Alkenes
6 marks · NormalCombust ethene in air. Write the balanced symbol equation for this reaction.
Describe in detail how you would use bromine water to distinguish between ethene and ethane.
Explain why only one of these gases decolourises bromine water.
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) \ce{C2H4 (g) + 3O2 (g) -> 2CO2 (g) + 2H2O (l)}
(b) Bubble each gas through separate samples of bromine water. Ethene will decolourise the orange bromine water, while ethane will not.
(c) Ethene is an alkene, which is unsaturated and contains a C=C bond that reacts with bromine water, causing decolourisation. Ethane is a saturated alkane and does not react with bromine water, so the colour remains.
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Marking points
M1 (a) · 1 mark · Correct reactants and products (C2H4, O2, CO2, H2O)
A1 (a) · 1 mark · Correct balancing of equation
B1 (b) · 1 mark · Describes adding bromine water to both gases
B2 (b) · 1 mark · States ethene decolourises bromine water, ethane does not
B3 (c) · 1 mark · Explains ethene contains C=C bond, reacts with bromine
B4 (c) · 1 mark · Explains ethane is saturated and does not react
Worked solution
a) The balanced equation is:
b) Bubble ethene through bromine water and the orange colour will disappear. Bubble ethane through another sample and the colour will remain.
c) Ethene reacts because it is unsaturated (has a C=C double bond), which can break and allow bromine to add. Ethane has only single C–C bonds, so it cannot react with bromine water.