AQA GCSE Chemistry Foundation (8462), Foundation tier · Organic chemistry › Carbon compounds as fuels and feedstock
Practise Cracking and alkenes. 8 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.
Why and how large hydrocarbon molecules are cracked into smaller, more useful ones, the conditions for catalytic and steam cracking, the bromine water test for alkenes, and balancing cracking equations. Expect supply-and-demand data questions, equations to complete and questions on the lab cracking experiment.
Key facts
Cracking = breaking large hydrocarbon molecules into smaller, more useful ones (thermal decomposition)
Catalytic cracking: vaporise, then pass over a hot catalyst
Steam cracking: vaporise, mix with steam, very high temperature
Products: smaller alkanes + alkenes
Bromine water + alkene: orange → colourless
Alkenes are more reactive than alkanes
Uses: small alkanes as fuels; alkenes to make polymers and other chemicals
Balancing: same total C and same total H on both sides
Alkenes: C=C double bond, general formula CnH2n
Notes
What cracking is
diagram
Cracking breaks down large (long-chain) hydrocarbon molecules into smaller, more useful molecules.
It is a thermal decomposition: heat is used to break the molecules apart.
Catalytic cracking: the hydrocarbons are heated to vaporise them, and the vapour is passed over a hot catalyst. You do not need to name the catalyst.
Steam cracking: the hydrocarbon vapour is mixed with steam and heated to a very high temperature.
In the lab, paraffin vapour can be passed over heated pieces of broken porous pot, which act as the catalyst. Take the delivery tube out of the water before you stop heating, or cold water is sucked back into the hot tube.
Cracking liquid paraffin in the lab. Take the delivery tube out of the water before you stop heating, to stop suck-back.
The products
The products are smaller alkanes and another type of hydrocarbon called alkenes.
Example: C10H22 → C8H18 + C2H4 (an alkane cracks to a smaller alkane and ethene).
In a balanced cracking equation, the total number of carbon atoms and the total number of hydrogen atoms are the same on both sides.
Alkenes contain a C=C double bond and have the general formula CnH2n, so you can spot the alkene among the products.
Alkenes are more reactive than alkanes.
Testing for alkenes
Shake the substance with bromine water.
Alkene present: the bromine water turns from orange to colourless.
Alkane: no change; the bromine water stays orange.
The bromine adds across the C=C double bond, forming a colourless product.
Why crack hydrocarbons?
Fractional distillation gives more long-chain hydrocarbons than are needed, and not enough short-chain ones.
There is a high demand for fuels with small molecules, so some of the products of cracking are used as fuels, e.g. petrol.
The alkenes are used to make polymers and as starting materials for many other chemicals.
Modern life depends on hydrocarbons: fuels for transport and heating, and materials such as polymers, solvents, lubricants and detergents.
How to answer each type of question
Complete a cracking equation
1 to 2 marksGrade 5
Subtract the carbon atoms you can see on the right from the carbon atoms on the left.
Do the same for the hydrogen atoms.
If there is a number in front of the missing formula, divide the missing atoms by that number.
Catalytic cracking: the hydrocarbon is heated to vaporise it, and the vapour is passed over a hot catalyst.
Steam cracking: the vapour is mixed with steam and heated to a very high temperature.
You do not need to name the catalyst.
Example. Describe the conditions used in catalytic cracking and in steam cracking.
Show the model answerHide the model answer
Catalytic cracking: the hydrocarbon is heated so that it vaporises (1) and the vapour is passed over a hot catalyst (1). Steam cracking: the hydrocarbon vapour is mixed with steam (1) and heated to a very high temperature (1).
Test for an alkene and the lab cracking experiment
1 to 3 marksGrade 4
Name the reagent: bromine water.
Give the whole colour change: orange to colourless.
In the lab experiment, the delivery tube is removed from the water before heating stops, to prevent suck-back.
Example. A student cracks liquid paraffin and collects the gas produced over water. (a) Describe a test to show that the gas contains an alkene. Give the result. (b) Why must the student take the delivery tube out of the water before stopping heating?
Show the model answerHide the model answer
(a) Shake the gas with bromine water (1). It turns from orange to colourless (1). (b) Otherwise cold water is sucked back into the hot boiling tube, which could crack it (1).
Don’t lose marks
Writing 'clear' instead of 'colourless'. Clear means see-through, and orange bromine water is already clear.
Getting the colour change backwards. Bromine water starts orange and ends colourless with an alkene.
Describing fractional distillation when asked about cracking. Cracking breaks molecules; distillation only separates them.
Leaving out the heating: both methods start by heating the hydrocarbons to vaporise them.
Not checking that the carbon and hydrogen totals match in a cracking equation.
More tips
Memory tricks
Cracking turns big molecules into small ones: small alkanes for fuel, alkenes for polymers.
Catalytic cracking uses a catalyst; steam cracking uses steam. The name tells you the key condition.
Bromine water: an alkene takes the orange colour away.
Cracking equations are just counting: C on the left = C on the right, H on the left = H on the right.
Exam technique
In data questions, quote the numbers for both supply and demand. 'Petrol is in high demand' on its own gets little credit.
'Give the result' means the whole colour change: the starting colour and the final colour.
Learn the two cracking methods as a pair so you can compare them: both vaporise the hydrocarbon; one uses a catalyst, the other steam and a very high temperature.
'Explain how modern life depends on hydrocarbons' needs examples: petrol and diesel oil for transport, polymers, solvents, lubricants and detergents.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Define crackingBreaking down large hydrocarbon molecules into smaller, more useful molecules.
Grade 4
Name the two types of productCracking produces smaller alkanes and another type of hydrocarbon called alkenes.
Grade 4
Describe the bromine water test for alkenesShake with bromine water: it turns from orange to colourless if an alkene is present.
Grade 5
Describe the conditions for catalytic and steam crackingCatalytic: vapour passed over a hot catalyst. Steam: vapour mixed with steam and heated to a very high temperature.
Grade 5
Give uses of the products of crackingSmaller alkanes are used as fuels such as petrol; alkenes are used to make polymers and many other chemicals.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
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
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