Fractional distillation and petrochemicals — AQA GCSE Chemistry
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Fractional distillation and petrochemicals explained
Fractional distillation and petrochemicals covers 6 specification points, including The many hydrocarbons in crude oil may be… and The fractions can be processed to produce….
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Open a point below for its explanation and revision guidance.
The many hydrocarbons in crude oil may be separated into fractions, each of which contains molecules with a similar number of carbon atoms, by fractional distillation.
Crude oil is a mixture of many hydrocarbons, compounds of carbon and hydrogen only. Because the molecules differ in size, they have different boiling points, so the mixture can be separated by fractional distillation. The crude oil is heated until most of it vaporises, and the vapour enters a tall fractionating column that is hot at the bottom and cooler at the top. As the vapour rises it cools; molecules with many carbon atoms condense at lower, hotter levels and are tapped off, while smaller molecules travel higher before condensing. Each collected fraction is a mixture of hydrocarbons with a similar number of carbon atoms and therefore a similar boiling point range, not a single pure compound.
The fractions can be processed to produce fuels and feedstock for the petrochemical industry.
The fractions obtained from crude oil are not wasted; they are processed in several ways to make useful products. Some fractions are used directly as fuels, for example petrol for cars and diesel oil for lorries. Other fractions are used as feedstock, meaning they are the starting materials for the petrochemical industry. Feedstock fractions can be cracked to produce smaller, more useful molecules, including alkenes, which are then used to make polymers and other chemicals. Processing therefore links the separation of crude oil to the manufacture of fuels and to a wide range of petrochemical products, showing that crude oil is a valuable raw material rather than just a fuel source.
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.
Modern transport, heating and industry rely heavily on fuels obtained from crude oil. Fractional distillation separates crude oil into fractions, and several of these fractions are used directly as fuels. Petrol is used in cars, diesel oil in lorries and some cars, kerosene in aircraft, heavy fuel oil in ships and power stations, and liquefied petroleum gases for heating and cooking. Each fuel is a mixture of hydrocarbons with a similar number of carbon atoms, which gives it suitable properties such as a particular boiling point range and how easily it burns. This dependence means crude oil is a finite resource, so alternative energy sources and conservation are important considerations.
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, especially feedstock obtained by fractional distillation and cracking, to manufacture a wide range of materials. Solvents dissolve other substances and are used in paints and cleaning products. Lubricants reduce friction between moving parts in engines and machines. Polymers are large molecules made from small alkene monomers and are used in plastics, fibres and packaging. Detergents help water to remove grease and dirt and are used in cleaning products. These materials are essential to modern life, and their production shows how crude oil is a source of chemicals as well as fuels.
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 it forms four covalent bonds and can bond to other carbon atoms in chains, branches and rings. This catenation, combined with bonding to hydrogen, oxygen, nitrogen and halogens, produces millions of natural compounds such as carbohydrates and proteins, and synthetic compounds such as plastics, medicines and fuels. Because carbon consistently forms four bonds, compounds group into homologous series: families with the same general formula and functional group, in which each member differs by one CH₂ unit, for example alkanes (CₙH₂ₙ₊₂), alkenes (CₙH₂ₙ) and alcohols (CₙH₂ₙ₊₁OH). Similar bonding therefore explains both the huge number of carbon compounds and the family relationships between them.
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. It is heated until most fractions evaporate, and the vapours enter a fractionating column that is hot at the bottom and cooler towards the top. As a vapour rises it cools; when it reaches a region below its boiling point it condenses and is tapped off as a liquid. Fractions with low boiling points, made of small molecules, rise far up the column before condensing, while fractions with high boiling points condense near the bottom. Each fraction is a mixture of hydrocarbons with a similar boiling-point range, not a single pure compound. The process separates the mixture because evaporation and condensation occur repeatedly at different temperatures.
Your focus
- Describe how crude oil is separated into fractions by fractional distillation.
- Explain how the number of carbon atoms in a hydrocarbon molecule affects its boiling point and the height at which it is collected.
- State that each fraction is a mixture of hydrocarbons with a similar number of carbon atoms.
Show all 18 objectives
- State that fractions from crude oil can be processed into fuels and feedstock.
- Describe cracking as a process that produces smaller molecules, including alkenes, from larger hydrocarbon fractions.
- Explain how processed fractions are used by the petrochemical industry.
- Name the main fuels produced from crude oil, including petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases.
- Describe how these fuels are obtained from crude oil by fractional distillation.
- Relate the properties of each fraction to its use as a fuel.
- Name examples of useful materials produced by the petrochemical industry, including solvents, lubricants, polymers and detergents.
- Describe the function of each type of petrochemical material.
- Explain that these materials are made from crude-oil-derived fractions and feedstock.
- State that carbon forms four covalent bonds and can bond to other carbon atoms.
- Describe how chains, branches, rings and bonding to other elements create a vast range of carbon compounds.
- Identify members of a homologous series from a shared general formula and functional group.
- Describe the apparatus and temperature gradient of a fractionating column.
- Explain how evaporation and condensation separate crude oil into fractions.
- Relate the boiling point of a fraction to where it condenses in the column.
Fractional distillation and petrochemicals exam tips
Marking Points
- Crude oil is a mixture of many different hydrocarbons, so it has a range of boiling points rather than one sharp boiling point.
- 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 near the bottom and cooler towards the top.
- Hydrocarbons with larger molecules have higher boiling points and condense lower down the column.
- Hydrocarbons with smaller molecules have lower boiling points and rise further before condensing.
- Each fraction is still a mixture of hydrocarbons with similar boiling points, not a pure substance.
- Fractions from crude oil can be used directly as fuels, such as petrol, diesel oil 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 and other petrochemical products.
- Processing fractions increases their usefulness because supply of fractions does not always match demand.
- Petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases are all produced from crude oil.
- These fuels are fractions obtained by fractional distillation of crude oil.
- Each fuel is a mixture of hydrocarbons with a similar number of carbon atoms and a similar boiling point range.
- Different fuels suit different uses, for example petrol for cars, kerosene for aircraft and heavy fuel oil for ships.
- Crude oil is a finite resource, so reliance on these fuels has economic and environmental implications.
- The petrochemical industry produces useful materials from crude oil fractions and feedstock.
- Solvents are substances used to dissolve other substances, for example in paints and cleaning products.
- Lubricants reduce friction between moving surfaces, for example in engines and machinery.
- Polymers are large molecules made by joining many small alkene monomers together.
- Detergents help water to remove grease and dirt and are used in cleaning products.
- These materials are important to modern life and are made from crude-oil-derived chemicals.
- Carbon atoms have four outer-shell electrons and form four covalent bonds, giving each carbon atom a full outer shell in stable molecules.
- Carbon atoms bond to one another, forming chains, branched chains and rings of varying length, which generates structural variety.
- Carbon also bonds to other elements such as hydrogen, oxygen, nitrogen and halogens, adding further variety to natural and synthetic compounds.
- Compounds are grouped into homologous series, families with the same general formula and functional group, whose members differ by one CH₂ unit.
- Examples include alkanes with general formula CₙH₂ₙ₊₂ and alkenes with general formula CₙH₂ₙ.
- Natural examples include carbohydrates and proteins; synthetic examples include plastics and medicines, showing the range of carbon chemistry.
- Crude oil is heated so that the hydrocarbons evaporate and rise as vapours into the fractionating column.
- The column has a temperature gradient, hottest at the bottom and coolest at the top.
- A vapour condenses when it reaches a level where the temperature is below its boiling point, and the liquid fraction is collected.
- Hydrocarbons with low boiling points, typically small molecules, travel further up the column before condensing.
- Hydrocarbons with high boiling points condense nearer the bottom of the column.
- Each collected fraction is a mixture of hydrocarbons with a similar boiling-point range rather than a pure substance.
Examiner Tips
- 💡Link molecular size to boiling point and then to the height at which a fraction is collected, so your explanation forms a clear chain of reasoning.
- 💡Use the term fraction precisely: a group of hydrocarbons with a similar number of carbon atoms and a similar boiling point range.
- 💡When describing the column, state the temperature gradient first, then explain how rising vapour cools and condenses at different heights.
- 💡Use the word feedstock correctly and explain it as a starting material for the petrochemical industry.
- 💡When asked about processing, mention cracking as a key example that turns larger fractions into smaller, more useful molecules.
- 💡Link each processed product to its use, such as alkenes for polymers or hydrogen for other industrial processes.
- 💡Learn the main fractions and one typical use for each, so you can match fuel to purpose accurately.
- 💡When listing fuels, use the names given in the specification, such as petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases.
- 💡Link the use of a fuel to its properties, such as how easily it ignites or how cleanly it burns.
- 💡Give a clear example for each product type, such as a solvent in paint or a polymer in plastic packaging.
- 💡Link each petrochemical product to its function, so your answer shows understanding rather than just naming the product.
- 💡Remember that alkenes from cracking are the starting point for many polymers, so mention this link when relevant.
- 💡Link the four bonds of carbon explicitly to the idea of chains, rings and bonding to other elements when explaining variety.
- 💡When asked about families, name the homologous series and give its general formula rather than listing unrelated compounds.
- 💡Use one natural and one synthetic example to show the breadth of carbon compounds in extended answers.
- 💡Refer to the temperature gradient of the column and link it to where each fraction condenses.
- 💡Use the terms evaporate and condense explicitly, since the question asks for an explanation in terms of both processes.
- 💡Compare small and large molecules in one sentence to show the link between boiling point and position in the column.
Common Mistakes
- Saying fractional distillation separates crude oil into pure hydrocarbons; correct this by stating that each fraction is a mixture of hydrocarbons with a similar number of carbon atoms.
- Reversing the temperature gradient by saying the column is cool at the bottom and hot at the top; correct this by stating that the bottom is hottest and the top is coolest.
- Confusing condensation with boiling when explaining where a fraction leaves the column; correct this by saying larger molecules condense lower down and smaller molecules condense higher up.
- Saying fractions are burned only as fuels; correct this by also stating that fractions can be used as feedstock for the petrochemical industry.
- Confusing feedstock with fuel; correct this by explaining that feedstock is a starting material for making other chemicals, not necessarily something burned for energy.
- Thinking cracking adds carbon atoms to molecules; correct this by stating that cracking breaks larger molecules into smaller ones.
- Naming individual pure compounds as the fuels; correct this by stating that each fuel is a mixture of hydrocarbons with a similar boiling point range.
- Mixing up the uses of fractions, such as saying kerosene is used in cars; correct this by matching kerosene to aircraft fuel and petrol to cars.
- Forgetting that liquefied petroleum gases are also obtained from crude oil; correct this by including them in the list of crude-oil-derived fuels.
- Thinking polymers are found naturally in crude oil; correct this by stating that polymers are manufactured from alkene monomers produced by cracking.
- Confusing lubricants with fuels; correct this by explaining that lubricants reduce friction rather than burn to release energy.
- Describing detergents as substances that dissolve grease completely; correct this by saying detergents help water to remove grease and dirt.
- Thinking carbon forms ionic bonds in these compounds; the correction is that carbon forms covalent bonds by sharing electrons.
- Believing each member of a homologous series has a different functional group; the correction is that the functional group is the same and the chain length changes by one CH₂ unit.
- Assuming all carbon compounds are synthetic; the correction is that many, such as carbohydrates and proteins, occur naturally.
- Saying fractions are separated by boiling point alone; the correction is that both evaporation and condensation are needed, with condensation occurring at a level where the temperature falls below the boiling point.
- Stating that larger molecules rise to the top; the correction is that larger molecules have higher boiling points and condense lower down.
- Describing each fraction as a pure compound; the correction is that a fraction is a mixture of hydrocarbons with a similar boiling-point range.