7.2 Fractional distillation of crude oil
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Which fraction is collected at the top of the fractionating column?
Choose one answer.
Correct Not quite: the answer is Refinery gases
Mark scheme
A
Model answer
A: refinery gases have the shortest chains and lowest boiling points.
Describe how crude oil is separated into fractions in a fractionating column.
Mark scheme
- crude oil is heated / evaporated (1)
- column is hotter at the bottom and cooler at the top (1)
- vapours rise and cool (1)
- each fraction condenses at the level where the temperature is below its boiling point / is collected at different heights (1)
Model answer
The crude oil is heated until it evaporates, and the vapour enters the bottom of the column. The column is hot at the bottom and cool at the top. As vapours rise they cool, and each fraction condenses at the height where the temperature falls below its boiling point, where it is collected.
Explain why petrol is collected higher up the column than diesel oil.
Mark scheme
- petrol molecules have shorter chains / fewer carbon atoms (1)
- so lower boiling points, condensing at a cooler, higher level (1)
Model answer
Petrol has shorter molecules with lower boiling points, so it stays a vapour longer and condenses higher up where it is cooler.
Give the use of kerosene and of bitumen.
Mark scheme
- kerosene: aircraft fuel (1)
- bitumen: roads / roofs (1)
Model answer
Kerosene is used as aircraft fuel; bitumen is used to surface roads and roofs.
What is meant by a feedstock for the petrochemical industry?
Mark scheme
- a raw material used to make other useful substances, e.g. polymers, solvents (1)
Model answer
A raw material that is converted into other products such as polymers, solvents and detergents.
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