Cracking and alkenes — AQA GCSE Chemistry
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Cracking and alkenes explained
Cracking and alkenes covers 11 specification points, including Hydrocarbons can be broken down (cracked) to… and Cracking can be done by various methods….
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Hydrocarbons can be broken down (cracked) to produce smaller, more useful molecules.
Cracking breaks large hydrocarbon molecules into smaller ones that are more useful as fuels and as starting materials for making polymers and other chemicals. It can be done by heating the vaporised hydrocarbon with a catalyst, called catalytic cracking, or by mixing the vapour with steam and heating to a very high temperature, called steam cracking. The products include smaller alkanes and alkenes; alkenes contain a carbon-to-carbon double bond and are more reactive than alkanes. For example, cracking decane can give octane and ethene. Ethene can be tested by shaking it with bromine water, which changes from orange to colourless. Cracking therefore helps match the supply of hydrocarbon fractions to demand, since longer-chain fractions are less useful as fuels.
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. Because long-chain fractions from fractional distillation are in low demand, cracking converts them into shorter alkanes and alkenes. Two named industrial methods are catalytic cracking, which passes vaporised hydrocarbons over a hot catalyst, and steam cracking, which mixes them with steam at high temperature. For example, decane can be cracked into octane and ethene. Both methods break C–C and C–H bonds using energy, so they are endothermic overall. The choice of method depends on the feedstock and the product mixture wanted. Understanding both routes lets you explain why refineries can match supply of fuels and alkenes to demand.
Students should be able to describe in general terms the conditions used for catalytic cracking and steam cracking.
To describe cracking conditions, compare temperature, catalyst and feedstock state. Catalytic cracking vaporises long-chain hydrocarbons and passes them over a hot catalyst, typically a zeolite, at a moderately high temperature; the catalyst lowers the energy needed and speeds up the reaction. Steam cracking mixes the hydrocarbon feedstock with steam at a very high temperature without a catalyst. In both methods the feedstock must be gaseous or vaporised so molecules can react. For example, a refinery may crack decane over a zeolite, or steam-crack naphtha to make ethene. You do not need exact numerical values, but you should state that temperature is high, that catalytic cracking uses a catalyst, and that steam cracking uses steam.
The products of cracking include alkanes and another type of hydrocarbon called alkenes.
Cracking does not give a single product; it gives a mixture of smaller hydrocarbons. These products belong to two homologous series: alkanes and alkenes. Alkanes have only single carbon–carbon bonds and are saturated; alkenes contain at least one carbon–carbon double bond and are unsaturated. For example, cracking decane can produce octane, an alkane, and ethene, an alkene. The alkane products are useful as fuels, while the alkene products are reactive starting materials for making polymers such as poly(ethene). Because a mixture forms, the products must be separated, often by fractional distillation. Recognising the two product families helps you predict formulae and explain why cracking is valuable to industry.
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, whose molecules have only single bonds. A key test uses bromine water, an orange-brown solution. When shaken with an alkene, the bromine water is decolourised, turning colourless, because the alkene adds bromine across its double bond in an addition reaction. An alkane does not react in this way, so the bromine water stays orange-brown. For example, ethene decolourises bromine water, while ethane does not. This test distinguishes unsaturated from saturated hydrocarbons. Remember that the colour change is from orange-brown to colourless, and that the test is carried out at room temperature by shaking the mixture.
Students should be able to recall the colour change when bromine water reacts with an alkene.
Bromine water is an orange solution used to test for a carbon–carbon double bond. When shaken with an alkene such as ethene, the bromine adds across the double bond in an addition reaction, so the orange colour is decolourised and the mixture turns colourless. An alkane such as ethane has no double bond, so no addition occurs and the orange colour remains. To perform the test, add a few drops of bromine water to the sample, shake, and record the colour before and after. The test is assessed by asking you to state the colour change from orange to colourless for an alkene, and to explain that the double bond is responsible.
There is a high demand for fuels with small molecules and so some of the products of cracking are useful as fuels.
Crude oil fractions contain many large hydrocarbon molecules that are not ideal as fuels because they are viscous, hard to ignite and burn less cleanly. Cracking breaks these large molecules into smaller ones, some of which are alkanes with small molecules such as methane, ethane, propane and butane. These small alkanes ignite easily, burn with a clean flame and can be stored and transported as gases or volatile liquids, so they are in high demand as fuels for homes, cooking, heating and vehicles. The process also produces alkenes, which are valuable as chemical feedstocks. Demand for small-molecule fuels therefore drives cracking, and the products are separated and used according to their properties.
Alkenes are used to produce polymers and as starting materials for the production of many other chemicals.
Alkenes contain a carbon–carbon double bond, which makes them reactive and valuable in the chemical industry. Their main use is to produce polymers by addition polymerisation: many alkene monomers join together to form a long-chain polymer such as poly(ethene) from ethene or poly(propene) from propene. These polymers are used for bags, bottles, pipes and fibres. Alkenes are also starting materials, or feedstocks, for making many other chemicals, including ethanol, ethanoic acid, antifreeze and detergents. The double bond allows addition reactions that convert alkenes into a wide range of products, so cracking is important because it supplies the alkenes needed by industry.
Students should be able to balance chemical equations as examples of cracking given the formulae of the reactants and products.
A cracking equation must conserve atoms: the number of each type of atom on the left must equal the number on the right. Given the formulae of the reactant and products, balance by adjusting the large numbers in front of formulae, never by changing the formulae themselves. For example, C₁₀H₂₂ can crack to give C₈H₁₈ and C₂H₄, which is already balanced: 10 carbon atoms and 22 hydrogen atoms on each side. If a product is doubled, such as 2C₂H₄, then the hydrogen count changes and other coefficients may need adjusting. Work atom by atom, starting with carbon and then hydrogen, and check the total at the end.
Students should be able to give examples to illustrate the usefulness of cracking.
Cracking turns large, less useful alkanes into smaller, more useful molecules. A long-chain fraction such as decane can be cracked to give octane, used in petrol, plus ethene, which is used to make poly(ethene). This matters because demand for petrol and for polymers is high, while long-chain fractions from crude oil are in surplus. Cracking may be thermal or catalytic, and it produces alkanes plus alkenes. The alkenes are the reactive products that feed the polymer industry, and the shorter alkanes are cleaner-burning fuels. A good answer names a starting material, a product and its use, and links the process to matching supply with demand.
They should also be able to explain how modern life depends on the uses of hydrocarbons.
Hydrocarbons are compounds of carbon and hydrogen only, and they underpin much of modern life. Fractions from crude oil provide fuels for cars, lorries, ships and aircraft, and for heating and electricity generation. They are also feedstocks for the petrochemical industry, which makes plastics, synthetic fibres, detergents, medicines, lubricants and solvents. Alkenes from cracking are polymerised into materials such as poly(ethene) for bags and bottles. A strong explanation links a named hydrocarbon use to a benefit and recognises that crude oil is a finite resource, so alternatives and recycling are important.
Your focus
- Describe cracking as the breakdown of large hydrocarbons into smaller molecules.
- Name the conditions used for catalytic and steam cracking, including mixing with steam.
- Identify alkenes using bromine water and explain why they are more reactive than alkanes.
Show all 33 objectives
- State that cracking breaks large hydrocarbon molecules into smaller ones.
- Identify catalytic cracking and steam cracking as two methods of cracking.
- Describe one key condition used in each named cracking method.
- Describe the conditions used in catalytic cracking.
- Describe the conditions used in steam cracking.
- Compare the two methods by stating one similarity and one difference in their conditions.
- State that cracking produces a mixture of alkanes and alkenes.
- Distinguish alkanes from alkenes in terms of saturation and bonding.
- Give one use each for the alkane and alkene products of cracking.
- Describe the bromine water test and its colour change with alkenes.
- Explain why alkenes are more reactive than alkanes in terms of the double bond.
- Use the bromine water test to distinguish an alkene from an alkane.
- Recall that bromine water is orange and becomes colourless with an alkene.
- Describe the test procedure of adding bromine water and shaking.
- Explain the colour change in terms of addition across the C=C double bond.
- Explain why small-molecule fuels are in high demand.
- Describe how cracking produces smaller molecules that can be used as fuels.
- Relate the properties of small alkanes to their use as fuels.
- Describe how alkenes are used to produce polymers by addition polymerisation.
- Give examples of polymers and their alkene monomers.
- Explain why alkenes are useful starting materials for making other chemicals.
- Balance cracking equations by adjusting coefficients only.
- Count atoms of carbon and hydrogen on each side of an equation.
- Check a balanced equation to confirm conservation of atoms.
- State that cracking converts large alkanes into smaller, more useful molecules.
- Give a named example of cracking with at least one product and its use.
- Explain how cracking helps match the supply of fractions to demand for fuels and polymer feedstocks.
- Describe several uses of hydrocarbons in modern life.
- Explain how named products from hydrocarbons benefit people.
- Discuss the sustainability issues arising from dependence on finite crude oil.
Cracking and alkenes exam tips
Marking Points
- Cracking breaks large hydrocarbon molecules into smaller molecules.
- The products are smaller alkanes and alkenes.
- Cracking can be carried out by heating with a catalyst or by mixing with steam and heating to a high temperature.
- Alkenes contain a carbon-to-carbon double bond and are more reactive than alkanes.
- Smaller molecules are more useful as fuels and as feedstock for polymers.
- Ethene can be identified because it decolourises bromine water from orange to colourless.
- 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 with steam, without a catalyst, to crack hydrocarbons.
- Both methods produce a mixture of smaller alkanes and alkenes rather than a single pure product.
- Cracking is economically important because it converts surplus long-chain fractions into shorter fuels and reactive alkenes for polymers.
- Catalytic cracking: hydrocarbon vapour passed over a hot catalyst, commonly a zeolite.
- Catalytic cracking: a high temperature is used, but the catalyst allows reaction at a lower temperature than steam cracking.
- Steam cracking: hydrocarbon feedstock mixed with steam at a very high temperature.
- Steam cracking: no catalyst is used.
- In both methods the hydrocarbon feedstock is vaporised or gaseous so that molecules can react.
- Cracking produces smaller hydrocarbon molecules, not a single pure substance.
- The products 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.
- Alkane products are useful as fuels, while alkene products are used to make polymers and other chemicals.
- The mixture of products is separated, for example by fractional distillation.
- Alkenes are more reactive than alkanes because they contain a carbon–carbon double bond.
- Bromine water is an orange-brown solution used to test for alkenes.
- An alkene reacts with bromine water and decolourises it, giving a colourless solution.
- An alkane does not decolourise bromine water, so the orange-brown colour remains.
- The reaction of an alkene with bromine is an addition reaction across the double bond.
- Bromine water is orange (or orange-brown) before the test.
- An alkene decolourises bromine water, turning it colourless.
- The colour change is from orange to colourless.
- The change occurs because bromine adds across the C=C double bond.
- An alkane gives no colour change and stays orange, so the test distinguishes alkenes from alkanes.
- The test is an addition reaction, not a substitution or combustion reaction.
- Large hydrocarbon molecules in crude oil are less useful as fuels because they are viscous and difficult to ignite.
- Cracking breaks large molecules into smaller molecules.
- Small-molecule alkanes such as methane, ethane, propane and butane burn readily and are useful fuels.
- There is high demand for fuels with small molecules for heating, cooking and transport.
- Cracking produces a mixture of smaller alkanes and alkenes, and the alkanes can be used as fuels.
- The products are separated before use, so useful fuels are obtained from the mixture.
- Alkenes contain a C=C double bond that makes them reactive.
- Alkenes undergo addition polymerisation to form polymers.
- Ethene forms poly(ethene) and propene forms poly(propene) as examples.
- Polymers made from alkenes are used for packaging, bottles, pipes and fibres.
- Alkenes are feedstocks for making other chemicals such as ethanol and ethanoic acid.
- Cracking supplies alkenes because they are in demand as chemical starting materials.
- The number of atoms of each element must be the same on both sides of the equation.
- Only coefficients in front of formulae may be changed when balancing.
- Subscripts within a formula must not be altered to balance an equation.
- Carbon atoms are usually balanced first, then hydrogen atoms.
- A final check confirms that the total atoms of each element match on both sides.
- State symbols may be added if required, but balancing concerns the formulae and coefficients.
- Cracking converts large alkane molecules into smaller molecules, including alkenes, which are more useful.
- An example is cracking decane to produce octane for petrol and ethene for making poly(ethene).
- Alkenes from cracking are used to make polymers such as poly(ethene) and poly(propene).
- Shorter-chain alkanes produced by cracking are used as fuels because they burn more cleanly than heavy fractions.
- Cracking helps match supply to demand: heavy fractions are in surplus while fuels and polymer feedstocks are in demand.
- Award credit for a named product and a specific use, not just the word cracking.
- Hydrocarbons are used as fuels for transport, heating and electricity generation.
- Hydrocarbons are feedstocks for making polymers such as poly(ethene) and poly(propene).
- Petrochemicals from hydrocarbons are used in medicines, detergents, lubricants, solvents and synthetic fibres.
- Modern life depends on these uses for transport, clothing, packaging, healthcare and communication.
- Crude oil is a finite resource, so dependence raises issues of sustainability, recycling and alternative feedstocks.
- Credit answers that name a specific product and explain the benefit it brings.
Examiner Tips
- 💡Name the type of cracking, catalytic or steam, and give the essential condition such as a catalyst or mixing with steam and heating to a high temperature.
- 💡State that the products are smaller alkanes and alkenes, and give a named example such as ethene.
- 💡When describing the bromine water test, give the starting colour and the final colour in the correct order.
- 💡Name both methods and give one condition for each, such as a hot catalyst for catalytic cracking and steam with high temperature for steam cracking.
- 💡Use the word hydrocarbon accurately and refer to large molecules being broken into smaller ones.
- 💡Link cracking to supply and demand: long-chain fractions are less useful, short chains and alkenes are more useful.
- 💡Use a two-column comparison in your notes: method, catalyst, steam, temperature.
- 💡When asked to describe conditions, give the method name plus at least one condition for each.
- 💡Avoid quoting precise temperatures unless the question supplies them; general terms are sufficient.
- 💡When naming products, state clearly which is the alkane and which is the alkene.
- 💡Use the general formulae CₙH₂ₙ₊₂ for alkanes and CₙH₂ₙ for alkenes to check a suggested product.
- 💡Link each product family to a use: alkanes as fuels, alkenes as polymer feedstocks.
- 💡State the starting colour and the final colour: orange-brown to colourless.
- 💡Mention shaking the test tube so the hydrocarbon and bromine water mix.
- 💡Use the result to classify the hydrocarbon as unsaturated (alkene) or saturated (alkane).
- 💡Learn the exact starting colour as orange, not just 'coloured'.
- 💡State both the before and after colours to secure the colour-change mark.
- 💡Link the colour change to the C=C double bond rather than to 'bromine reacting' vaguely.
- 💡Name at least two small alkanes to show the products are useful fuels.
- 💡Contrast the properties of large and small molecules to justify the demand.
- 💡Mention that cracking gives a mixture, so separation is needed before the fuels are used.
- 💡Give a named polymer and its monomer to show the link clearly.
- 💡Use the term feedstock when describing alkenes as starting materials.
- 💡Link the reactivity of the double bond to both polymerisation and other addition reactions.
- 💡Count atoms in a table before and after to avoid losing track.
- 💡Balance carbon first, then hydrogen, and recheck both at the end.
- 💡Never alter a formula to make an equation balance; adjust only the coefficients.
- 💡Name the starting alkane, the products and one use for each product.
- 💡Link the example to supply and demand to show why cracking is useful.
- 💡Use the terms thermal cracking and catalytic cracking correctly if you mention conditions.
- 💡Name at least two different categories of use, such as fuel and feedstock.
- 💡Link each use to a benefit for people, for example transport or healthcare.
- 💡Mention sustainability to show awareness that crude oil is finite.
Common Mistakes
- Saying cracking joins small molecules together; correct this by stating that cracking breaks large molecules down into smaller ones.
- Stating that cracking produces only alkanes; correct this by stating that alkenes are also produced.
- Describing the bromine water test as turning from colourless to orange; correct this by stating that bromine water changes from orange to colourless.
- Thinking cracking is a separation process like fractional distillation; correct this by stressing that new smaller molecules are made by breaking bonds.
- Believing cracking needs no energy input; correct this by noting that bond breaking makes the process endothermic overall.
- Assuming only one product forms; correct this by stating that a mixture of alkanes and alkenes is produced.
- Swapping the conditions, for example saying steam cracking uses a catalyst; correct this by linking catalyst only with catalytic cracking.
- Giving exact temperatures as if they were fixed facts; correct this by describing conditions generally as high or very high temperature.
- Forgetting that the feedstock is vaporised; correct this by stating that gaseous hydrocarbon molecules contact the catalyst or steam.
- Saying cracking produces only alkenes; correct this by stating that alkanes are also produced.
- Confusing the terms saturated and unsaturated; correct this by linking saturated to single bonds in alkanes and unsaturated to a double bond in alkenes.
- Writing alkane and alkene formulae incorrectly; correct this by using displayed or structural formulae to show the double bond in alkenes.
- Saying bromine water turns clear rather than colourless; correct this by using the term colourless for the observed result.
- Claiming alkanes also decolourise bromine water; correct this by stating that alkanes do not react and the colour persists.
- Describing the test as adding bromine gas rather than bromine water; correct this by naming bromine water as the reagent.
- Saying the colour change is from colourless to orange; the correction is that bromine water starts orange and becomes colourless with an alkene.
- Confusing bromine water with limewater; the correction is that limewater tests for carbon dioxide, while bromine water tests for alkenes.
- Claiming alkanes also decolourise bromine water; the correction is that alkanes have no C=C bond and leave the orange colour unchanged.
- Thinking all cracking products are alkenes; the correction is that cracking produces both smaller alkanes and alkenes.
- Assuming large molecules are in higher demand as fuels; the correction is that small molecules are in higher demand because they ignite and burn more easily.
- Confusing cracking with fractional distillation; the correction is that cracking is a chemical breakdown, while fractional distillation separates without chemical change.
- Saying alkenes are used to make fuels only; the correction is that alkenes are mainly used to make polymers and other chemicals.
- Confusing addition polymerisation with condensation polymerisation; the correction is that alkene polymerisation is addition, with no small molecule eliminated.
- Writing that polymers are made from alkanes; the correction is that the C=C double bond in alkenes allows addition polymerisation.
- Changing a subscript in a formula, such as turning C₂H₄ into C₂H₆; the correction is to change only the coefficient in front of the formula.
- Balancing carbon but forgetting hydrogen; the correction is to check every element, not just the first one.
- Writing a coefficient as a subscript, such as C₈H₁₈₂; the correction is to place the coefficient before the formula, for example 2C₈H₁₈.
- Saying cracking makes only alkenes; correction: cracking produces alkanes and alkenes.
- Giving no named product or use; correction: state a product such as ethene and its use in making poly(ethene).
- Confusing cracking with fractional distillation; correction: fractional distillation separates crude oil without changing molecules, while cracking breaks large molecules into smaller ones.
- Saying hydrocarbons are only fuels; correction: they are also feedstocks for plastics, medicines and many other products.
- Claiming crude oil is renewable; correction: crude oil is finite and will eventually run out.
- Listing products without linking them to a benefit; correction: explain how each product supports modern life, such as packaging preserving food.