Fractional distillation and petrochemicals — AQA GCSE Combined Science
Test yourself on Fractional distillation and petrochemicals with AQA GCSE practice questions.
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Fractional distillation and petrochemicals explained
Crude oil is a mixture of many hydrocarbons, compounds containing only carbon and hydrogen.
Read the full explanation
Because these molecules have different numbers of carbon atoms, they have different boiling points. Fractional distillation separates the mixture into fractions, each containing molecules with a similar number of carbon atoms. The crude oil is heated and enters a tall fractionating column as a vapour. The column is hotter at the bottom and cooler at the top. Hydrocarbons with small molecules, low boiling points and weak intermolecular forces rise far up the column before condensing. Larger molecules with higher boiling points condense lower down. Each fraction is collected and used as a fuel or as feedstock.
The fractions can be processed to produce fuels and feedstock for the petrochemical industry.
Fractions obtained from crude oil are not wasted; they are processed to make useful products. Many fractions are used directly as fuels, such as petrol for cars, diesel for lorries and kerosene for aircraft. Other fractions are used as feedstock, meaning they are starting materials for the petrochemical industry. Feedstock molecules can be cracked to produce smaller, more reactive alkenes such as ethene. These alkenes are then used to make polymers, solvents, lubricants and detergents. Processing therefore links the separation of crude oil to the manufacture of everyday materials, and it also helps match supply to demand because some fractions are more useful than others.
Many of the fuels on which we depend for our modern lifestyle, such as petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases, are produced from crude oil.
Crude oil is a finite mixture of hydrocarbons. It is separated by fractional distillation because the fractions have different boiling points. The column is hot at the bottom and cooler at the top. Each fraction condenses at a different height and is tapped off. Petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases are examples of these fractions. They are fuels for cars, lorries, aircraft, ships and heating. Smaller molecules have lower boiling points, are more volatile, flow more easily and burn more cleanly. Larger molecules have higher boiling points, are more viscous and burn with a smoky flame. This explains why different fractions suit different uses.
Many useful materials on which modern life depends are produced by the petrochemical industry, such as solvents, lubricants, polymers, detergents.
The petrochemical industry uses fractions from crude oil as feedstocks. These hydrocarbons are cracked or chemically processed to make new substances. Solvents dissolve other substances, for example in paints and dry cleaning. Lubricants reduce friction between moving parts, such as engine oil. Polymers are large molecules made from small monomer units, used in plastics and fibres. Detergents help water remove grease and dirt, for example in washing-up liquid. Many of these materials are not fuels; they are useful products on which modern life depends. The industry also produces medicines, cosmetics and fertilisers, showing the wide importance of petrochemicals.
The vast array of natural and synthetic carbon compounds occur due to the ability of carbon atoms to form families of similar compounds.
Carbon has four electrons in its outer shell, so each carbon atom can form four covalent bonds. This lets carbon atoms bond to one another in chains, branches and rings, and also bond to hydrogen, oxygen, nitrogen and halogens. Because the same small set of bonding patterns repeats, carbon compounds fall into families called homologous series, such as alkanes (CₙH₂ₙ₊₂), alkenes (CₙH₂ₙ), alcohols (CₙH₂ₙ₊₁OH) and carboxylic acids (CₙH₂ₙ₊₁COOH). Natural compounds come from living things, for example the hydrocarbons in crude oil and the carbohydrates in plants; synthetic compounds are made by humans, for example plastics such as poly(ethene) and medicines such as aspirin. Millions of different compounds are possible because the chain length, branching and functional group can all vary.
Students should be able to explain how fractional distillation works in terms of evaporation and condensation.
Crude oil is a mixture of hydrocarbons with different boiling points. Fractional distillation separates them in a fractionating column that is hot at the bottom and cooler at the top. The crude oil is heated until most of it evaporates. Vapour rises up the column and cools as it goes. A hydrocarbon condenses when the temperature falls below its boiling point, so it leaves the column as a liquid at that level. Hydrocarbons with low boiling points, such as short-chain alkanes, stay as gases and rise to the top, where they are collected. Hydrocarbons with high boiling points condense near the bottom. Each collected group is a fraction, not a pure substance, and the fractions have different uses such as petrol, diesel and bitumen.
Your focus
- Describe how crude oil is separated into fractions by fractional distillation.
- Explain why fractions condense at different heights in the fractionating column.
- Relate the number of carbon atoms in a hydrocarbon molecule to its boiling point.
Show all 18 objectives
- Describe how fractions from crude oil are processed into fuels and feedstock.
- Explain the role of cracking in producing useful alkenes for the petrochemical industry.
- Give examples of products made from fractions and from cracked hydrocarbons.
- Name the main fractions obtained from crude oil, including petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases.
- Describe how fractional distillation separates crude oil into fractions with different boiling point ranges.
- Explain how the size of hydrocarbon molecules affects boiling point, volatility, viscosity and ease of burning, and link this to the use of each fraction as a fuel.
- Identify solvents, lubricants, polymers and detergents as useful materials produced by the petrochemical industry.
- Describe the function of each type of petrochemical product and give a common example of its use.
- Explain that crude oil fractions are used as feedstocks to make many materials on which modern life depends, not only fuels.
- Describe how carbon's four covalent bonds allow chains, branches and rings to form.
- Identify a homologous series from its general formula and a named member.
- Classify given carbon compounds as natural or synthetic and justify the classification.
- Describe the sequence of evaporation, rising, cooling and condensation in fractional distillation.
- Relate the boiling point of a hydrocarbon to where it is collected in the column.
- Explain why each fraction is a mixture rather than a pure substance.
Fractional distillation and petrochemicals exam tips
Marking Points
- Crude oil is a mixture of many hydrocarbons, which are compounds of carbon and hydrogen only.
- Fractional distillation separates the mixture into fractions, each containing molecules with a similar number of carbon atoms.
- The fractionating column has a temperature gradient: hot at the bottom and cooler towards the top.
- Hydrocarbons with smaller molecules have lower boiling points and condense higher up the column.
- Hydrocarbons with larger molecules have higher boiling points and condense lower down the column.
- Each fraction is collected separately and can be used as a fuel or as feedstock for the petrochemical industry.
- Fractions from crude oil can be used directly as fuels, for example petrol, diesel and kerosene.
- Fractions can also be used as feedstock, which means starting materials for the petrochemical industry.
- Cracking converts larger hydrocarbon molecules into smaller, more useful molecules, including alkenes.
- Alkenes produced by cracking are used to make polymers, solvents, lubricants and detergents.
- Processing helps match the supply of fractions to demand for particular fuels and chemicals.
- Crude oil is a mixture of hydrocarbons that must be separated into fractions with different boiling point ranges.
- Fractional distillation uses a fractionating column with a temperature gradient: hot at the bottom, cooler at the top.
- Each fraction condenses at a different height because the hydrocarbons in it have a specific boiling point range.
- Petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases are named fractions obtained from crude oil.
- The fractions are used as fuels because they release energy when burned, supporting transport, heating and industry.
- Smaller hydrocarbon molecules have lower boiling points, are more volatile and ignite more easily than larger molecules.
- The petrochemical industry uses hydrocarbons from crude oil as raw materials or feedstocks.
- Solvents are useful petrochemical products that dissolve other substances, for example in paints or cleaning fluids.
- Lubricants are petrochemical products that reduce friction between moving surfaces, such as engine oils and greases.
- Polymers are large molecules made by joining many small monomer units, and are used to make plastics and synthetic fibres.
- Detergents are petrochemical products that help water to remove grease and dirt from surfaces.
- Many useful materials for modern life, including medicines, cosmetics and fertilisers, also come from the petrochemical industry.
- Carbon atoms each form four covalent bonds, which allows carbon to bond to other carbon atoms and to elements such as hydrogen, oxygen, nitrogen and chlorine.
- Carbon atoms can join into chains of different lengths, and the chains may be straight, branched or arranged in rings.
- A homologous series is a family of compounds with the same general formula and similar chemical properties, in which each member differs from the next by a CH₂ unit.
- Natural carbon compounds are obtained from living organisms or geological sources, for example hydrocarbons in crude oil and carbohydrates in plants.
- Synthetic carbon compounds are manufactured by humans, for example poly(ethene) from ethene and aspirin from salicylic acid.
- The variety of possible chain lengths, branching patterns and functional groups explains why so many different carbon compounds exist.
- Crude oil is heated so that the hydrocarbons evaporate and rise up the fractionating column.
- The column has a temperature gradient, being hotter at the bottom and cooler at the top.
- As vapour rises it cools; a hydrocarbon condenses when the temperature drops below its boiling point.
- Hydrocarbons with low boiling points travel further up the column and are collected near the top, often as gases.
- Hydrocarbons with high boiling points condense lower down the column and are collected near the bottom.
- Each fraction is a mixture of hydrocarbons with a similar boiling point range, not a single pure compound.
Examiner Tips
- 💡Link boiling point to molecular size: more carbon atoms means stronger intermolecular forces and a higher boiling point.
- 💡Describe the process in order: vaporisation, rising through the column, condensation at different heights, collection.
- 💡Use the phrase similar number of carbon atoms when defining a fraction, rather than identical molecules.
- 💡Give named examples of fuels from fractions, such as petrol, diesel or kerosene, to support your answer.
- 💡Define feedstock clearly as a starting material for the petrochemical industry.
- 💡Link cracking to the production of alkenes such as ethene, then link alkenes to polymers.
- 💡Link each named fuel to its use, for example kerosene for aircraft fuel and heavy fuel oil for ships, to show understanding of why the fraction is suitable.
- 💡When describing the column, state clearly that it is hot at the bottom and cooler at the top, and that fractions condense at different heights.
- 💡Use comparative language such as lower boiling point, more volatile and more viscous to explain trends rather than simply listing fractions.
- 💡Give a named example for each product type, such as ethanol as a solvent or poly(ethene) as a polymer, to make your answer specific.
- 💡Link each product to its function, for example lubricants reduce friction and detergents help remove grease, rather than just naming the product.
- 💡Use the term feedstock when explaining that crude oil fractions are starting materials for making new substances in the petrochemical industry.
- 💡When asked why carbon forms so many compounds, link the answer to carbon's four covalent bonds and its ability to form chains, branches and rings.
- 💡Give one named natural example and one named synthetic example, and state clearly which is which.
- 💡If a general formula is given, check it against a named member, for example C₃H₈ fits CₙH₂ₙ₊₂ for an alkane.
- 💡Use the words evaporate, rise, cool and condense in the correct order when describing the process.
- 💡Link boiling point to position in the column: lower boiling point means collected higher up.
- 💡State that a fraction is a mixture, and give one use for a named fraction to show understanding.
Common Mistakes
- Thinking a fraction is a single pure compound; correct this by stating that a fraction is a mixture of hydrocarbons with a similar number of carbon atoms.
- Believing that fractional distillation breaks large molecules into smaller ones; correct this by explaining that it is a physical separation by boiling point, not a chemical reaction.
- Confusing the temperature gradient, for example saying the top is hottest; correct this by stating that the bottom is hottest and the top is coolest.
- Confusing fuel and feedstock; correct this by stating that fuels are burned for energy while feedstock is a raw material for making other chemicals.
- Thinking all fractions are burned as fuels; correct this by explaining that some fractions are processed into petrochemical products.
- Believing cracking is a separation method; correct this by stating that cracking is a chemical reaction that breaks larger molecules into smaller ones.
- Thinking crude oil is a single compound rather than a mixture. Correction: crude oil is a mixture of many hydrocarbons with different boiling points.
- Believing fractional distillation chemically changes the hydrocarbons. Correction: it is a physical separation process; the molecules themselves are unchanged.
- Confusing the order of fractions in the column. Correction: smaller molecules with lower boiling points leave near the top, while larger molecules with higher boiling points leave near the bottom.
- Thinking all crude oil products are fuels. Correction: many fractions are used as feedstocks to make non-fuel materials such as solvents, lubricants, polymers and detergents.
- Confusing polymers with the small molecules they are made from. Correction: polymers are large molecules built from many small monomer units joined together.
- Assuming detergents work by dissolving grease in water alone. Correction: detergents help water to remove grease and dirt by interacting with both water and grease.
- Thinking carbon forms only one bond: carbon has four outer electrons, so it forms four covalent bonds, as in methane CH₄.
- Confusing natural with synthetic: natural compounds are produced by living things or found in nature, whereas synthetic compounds are made by humans, even if they copy a natural substance.
- Assuming a homologous series contains identical compounds: members share a general formula and similar reactions but have different numbers of carbon atoms, so their physical properties change gradually.
- Saying hydrocarbons melt in the column: the change is condensation from vapour to liquid, not melting from solid to liquid.
- Thinking all fractions have the same boiling point: a fraction is a mixture with a boiling point range, and different fractions have different ranges.
- Reversing the temperature trend: the column is hottest at the bottom and coolest at the top, so low-boiling hydrocarbons rise furthest.