7.4 Cracking and alkenes
5 exam questions worth 8 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What is seen when bromine water is shaken with an alkene?
Choose one answer.
Correct Not quite: the answer is It turns from orange to colourless
Mark scheme
C
Model answer
C: alkenes decolourise bromine water, turning it from orange to colourless.
Complete the cracking equation: C12H26 → C10H22 + ...
Mark scheme
- C2H4 (1)
Model answer
C2H4 (ethene).
Explain why oil companies crack long-chain hydrocarbons. Use ideas about supply and demand.
Mark scheme
- crude oil contains more long-chain hydrocarbons than are needed / supply exceeds demand (1)
- demand for short chains such as petrol is greater than the supply (1)
- cracking converts long chains into more useful short chains (and alkenes for polymers) (1)
Model answer
Crude oil contains more long-chain hydrocarbons than are wanted, but not enough short-chain fuels such as petrol. Cracking converts the surplus long chains into shorter alkanes for fuel and alkenes for making polymers.
Describe the two conditions used for cracking.
Mark scheme
- catalytic: vapour passed over a hot catalyst (1)
- steam: mixed with steam at a very high temperature (1)
Model answer
Catalytic cracking passes the vapour over a hot catalyst; steam cracking mixes it with steam at a very high temperature.
In the laboratory cracking experiment, explain why the delivery tube must be removed from the water before heating stops.
Mark scheme
- to prevent suck-back (cold water being sucked into the hot tube, which could crack it) (1)
Model answer
To stop cold water being sucked back into the hot boiling tube, which could shatter it.
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