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7.1.4Cracking and alkenes

AQA GCSE Chemistry (8462), Higher tier · Organic chemistry › Carbon compounds as fuels and feedstock

Practise Cracking and alkenes. 14 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.

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What is meant by cracking?
Breaking down large hydrocarbon molecules into smaller, more useful molecules.
Complete the sentence.
In catalytic cracking, the hydrocarbon is heated to vaporise it, and the vapour is passed over a hot ________.
catalyst
Long-chain alkanes are cracked to produce shorter, more useful molecules. Complete the equation.
C14H30 → C8H18 + .....C2H4
3
Write two different equations for cracking decane, C10H22, in which propene, C3H6, is one of the products.
C10H22 → C7H16 + C3H6 and C10H22 → C4H10 + 2C3H6
Cracking produces shorter alkanes and another type of hydrocarbon.
Name this other type of hydrocarbon.
Alkenes

Sample questions

Written for this site in the style of AQA exam questions. They are not taken from real past papers.

Question 1Easy5 marks
(a) What is meant by cracking?[1]
(b) Cracking produces two types of hydrocarbon.
Name the two types.[2]
(c) Give the test for an alkene and the result.[2]
Show the answer and mark scheme
(a) Answer: Breaking down large hydrocarbon molecules into smaller, more useful molecules.
  • breaking down large hydrocarbon molecules into smaller (more useful) molecules
(b) Answer: Alkanes and alkenes
  • alkanes
  • alkenes
(c) Answer: Add bromine water: it changes from orange to colourless.
  • add bromine water
  • orange to colourless
Question 2Medium6 marks
Long-chain alkanes are cracked to produce shorter, more useful molecules.
(a) Complete the equation.
C14H30 → C8H18 + .....C2H4[1]
(b) Complete the equation.
C16H34 → C10H22 + 2 ________[1]
(c) Complete the equation.
C12H26 → C4H10 + ________[1]
(d) Cracking is an example of thermal decomposition.
What is meant by thermal decomposition?[1]
(e) Is cracking exothermic or endothermic?
Give a reason for your answer.[2]
Show the answer and mark scheme
(a) Answer: 3
  • 3
(b) Answer: C3H6
  • C3H6
(c) Answer: C8H16
  • C8H16
(d) Answer: Breaking down a compound into simpler substances by heating it.
  • breaking down a compound into simpler substances by heating
(e) Answer: Endothermic: energy must be supplied (by heating) to break bonds.
  • endothermic
  • energy / heat must be supplied (continuously) to break bonds
Question 3Hard7 marks
An alkane was cracked. The only products were ethene, C2H4, and hexane, C6H14.
alkane → 2C2H4 + C6H14
(a) Determine the formula of the alkane.[1]
(b) Calculate the maximum mass of ethene that could be produced from 71 kg of this alkane.
Relative atomic masses (Ar): H = 1   C = 12[3]
(c) In practice, less ethene than this is produced.
Suggest one reason why.[1]
(d) Describe a test to show that ethene is present in the products.[2]
Show the answer and mark scheme
(a) Answer: C10H22
  • C10H22
(b) Answer: 28 kg
  • Mr of C10H22 = 142 and Mr of C2H4 = 28
  • 71 000 ÷ 142 = 500 (mol), so 1000 mol of ethene (1 : 2)
  • 1000 × 28 = 28 000 g = 28 (kg)
(c) Answer: The alkane also cracks in other ways, giving different products.
  • the alkane also cracks in other ways (giving different products) / not all of the alkane cracks
(d) Answer: Add bromine water: it turns from orange to colourless.
  • add bromine water
  • orange to colourless

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