Cracking and alkenes — AQA GCSE Combined Science
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Cracking and alkenes explained
Cracking is a thermal decomposition reaction in which large hydrocarbon molecules are broken down into smaller, more useful molecules.
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It is used because long-chain hydrocarbons from crude oil are less useful as fuels, while short-chain alkanes are in high demand and alkenes are needed to make polymers. Cracking can be done by heating the hydrocarbon with a catalyst (catalytic cracking) or by heating it to a high temperature with steam (steam cracking). The products include smaller alkanes and alkenes. For example, decane can crack to produce octane and ethene. Alkenes are unsaturated hydrocarbons with a C=C double bond and are more reactive than alkanes, so they are useful for making polymers such as poly(ethene).
Cracking can be done by various methods including catalytic cracking and steam cracking.
Cracking is thermal decomposition of large, less useful hydrocarbon molecules into smaller, more useful ones, often producing alkenes as well as shorter alkanes. It can be done by various methods, including catalytic cracking and steam cracking. In catalytic cracking, vaporised long-chain hydrocarbons pass over a hot catalyst such as zeolite, giving shorter chains and alkenes. In steam cracking, the feedstock is mixed with steam and heated strongly without a catalyst. Both methods break C–C and C–H bonds, so products include alkanes and alkenes; for example, decane can crack to octane and ethene. Alkenes are valuable because they are used to make polymers, so cracking helps match supply to demand for fuels and petrochemicals.
Students should be able to describe in general terms the conditions used for catalytic cracking and steam cracking.
To describe cracking conditions in general terms, compare the two methods. Catalytic cracking passes vaporised long-chain hydrocarbons over a hot catalyst, commonly a zeolite, at a high temperature; the catalyst speeds up the breakdown. Steam cracking mixes the hydrocarbon feedstock with steam and heats it strongly, without a catalyst. Both methods use high temperature because breaking covalent C–C and C–H bonds needs energy. The conditions are chosen to produce shorter-chain alkanes and alkenes, such as ethene, which are more useful as fuels and as monomers for polymers. A good answer names the feedstock state, the presence or absence of a catalyst, the role of steam, and the need for strong heating, without inventing exact industrial temperatures or pressures.
The products of cracking include alkanes and another type of hydrocarbon called alkenes.
Cracking breaks large, less useful hydrocarbon molecules into smaller, more useful ones. The products are a mixture: shorter-chain alkanes and a different family called alkenes. Alkanes have only single carbon–carbon bonds and are saturated; alkenes contain a carbon–carbon double bond and are unsaturated. For example, cracking decane, C₁₀H₂₂, can give octane, C₈H₁₈, and ethene, C₂H₄. The alkane product is useful as a fuel, while the alkene is a feedstock for polymers. You must be able to recognise both families from their names and formulae, and explain that cracking produces a mixture that is separated by fractional distillation. The key idea is that cracking does not make a single product; it makes a range of smaller molecules, including at least one alkane and at least one alkene.
Alkenes are more reactive than alkanes and react with bromine water, which is used as a test for alkenes.
Alkenes contain a carbon–carbon double bond, which makes them more reactive than alkanes. A simple test uses bromine water, an orange solution. When an alkene is shaken with bromine water, the orange colour disappears and the solution becomes colourless. This decolourisation happens because the alkene reacts with bromine in an addition reaction, breaking the double bond. Alkanes do not react in this way under the same conditions, so the orange colour remains. The test is therefore used to distinguish alkenes from alkanes. You should be able to describe the test, state the colour change from orange to colourless, and explain that it is the double bond that causes the reaction. Remember that the test is qualitative: it shows the presence of an alkene but does not identify which alkene.
Students should be able to recall the colour change when bromine water reacts with an alkene.
Bromine water is an orange solution used as a test for unsaturation. When shaken with an alkene such as ethene, the C=C double bond reacts with bromine in an addition reaction, so the orange colour is decolourised and the mixture turns colourless. An alkane such as ethane has only single C–C bonds and does not react this way under the same conditions, so the orange colour persists. To perform the test, add a few drops of bromine water to the sample, shake, and observe. The colour change from orange to colourless shows an alkene is present. This links the functional group to a visible result and is assessed by asking you to state the starting and finishing colours accurately.
There is a high demand for fuels with small molecules and so some of the products of cracking are useful as fuels.
Cracking breaks large hydrocarbon molecules into smaller, more useful ones. Small molecules such as methane, ethane, propane and butane have weak intermolecular forces, so they have low boiling points and are gases or volatile liquids that ignite and burn easily. This makes them valuable as fuels for cooking, heating and transport. Demand for these small-molecule fuels is high because they release energy readily when burned. Cracking therefore supplies fuels such as petrol and diesel components as well as alkenes for making polymers. For example, decane can be cracked to produce octane and ethene; the octane is a useful fuel while the ethene is a feedstock for plastics.
Alkenes are used to produce polymers and as starting materials for the production of many other chemicals.
Alkenes are unsaturated hydrocarbons containing a C=C double bond, making them far more reactive than alkanes. This reactivity is exploited in the chemical industry. Alkenes are used to produce polymers and act as starting materials for many other chemicals. For example, ethene (molecular formula C₂H₄, empirical formula CH₂) can be used to make poly(ethene) for carrier bags and bottles, or converted into other useful compounds like ethanol. Cracking long-chain alkanes supplies these essential alkenes. Understanding that alkenes serve as versatile chemical feedstocks is key, as their double bond allows them to undergo reactions that alkanes cannot, forming the basis of many everyday materials.
Students should be able to balance chemical equations as examples of cracking given the formulae of the reactants and products.
Cracking breaks a long-chain alkane into a shorter, more useful alkane and an alkene. A word equation such as decane → octane + ethene becomes a balanced symbol equation by writing correct formulae and then adjusting coefficients so every element has the same number of atoms on each side. For example, C₁₀H₂₂ → C₈H₁₈ + C₂H₄ is already balanced: carbon 10 = 8 + 2 and hydrogen 22 = 18 + 4. If the products given do not balance, only the large numbers in front of formulae may be changed; formulae themselves must not be altered. Check each element in turn, starting with carbon and hydrogen, and remember cracking must also produce at least one alkene.
Students should be able to give examples to illustrate the usefulness of cracking. They should also be able to explain how modern life depends on the uses of hydrocarbons.
Cracking turns long-chain hydrocarbons from crude oil into shorter, more useful molecules, including alkenes. For example, decane can be cracked into octane and ethene: C₁₀H₂₂ → C₈H₁₈ + C₂H₄. The shorter alkanes make fuels such as petrol, which burn more cleanly and are in high demand, while alkenes are reactive starting materials for polymers such as poly(ethene) and for other chemicals. Modern life depends on hydrocarbons because fuels power transport, heating and electricity generation, and petrochemical feedstocks provide plastics, medicines, detergents, lubricants, solvents and synthetic fibres. Without cracking, supplies of short-chain fuels and alkenes would be limited, so petrol, plastics and many everyday products would be scarcer and more expensive.
Your focus
- Describe cracking as a thermal decomposition reaction that breaks large hydrocarbons into smaller molecules.
- Identify the products of cracking as smaller alkanes and alkenes, and recognise the C=C double bond in alkenes.
- Explain why cracking is economically important for producing useful fuels and polymer feedstocks.
Show all 30 objectives
- State that cracking breaks large hydrocarbon molecules into smaller ones.
- Identify catalytic cracking and steam cracking as two methods of cracking.
- Describe the products of cracking as shorter alkanes and alkenes.
- Describe the general conditions for catalytic cracking, including a hot catalyst and high temperature.
- Describe the general conditions for steam cracking, including steam and high temperature without a catalyst.
- Compare the two cracking methods by identifying the key difference in their conditions.
- State that cracking produces a mixture of smaller hydrocarbons including alkanes and alkenes.
- Identify an alkane and an alkene from a given name or formula.
- Explain why cracking is useful in the petrochemical industry.
- Describe the bromine water test for alkenes, including the colour change.
- Explain why alkenes react with bromine water but alkanes do not.
- Use the test to distinguish an alkene from an alkane.
- State the colour of bromine water before and after reaction with an alkene.
- Describe the test procedure of adding bromine water and shaking.
- Use the result to classify a hydrocarbon as an alkene or an alkane.
- Explain why small hydrocarbon molecules are useful as fuels.
- Describe how cracking produces smaller molecules from larger ones.
- Relate the products of cracking to their uses as fuels and as feedstocks for polymers.
- State that alkenes are unsaturated hydrocarbons containing a C=C double bond.
- Identify the molecular and empirical formulae of simple alkenes like ethene.
- Explain that alkenes are used to produce polymers and as starting materials to produce other chemicals.
- Write correct formulae for the reactants and products in a cracking reaction.
- Balance a cracking equation by adding whole-number coefficients without changing formulae.
- Check a balanced cracking equation by counting carbon and hydrogen atoms on each side.
- Give at least one example of cracking and identify the useful products formed.
- Explain how cracking increases the supply of short-chain fuels and reactive alkenes.
- Describe how modern life depends on hydrocarbons for fuels and for making everyday materials.
Cracking and alkenes exam tips
Marking Points
- Cracking is a thermal decomposition reaction: large hydrocarbon molecules are broken down into smaller molecules by heating.
- Cracking produces smaller alkanes and alkenes; alkenes are unsaturated and contain a carbon-to-carbon double bond, C=C.
- Cracking is important because it converts less useful long-chain hydrocarbons into shorter-chain alkanes that are better fuels and alkenes that are used to make polymers.
- Catalytic cracking uses a catalyst and heat, while steam cracking uses high temperature and steam; both break large molecules into smaller ones.
- A typical example is decane cracking to form octane and ethene: C₁₀H₂₂ → C₈H₁₈ + C₂H₄, showing that the products are smaller molecules.
- Cracking is the breakdown of large hydrocarbon molecules into smaller molecules, usually by heating.
- Catalytic cracking uses a hot catalyst, often a zeolite, to speed up the breakdown of vaporised long-chain hydrocarbons.
- Steam cracking uses high temperature and steam, without a catalyst, to crack hydrocarbons.
- Products of cracking include shorter-chain alkanes and alkenes, which are more useful than the original large molecules.
- Alkenes produced by cracking are important as monomers for making polymers.
- Cracking is a form of thermal decomposition and requires an input of energy to break covalent bonds.
- Catalytic cracking uses a hot catalyst, often a zeolite, and a high temperature.
- In catalytic cracking, long-chain hydrocarbons are vaporised before passing over the catalyst.
- Steam cracking uses steam and a high temperature, without a catalyst.
- Both methods require strong heating because covalent bonds in hydrocarbon molecules must be broken.
- The conditions are chosen to produce shorter-chain alkanes and alkenes, including ethene.
- General descriptions should identify the key difference: catalyst present in catalytic cracking, steam present in steam cracking.
- Cracking converts large hydrocarbon molecules into smaller molecules that are more useful as fuels or chemical feedstocks.
- The products of cracking always include alkanes, which are saturated hydrocarbons with only single carbon–carbon bonds.
- The products also include alkenes, which are unsaturated hydrocarbons containing a carbon–carbon double bond.
- A named example such as decane cracking to octane and ethene shows one alkane and one alkene product.
- Alkenes are a different homologous series from alkanes, with the general formula CₙH₂ₙ for alkenes and CₙH₂ₙ₊₂ for alkanes.
- The mixture of products is separated by fractional distillation because the products have different boiling points.
- Alkenes are more reactive than alkanes because they contain a carbon–carbon double bond.
- Bromine water is an orange solution used to test for the presence of an alkene.
- When an alkene is added to bromine water and shaken, the orange colour is lost and the solution becomes colourless.
- The reaction is an addition reaction in which bromine adds across the carbon–carbon double bond.
- Alkanes do not decolourise bromine water under the same conditions, so the orange colour remains.
- The test is qualitative and shows that an alkene is present, but it does not identify the specific alkene.
- Bromine water is orange (or orange-brown/yellow-orange) before the test.
- An alkene reacts with bromine water and the colour is lost.
- The final observation is colourless, not clear or white.
- The reaction is an addition reaction across the C=C double bond.
- An alkane does not decolourise bromine water, so the orange colour remains.
- The test distinguishes unsaturated alkenes from saturated alkanes.
- Cracking converts large hydrocarbon molecules into smaller molecules.
- Small molecules have weaker intermolecular forces and lower boiling points than large molecules.
- Small-molecule fuels are gases or volatile liquids that burn readily, so they are in high demand.
- Some cracking products, such as petrol components, are used as fuels.
- Cracking also produces alkenes, which are useful for making polymers.
- The demand for small-molecule fuels is a reason why cracking is carried out industrially.
- Alkenes contain a carbon–carbon double bond (C=C) and are described as unsaturated.
- Alkenes are more reactive than alkanes due to the presence of the C=C double bond.
- Ethene has the molecular formula C₂H₄ and the empirical formula CH₂.
- Alkenes are used as starting materials (feedstocks) for making other chemicals, such as ethanol.
- Alkenes are used to produce polymers, such as poly(ethene) from ethene.
- Write the correct formulae of the reactant alkane and the product alkane and alkene before attempting to balance.
- Balance by placing whole-number coefficients in front of formulae; never change a formula's subscripts to make an equation balance.
- Check carbon atoms first, then hydrogen atoms, and confirm the totals are equal on both sides.
- Recognise that a valid cracking equation produces a shorter alkane and an alkene, so the products must include a C=C-containing molecule.
- For C₁₀H₂₂ → C₈H₁₈ + C₂H₄, verify carbon: 10 = 8 + 2, and hydrogen: 22 = 18 + 4.
- If coefficients are needed, for example 2C₈H₁₈, multiply every atom in that formula by the coefficient when checking.
- Cracking converts long-chain hydrocarbon molecules into shorter-chain molecules, which are more useful as fuels.
- Cracking produces alkenes, which are reactive and can be used to make polymers such as poly(ethene).
- A valid example is cracking decane to give octane and ethene, showing a shorter alkane fuel and an alkene feedstock.
- Shorter-chain hydrocarbons are in greater demand as fuels because they ignite and burn more easily than long-chain hydrocarbons.
- Modern life depends on hydrocarbons as fuels for transport, heating and electricity generation.
- Hydrocarbon feedstocks are used to make plastics, medicines, detergents, lubricants, solvents and synthetic fibres.
- A consequence of limited cracking would be reduced supplies of petrol and petrochemical products, raising costs.
Examiner Tips
- 💡When explaining why cracking is used, refer to supply and demand: long-chain hydrocarbons are in less demand, while short-chain fuels and alkenes for polymers are in higher demand.
- 💡If asked to name products, check whether the question wants an alkane, an alkene or both; give the correct molecular formula where possible.
- 💡Use the term 'thermal decomposition' and state that no oxygen is needed, to distinguish cracking from burning.
- 💡Name the method and state its key condition, such as catalyst or steam, rather than saying only 'heat'.
- 💡Use a balanced equation or a labelled example to show that atoms are conserved during cracking.
- 💡Link the products to a use, such as alkenes for polymers, to show why cracking is economically important.
- 💡Use a comparison table or two labelled bullet points to contrast catalytic cracking and steam cracking clearly.
- 💡Include the word 'catalyst' only for catalytic cracking and 'steam' only for steam cracking to avoid mixing the methods.
- 💡Link each condition to its purpose, such as high temperature to break bonds or catalyst to speed up the reaction.
- 💡When asked for the products of cracking, name at least one alkane and one alkene and state that a mixture forms.
- 💡Use displayed or structural formulae to show the double bond in an alkene if the question asks you to explain unsaturation.
- 💡Link cracking to the demand for shorter-chain fuels and to polymer production to show why the process is useful.
- 💡State the starting colour and the final colour precisely: orange to colourless.
- 💡Link the colour change to the reaction of bromine with the carbon–carbon double bond.
- 💡If asked to compare, say that alkanes do not decolourise bromine water, so the test distinguishes the two families.
- 💡Learn both colours as a pair: orange to colourless.
- 💡Name the substance being tested, for example ethene, and state that it is an alkene.
- 💡If asked to compare, state that the alkane stays orange while the alkene goes colourless.
- 💡Use the term decolourised to show precise scientific vocabulary.
- 💡Link the small size of the molecule to weak intermolecular forces and a low boiling point.
- 💡Give a named example of a small-molecule fuel, such as methane or petrol.
- 💡Mention both products of cracking: fuels and alkenes for polymers.
- 💡Use the phrase high demand for fuels with small molecules when explaining why cracking is useful.
- 💡When asked why alkenes are useful, link the answer to their reactivity and use as starting materials for polymers and other chemicals.
- 💡Use the phrase 'starting material' or 'feedstock' when describing alkenes being converted into other chemicals.
- 💡Count and write the number of each type of atom on both sides before and after balancing so your working is clear.
- 💡Balance carbon first, then hydrogen, and recheck both at the end.
- 💡If the equation is already balanced, say so and show the atom counts rather than adding unnecessary coefficients.
- 💡Link each example of cracking to a named useful product, such as petrol or poly(ethene), rather than describing cracking in general terms.
- 💡When explaining dependence on hydrocarbons, cover both fuels and feedstocks, giving at least one everyday product for each category.
- 💡Use a balanced symbol equation or a labelled flow diagram to show a cracking example clearly and accurately.
Common Mistakes
- Error: saying cracking is the same as combustion. Correction: cracking is thermal decomposition without oxygen, whereas combustion reacts a fuel with oxygen.
- Error: writing that cracking produces only alkenes. Correction: cracking produces a mixture of smaller alkanes and alkenes.
- Error: confusing saturated and unsaturated hydrocarbons. Correction: alkanes are saturated with single C–C bonds; alkenes are unsaturated and contain a C=C double bond.
- Thinking cracking only produces alkenes; correction: it typically produces a mixture of shorter alkanes and alkenes.
- Confusing catalytic cracking with steam cracking; correction: catalytic cracking uses a catalyst, while steam cracking uses steam and high temperature without a catalyst.
- Writing that cracking adds hydrogen or removes carbon; correction: cracking rearranges the same atoms into smaller hydrocarbon molecules, so the total number of carbon and hydrogen atoms is conserved.
- Saying steam cracking uses a catalyst; correction: steam cracking uses steam and high temperature, not a catalyst.
- Giving exact temperatures or pressures as if they were fixed facts; correction: describe conditions in general terms such as 'high temperature' unless the specification requires a specific value.
- Forgetting that the feedstock is vaporised in catalytic cracking; correction: the hydrocarbon must be a gas so it can contact the catalyst surface.
- Thinking cracking produces only alkenes; correction: cracking produces a mixture that includes alkanes as well as alkenes.
- Confusing the general formulae; correction: alkanes are CₙH₂ₙ₊₂ and alkenes are CₙH₂ₙ.
- Believing alkenes are saturated; correction: alkenes are unsaturated because they contain a carbon–carbon double bond.
- Saying bromine water turns clear rather than colourless; correction: the correct observation is that the orange colour disappears and the solution becomes colourless.
- Thinking alkanes also decolourise bromine water; correction: alkanes do not react with bromine water in this test, so the orange colour persists.
- Confusing the test with a test for alkanes; correction: bromine water is used to test for alkenes, not alkanes.
- Saying the colour change is orange to clear: clear describes a transparent solution, not a loss of colour; the correct term is colourless.
- Saying the solution turns white: a white precipitate or cloudiness is not the observation; the orange colour is removed, giving a colourless solution.
- Claiming alkanes also decolourise bromine water: alkanes are saturated and do not react in this test, so the orange colour stays.
- Saying cracking only makes alkenes: cracking also produces smaller alkanes that are used as fuels.
- Confusing small molecules with low demand: small molecules are in high demand because they are useful fuels, not because they are scarce.
- Stating that large molecules burn better: large molecules have stronger intermolecular forces and are less volatile, so they are less useful as fuels.
- Writing that alkenes are saturated: alkenes are unsaturated because of the C=C double bond, whereas alkanes are saturated.
- Confusing molecular and empirical formulae: ethene's molecular formula is C₂H₄, but its empirical formula is CH₂.
- Assuming alkanes and alkenes have the same reactivity: alkenes are much more reactive due to the double bond.
- Changing a subscript in a formula, such as writing C₈H₂₀ instead of C₈H₁₈, to force the equation to balance; only coefficients may be changed.
- Forgetting to multiply all atoms by the coefficient when a product has a number in front of it.
- Producing only alkanes as products; a cracking equation must include an alkene such as ethene or propene.
- Thinking cracking joins small molecules together; correction: cracking breaks long-chain hydrocarbons into shorter, more useful molecules.
- Stating that cracking only makes fuels; correction: it also makes alkenes, which are used to make polymers and other chemicals.
- Claiming all crude oil fractions are equally useful; correction: long-chain fractions are less in demand, so cracking converts them into shorter-chain products that are more useful.