarbons.
Explain what is meant by 'unsaturated' in relation to alkenes.
State how alkenes differ in structure from 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 C=C double bond and less than maximum number of hydrogens (not saturated with hydrogens)
(b) — · must show Mentions double bond in alkenes vs. single bond in alkanes
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Marking points
M1 (a) · 1 mark · States that unsaturated means there are not the maximum number of hydrogen atoms possible (not saturated with hydrogen).
M2 (a) · 1 mark · Mentions that alkenes contain at least one C=C double bond.
B1 (b) · 1 mark · States that alkanes contain only single C–C bonds.
B2 (b) · 1 mark · States that alkenes contain at least one C=C double bond, differentiating them from alkanes.
Worked solution
a) 'Unsaturated' means the molecule does not have the maximum number of hydrogen atoms. Alkenes have fewer hydrogens than alkanes with the same number of carbon atoms because of the double bond.
They contain at least one carbon–carbon double bond (C=C).
b) Alkanes have only single C–C bonds, while alkenes have at least one C=C double bond.