Fractional distillation and crackingAQA A-Level Chemistry: Revision notes
Section 1
Alkanes and petroleum
Alkanes are saturated hydrocarbons. They contain only carbon and hydrogen (hydrocarbons) and have only single C–C bonds (saturated). Their general formula is .
Petroleum (crude oil) is a mixture consisting mainly of alkane hydrocarbons of many different chain lengths. It must be separated before use. The separation is done by fractional distillation.
'Saturated' does not mean 'only single bonds'. It means only single carbon-carbon bonds and the maximum number of hydrogen atoms. Always say hydrocarbon as well when defining an alkane.
Section 2
Fractional distillation of crude oil
The crude oil is heated until it vaporises, and the vapour enters the fractionating column near the bottom. The column is hot at the bottom and cooler at the top, so there is a temperature gradient.
Vapours rise up the column. Each hydrocarbon condenses when it reaches a part of the column that is cooler than its boiling point. It is then drawn off.
The boiling point depends on chain length. Longer molecules have more electrons and a larger surface area, so the London (dispersion) forces between them are stronger and more energy is needed to separate them. So long-chain molecules condense low in the column and short-chain molecules rise to the top.
Each fraction is a mixture of hydrocarbons with similar chain lengths and a similar boiling range. From top to bottom: refinery gas, petrol, kerosene, diesel, fuel oil, then bitumen or residue.
Boiling points of alkanes depend on the strength of London forces between molecules. Covalent bonds within the molecules are not broken when a hydrocarbon boils.
Section 3
Cracking: what and why
Cracking involves breaking C–C bonds in long-chain alkanes to make shorter, more useful alkanes and alkenes. For example:
C₁₄H₃₀ → C₈H₁₈ + 3C₂H₄
Always check that the carbon and hydrogen atoms balance. Cracking is needed because of economics:
- Demand for shorter-chain alkanes (petrol, motor fuels) is greater than the supply from fractional distillation.
- Long-chain fractions are in surplus and are less valuable.
- Cracking converts the surplus into valuable products, including alkenes that are the feedstock for polymers and other chemicals.
For 'explain the economic reasons for cracking', mention both supply and demand for the short-chain fractions and the value of the alkenes produced.
Section 4
Thermal and catalytic cracking
Thermal cracking takes place at high pressure and high temperature and produces a high percentage of alkenes. The mechanism is not required.
Catalytic cracking takes place at a slight pressure, a high temperature and in the presence of a zeolite catalyst. It is used mainly to produce motor fuels and aromatic hydrocarbons. The mechanism is not required.
Catalytic cracking needs only a slight pressure, so it uses less energy and costs less to run than high-pressure thermal cracking, and its products improve the quality of petrol.
Do not say catalytic cracking uses high pressure. It needs only a slight pressure; that is a main difference from thermal cracking.
Must know
- Alkanes are saturated hydrocarbons; petroleum is mainly alkanes.
- Fractional distillation separates by boiling point; longer chains have stronger London forces.
- Cracking breaks C–C bonds to make shorter alkanes and alkenes.
- Thermal: high pressure, high temperature, high percentage of alkenes.
- Catalytic: slight pressure, high temperature, zeolite catalyst; motor fuels and aromatic hydrocarbons.
- Economic reasons: demand for short chains, surplus of long chains, alkenes are valuable.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Fractional distillation and cracking
- A refinery separates crude oil in a fractionating column. The fractions collected, in no particular order, are refinery gas, petrol, kerosene, diesel and fuel oil. Each fraction is a mixture of alkanes.Explain why alkanes are described as saturated hydrocarbons.2 marks
- Tetradecane, C₁₄H₃₀, is a long-chain alkane that is cracked in an oil refinery. Cracking breaks C–C bonds in long-chain alkanes to form shorter, more useful molecules.Write an equation for the cracking of tetradecane to form octane and ethene as the only products.2 marks
- An oil refinery has a large surplus of long-chain hydrocarbons from fractional distillation. At the same time, demand for petrol and for alkenes used in the chemical industry is greater than the amounts the distillation alone provides.Explain the economic reasons for cracking long-chain alkanes.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).