Alkenes (Edexcel IGCSE Chemistry (Modular): Unit 1): Flashcards
Exam code: 4XCH1
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- FrontWhat are Alkenes?
Define unsaturated (as used for hydrocarbons).
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Cards in this collection (18)
Define unsaturated (as used for hydrocarbons).
An unsaturated hydrocarbon contains at least one carbon-carbon double bond (C=C). Alkenes are unsaturated because they contain a C=C bond.
Complete the table of the first four alkenes.
Name
Molecular formula
Ethene
Propene
But-1-ene
Pent-1-ene
Complete the table of the first four alkenes.
Name
Molecular formula
Ethene
C2H4
Propene
C3H6
But-1-ene
C4H8
Pent-1-ene
C5H10
What is the general formula of the alkenes?
The general formula of the alkenes is CnH2n. This means for every n carbon atoms, there are 2n hydrogen atoms.
True or False?
Alkenes are saturated hydrocarbons.
False.
Alkenes are unsaturated hydrocarbons. They contain a carbon-carbon double bond (C=C), which means they are not fully saturated with hydrogen.
What is the functional group of the alkenes?
The functional group of the alkenes is the carbon-carbon double bond (C=C). This group determines the chemical reactions that alkenes undergo.
All alkenes contain a .......... double bond, which is the .......... of the alkene.
All alkenes contain a carbon-carbon double bond, which is the functional group of the alkene.
Why are alkenes more reactive than alkanes?
Alkenes are more reactive because they contain a carbon-carbon double bond (C=C). This double bond can break open, allowing other atoms to bond to the carbon atoms.
True or False?
Ethene has the molecular formula C2H4.
True.
Ethene is the first alkene and follows the general formula CnH2n. With n = 2, we get C2H4.
What do the numbers in alkene names (such as but-1-ene or pent-2-ene) indicate?
The numbers indicate the position of the C=C double bond — specifically, which carbon atom the double bond begins on, counting from the left of the carbon chain.
What is an addition reaction?
An addition reaction is one in which atoms of a simple molecule add across a double bond, converting it to a single bond. No atoms are lost in the process.
What is formed when bromine reacts with ethene?
When bromine reacts with ethene, dibromoethane is formed. This is an example of an addition reaction in which bromine atoms add across the C=C double bond.
When bromine reacts with an alkene, the bromine atoms add across the .......... bond to form a ...........
When bromine reacts with an alkene, the bromine atoms add across the C=C bond to form a dibromoalkane.
True or False?
The reaction between bromine and an alkene is a substitution reaction.
False.
The reaction between bromine and an alkene is an addition reaction. Bromine atoms add across the C=C double bond — no atoms are removed.
Why does bromine water decolourise when added to an alkene?
Bromine water decolourises because the bromine atoms add across the C=C double bond in an addition reaction. The bromine is no longer free in solution, so the orange colour disappears.
Complete the table showing the result of the bromine water test.
Compound type
Colour before
Colour after
Alkane
Alkene
Complete the table showing the result of the bromine water test.
Compound type
Colour before
Colour after
Alkane
orange
orange
Alkene
orange
colourless
Why is bromine water used instead of bromine in the alkene test?
Bromine water is used instead of pure bromine because it is safer and easier to handle. It still contains bromine, which reacts with alkenes in the same way.
True or False?
Adding bromine water to an alkene causes the orange colour to disappear.
True.
Alkenes react with bromine water in an addition reaction. The bromine adds across the C=C double bond, leaving no free bromine in solution, so the colour is lost.
What happens when bromine water is added to an alkane?
When bromine water is added to an alkane, the solution remains orange. Alkanes are saturated and cannot react with bromine, so the bromine stays in solution.
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