Alkenes
6 marks · NormalDescribe what is observed when bromine water is added to an alkene and the mixture is shaken.
Write a balanced chemical equation for the reaction between but-2-ene and bromine.
Explain why the reaction you wrote in part b is called an addition reaction.
Describe what you would see if bromine water is added to hexane instead of but-2-ene and the mixture is shaken.
State the difference between complete combustion and addition reactions of alkenes.
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(a) Bromine water is decolourised (changes from orange to colourless).
(b) \ce{CH3CH=CHCH3 (l) + Br2(aq) -> CH3CHBrCHBrCH3 (l)}
(c) Because bromine atoms add across the double bond, forming a single product with new bonds.
(d) No colour change; bromine water remains orange.
(e) Combustion produces carbon dioxide and water; addition forms one product by breaking the double bond.
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Marking points
B1 (a) · 1 mark · Bromine water decolourised (orange to colourless)
B2 (b) · 1 mark · Correct reactants for but-2-ene and bromine
B3 (b) · 1 mark · Correct product formula
B4 (c) · 1 mark · Explains that atoms add across the double bond (add across C=C) forming one product
B5 (d) · 1 mark · Bromine water remains orange/no colour change
B6 (e) · 1 mark · Difference: combustion makes CO2 & water, addition forms one product by breaking double bond
Worked solution
a) When bromine water is shaken with an alkene, the orange colour disappears and the solution becomes colourless.
b) This is the addition of bromine to but-2-ene, producing 2,3-dibromobutane.
c) The reaction is called an addition reaction because bromine atoms add across the double bond, producing one product.
d) Hexane is a saturated hydrocarbon (alkane), so bromine water does not react. The orange colour remains.
e) Complete combustion forms carbon dioxide and water, whereas addition forms one product with atoms added across a double bond.