Unsaturated hydrocarbons
2 marks · HardExplain why alkenes are classified as unsaturated hydrocarbons and describe the significance of the C=C bond in their chemical reactivity.
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Alkenes are unsaturated hydrocarbons because they contain at least one C=C double bond, meaning they do not hold the maximum possible number of hydrogen atoms. The C=C bond is more reactive than a single C–C bond, allowing alkenes to undergo addition reactions.
Also accept: Alkenes contain a double bond so they are not saturated with hydrogen atoms. The C=C bond makes them more reactive and able to take part in addition reactions.
Common mistakes: Stating only that alkenes have fewer hydrogens (without mentioning the C=C bond); Saying alkenes contain a triple bond
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Marking points
M1 · 1 mark · States or explains that alkenes are unsaturated due to the presence of a C=C double bond (not saturated with hydrogens, or not the maximum number of hydrogens).
M2 · 1 mark · Describes the significance of the double bond for reactivity, e.g. allows addition reactions or is more reactive than single bonds.
Worked solution
Alkenes are called unsaturated hydrocarbons because the C=C double bond means they do not have the maximum possible number of hydrogen atoms attached to the carbon chain.
The C=C bond is the functional group for alkenes and is more reactive than a single C–C bond. It allows alkenes to take part in addition reactions where new atoms can be added across the double bond.