Revision notes for AQA GCSE Chemistry Cracking and alkenes. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Cracking and alkenes
What you'll learn
- Why long-chain hydrocarbons are cracked into smaller molecules.
- The general conditions for catalytic cracking and steam cracking.
- What alkenes are, why they are reactive, and how bromine water tests for them.
- How to balance simple cracking equations.
The starting point: hydrocarbons
Crude oil is a mixture of many different hydrocarbons. These molecules can have short chains or long chains of carbon atoms.
Definition
Hydrocarbon
A hydrocarbon is a compound made from hydrogen and carbon atoms only.
Many hydrocarbons in crude oil are alkanes. Alkanes are useful as fuels because they burn in oxygen and release energy.
Definition
Alkane
An alkane is a saturated hydrocarbon: it contains only single bonds between carbon atoms. Saturated means the carbon chain has as many hydrogen atoms attached as possible.
Why chain length matters
Short-chain hydrocarbons are in high demand because many are useful fuels, such as petrol and gases used for heating and cooking. They tend to have lower boiling points, flow more easily, and are easier to ignite than very long-chain hydrocarbons.
Long-chain hydrocarbons are less useful as fuels directly. They are often thick, have higher boiling points, and are harder to vaporise.
Key Idea
Supply and demand
Crude oil often contains more long-chain hydrocarbons than we need, but there is high demand for small fuel molecules. Cracking helps turn less useful large molecules into more useful smaller ones.
Example
Explaining why cracking is useful
A refinery has a surplus of long-chain hydrocarbons but needs more petrol-like fuels and chemical feedstocks.
- Compare the molecules: the surplus molecules are long-chain hydrocarbons, so they are less useful as direct fuels than shorter-chain hydrocarbons.
- Apply the effect of cracking: cracking breaks large hydrocarbon molecules into smaller hydrocarbons, increasing the supply of small molecules that can be used as fuels.
- Add the second benefit: cracking also produces alkenes, which are reactive starting materials for polymers and many other chemicals.
What cracking means
Definition
Cracking
Cracking is the thermal decomposition of long-chain hydrocarbons into smaller, more useful hydrocarbon molecules. Thermal decomposition means breaking down a substance using heat.
Cracking does not destroy atoms. The carbon and hydrogen atoms are rearranged into new molecules. The products include shorter alkanes and alkenes.
A possible cracking equation is:
C10H22(g)→C8H18(g)+C2H4(g)\mathrm{C_{10}H_{22}(g) \to C_8H_{18}(g) + C_2H_4(g)}C10H22(g)→C8H18(g)+C2H4(g)The number of carbon atoms and hydrogen atoms is the same on both sides, so the equation is balanced.
Common Mistake
Cracking is not combustion
Cracking breaks large hydrocarbons into smaller hydrocarbons. Combustion is burning a substance in oxygen. Do not describe cracking as “reacting with oxygen”.
Conditions for cracking
Cracking needs a lot of energy because strong covalent bonds in the hydrocarbon molecules must be broken.
Definition
Catalyst
A catalyst is a substance that speeds up a chemical reaction without being used up overall.
Catalytic cracking
In catalytic cracking, hydrocarbon vapour is passed over a hot catalyst. At GCSE, you can describe the conditions in general terms:
- high temperature
- a hot catalyst, such as aluminium oxide, silica, or a zeolite
- long-chain hydrocarbon vapour
The catalyst helps the reaction happen more easily, so catalytic cracking can be done at a lower temperature than steam cracking.
Steam cracking
In steam cracking, hydrocarbon vapour is mixed with steam and heated to a very high temperature. The key conditions are:
- very high temperature
- steam
- hydrocarbon vapour
Steam cracking is especially important industrially for producing alkenes.
Tip
Remember the two methods
Catalytic cracking uses a hot catalyst. Steam cracking uses steam and a very high temperature.
Balancing cracking equations
In cracking equations, you usually only need to balance carbon and hydrogen atoms. The formulas of the reactant and products may be given to you.
The important rule is conservation of atoms: atoms are not created or destroyed during a chemical reaction.
Example
Balancing a cracking equation
Balance this cracking equation:
C16H34(g)→C8H18(g)+C2H4(g)\mathrm{C_{16}H_{34}(g) \to C_8H_{18}(g) + C_2H_4(g)}C16H34(g)→C8H18(g)+C2H4(g)
- Count what is already fixed: the reactant has 16 carbon atoms and 34 hydrogen atoms. One octane molecule, C8H18(g)\mathrm{C_8H_{18}(g)}C8H18(g), accounts for 8 carbon atoms and 18 hydrogen atoms.
- Work out what is left for ethene: after making one octane molecule, there must still be 8 carbon atoms and 16 hydrogen atoms in the ethene molecules.
- Each ethene molecule, C2H4(g)\mathrm{C_2H_4(g)}C2H4(g), has 2 carbon atoms and 4 hydrogen atoms, so four ethene molecules provide 8 carbon atoms and 16 hydrogen atoms. The balanced equation is C16H34(g)→C8H18(g)+4C2H4(g)\mathrm{C_{16}H_{34}(g) \to C_8H_{18}(g) + 4C_2H_4(g)}C16H34(g)→C8H18(g)+4C2H4(g).
Common Mistake
Changing subscripts instead of coefficients
When balancing, change the big numbers in front of formulas, not the small numbers inside formulas. Changing C2H4\mathrm{C_2H_4}C2H4 to something else would change the substance.
Alkenes
Cracking produces alkenes as well as alkanes.
Definition
Alkene
An alkene is an unsaturated hydrocarbon that contains at least one carbon-carbon double bond, written as C=C. Unsaturated means the molecule can add more atoms by reacting at the double bond.
Alkenes are more reactive than alkanes because the C=C double bond can open up during reactions. This makes alkenes useful as chemical feedstocks.
For GCSE Chemistry, it is useful to recognise examples such as ethene, C2H4\mathrm{C_2H_4}C2H4, and propene, C3H6\mathrm{C_3H_6}C3H6. If you are doing Combined Science, you do not need to know the names or formulae of individual alkenes for this section.
A useful pattern is:
- alkanes have the general formula CnH2n+2\mathrm{C_nH_{2n+2}}CnH2n+2
- alkenes with one C=C double bond have the general formula CnH2n\mathrm{C_nH_{2n}}CnH2n
Example
Identifying an alkene from its formula
Decide whether C5H10\mathrm{C_5H_{10}}C5H10 fits the alkene pattern.
- Use the alkene general formula CnH2n\mathrm{C_nH_{2n}}CnH2n. If there are 5 carbon atoms, then the number of hydrogen atoms should be 2n=2×5=102n = 2 \times 5 = 102n=2×5=10.
- Compare with the formula given: C5H10\mathrm{C_5H_{10}}C5H10 has 10 hydrogen atoms, so it matches the alkene pattern.
- Check against the alkane pattern: an alkane with 5 carbon atoms would be C5H12\mathrm{C_5H_{12}}C5H12, so C5H10\mathrm{C_5H_{10}}C5H10 has fewer hydrogen atoms and is unsaturated.
Testing for alkenes with bromine water
Bromine water is an orange solution used to test for alkenes. When bromine water is shaken with an alkene, it is decolourised: it changes from orange to colourless.
Key Idea
Positive test for an alkene
An alkene turns orange bromine water colourless. An alkane does not react under normal test conditions, so the bromine water stays orange.
The reaction happens because bromine adds across the C=C double bond. For example:
C2H4(g)+Br2(aq)→C2H4Br2(l)\mathrm{C_2H_4(g) + Br_2(aq) \to C_2H_4Br_2(l)}C2H4(g)+Br2(aq)→C2H4Br2(l)Example
Interpreting bromine water results
A student tests two hydrocarbon samples with bromine water. Sample A turns bromine water colourless. Sample B leaves bromine water orange.
- Apply the positive test: turning bromine water from orange to colourless shows that Sample A contains a C=C double bond.
- Link this to the family: hydrocarbons with a C=C double bond are alkenes, so Sample A is likely to be an alkene.
- Compare Sample B: because the bromine water stays orange, Sample B does not show the alkene test result and is likely to be an alkane under these conditions.
Common Mistake
Saying clear instead of colourless
The expected observation is orange to colourless. “Clear” is not precise enough because an orange solution can still be transparent.
Why cracking matters in modern life
Cracking helps supply two very important groups of products.
Smaller alkanes as fuels
Some cracking products are short-chain alkanes. These are useful fuels for transport, heating, cooking, and electricity generation. Modern life depends heavily on these fuels, even though crude oil is a finite resource.
Alkenes as chemical feedstocks
Alkenes are used to make polymers and many other chemicals.
Definition
Polymer
A polymer is a very large molecule made when many small molecules, called monomers, join together.
For example, ethene can be used to make poly(ethene), a common plastic. Other alkene-derived chemicals are used in products such as packaging, synthetic fibres, detergents, solvents, and medical materials.
Key Idea
Fuel and feedstock
Cracking is valuable because it produces small alkanes for fuels and alkenes for making polymers and other chemicals.
Exam technique
In the exam
- For cracking conditions, state high temperature plus either a catalyst for catalytic cracking or steam for steam cracking.
- For balancing cracking equations, count carbon atoms and hydrogen atoms separately, then change coefficients, not formula subscripts.
- For the alkene test, give the exact colour change: bromine water turns from orange to colourless.
Self review
Check yourself
- Why are long-chain hydrocarbons cracked rather than all used directly as fuels?
- What conditions are used for catalytic cracking and steam cracking?
- Balance this equation: C12H26(g)→C8H18(g)+C2H4(g)\mathrm{C_{12}H_{26}(g) \to C_8H_{18}(g) + C_2H_4(g)}C12H26(g)→C8H18(g)+C2H4(g)
Carbon compounds as fuels and feedstock
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