AQA GCSE Chemistry Foundation (8462), Foundation tier · Organic chemistry › Carbon compounds as fuels and feedstock
Practise Fractional distillation and petrochemicals. 6 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How crude oil is separated into fractions by fractional distillation, and how the fractions are used as fuels and as feedstock for the petrochemical industry. Expect labelled diagrams of the column, 'explain how fractional distillation works' questions (often 4 to 6 marks) and questions on uses.
Key facts
Fraction = mixture of hydrocarbons with similar numbers of carbon atoms (similar boiling points)
Column: hot at the bottom, cool at the top
Evaporate, rise, then condense where the temperature is below the boiling point
Small molecules: low boiling points, collected at the top
Large molecules: high boiling points, collected at the bottom
Fuels: LPG, petrol, kerosene, diesel oil, heavy fuel oil
Feedstock = raw material used to make other chemicals
Notes
Fractional distillation
The hydrocarbons in crude oil have different boiling points, mainly because their molecules are different sizes.
Fractional distillation separates crude oil into fractions. Each fraction is a mixture of hydrocarbons with a similar number of carbon atoms, so they have similar boiling points.
1. The crude oil is heated until most of it evaporates (turns into vapour).
2. The vapour goes into the bottom of a fractionating column, which is hot at the bottom and cooler towards the top.
3. The vapours rise up the column. Each hydrocarbon condenses (turns back into a liquid) when it reaches a level where the temperature is below its boiling point.
4. The liquids are collected at different heights. Hydrocarbons with very low boiling points stay as gases and leave at the top; those with the highest boiling points are collected at the bottom.
It is a physical process: the molecules are separated, but none of them are broken up.
Where each fraction comes out
diagram
Near the top (cooler): small molecules with low boiling points. LPG leaves the very top as a gas; petrol condenses just below.
Lower down (hotter): larger molecules with higher boiling points, such as diesel oil and heavy fuel oil.
Order from top to bottom: LPG, petrol, kerosene, diesel oil, heavy fuel oil.
Each hydrocarbon condenses at the level where the temperature is just below its boiling point.
Going down the column, the molecules get bigger and the boiling points get higher.
Fuels and feedstock
The fractions are processed to make fuels and feedstock (raw materials) for the petrochemical industry.
Many fuels that modern life depends on come from crude oil: petrol (cars), diesel oil (cars, lorries and trains), kerosene (aircraft), heavy fuel oil (ships and power stations) and LPG (heating and cooking).
The petrochemical industry makes many useful materials: solvents, lubricants, polymers and detergents.
There is a vast range of natural and synthetic carbon compounds because carbon atoms can form families of similar compounds: carbon atoms bond to each other to make chains (and rings) of many different lengths.
How to answer each type of question
Recall: fuels and petrochemical products
1 to 2 marksGrade 3
Use the names in the specification: petrol, diesel oil, kerosene, heavy fuel oil, LPG.
For petrochemical products, choose from solvents, lubricants, polymers and detergents.
Give only as many answers as the question asks for.
Example. Give two useful materials, other than fuels, that are made by the petrochemical industry.
Show the model answerHide the model answer
Any two from: solvents (1), lubricants (1), polymers (1), detergents (1). Maximum 2 marks.
Use data to place a fraction or a hydrocarbon
1 to 2 marksGrade 5
Read the boiling range or the number of carbon atoms from the table.
A higher boiling point means lower down the column, where it is hotter.
Match a given boiling point to the range it falls inside.
Example. The table shows the boiling ranges of three fractions.
Fraction
Boiling range in °C
X
40 to 180
Y
180 to 260
Z
260 to 350
(a) Which fraction is collected highest up the column? Give a reason. (b) A hydrocarbon boils at 230 °C. Which fraction is it in?Show the model answerHide the model answer
(a) X (1). It has the lowest boiling points, and the top of the column is the coolest part (1). (b) Y (1)
Don’t lose marks
Saying the column is hottest at the top. It is hottest at the bottom, where the hot vapour enters.
Saying fractions 'evaporate at different heights'. They condense at different heights.
Saying fractional distillation breaks molecules into smaller ones. It only separates them; breaking molecules is cracking.
Calling a fraction a pure substance. Each fraction is still a mixture of hydrocarbons.
More tips
Memory tricks
Hot at the bottom, cool at the top: small molecules make it to the top before they condense.
Fuels from top to bottom: 'Lazy Pupils Keep Doing Homework' = LPG, petrol, kerosene, diesel oil, heavy fuel oil.
Evaporate on the way in, condense on the way out. Every explanation needs both words.
Size, boiling point, height: bigger molecule, higher boiling point, lower down the column.
Exam technique
In 6-mark questions, describe the process in order: heat, evaporate, enter the column, temperature gradient, rise, condense, collect. A logical sequence is part of the level.
Link three things for full marks: molecule size, boiling point and height in the column.
If the diagram labels the fractions with letters, refer to those letters in your answer.
Learn one use for each fuel. 'Used as a fuel' on its own rarely scores.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Name fuels produced from crude oilPetrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases (LPG).
Grade 3
Name materials made by the petrochemical industrySolvents, lubricants, polymers and detergents.
Grade 4
Define a fractionA mixture of hydrocarbons with a similar number of carbon atoms, and so similar boiling points.
Grade 5
Describe the temperature gradient in the columnThe fractionating column is hottest at the bottom and coolest at the top.
Grade 5
Explain what feedstock meansA raw material used to make other chemicals; fractions are feedstock for the petrochemical industry.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
The fractions from crude oil are processed to produce fuels and feedstock for the petrochemical industry. What is meant by feedstock?
A raw material used to make other chemicals.
Where in the fractionating column is the fraction with the lowest boiling points collected?
At the top
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy2 marks
(a) Which property mainly determines the height at which a hydrocarbon condenses inside a fractionating column? Tick (✓) one box.[1]
Its colour
Its boiling point
Its density as a solid
Its reactivity with bromine water
(b) Where in the fractionating column is the fraction with the lowest boiling points collected?[1]
Show the answer and mark scheme
(a)Answer: Its boiling point
(b)Answer: At the top
at the top (of the column)
Question 2Medium6 marks
The fractions from crude oil are processed to produce fuels and feedstock for the petrochemical industry.
(a) What is meant by feedstock?[1]
(b) Explain why there is a vast array of different carbon compounds.[2]
(c) Explain how modern life depends on the products from crude oil. Give examples in your answer.[3]
Show the answer and mark scheme
(a)Answer: A raw material used to make other chemicals.
a raw material used to make other chemicals / products
(b)Answer: Carbon atoms can bond to other carbon atoms, forming chains of different lengths and families of similar compounds.
carbon atoms can form (covalent) bonds with other carbon atoms
to make chains (or rings) of different lengths / families of similar compounds (homologous series)
(c)Answer: Fuels for transport (petrol, diesel oil, kerosene) and heating; polymers made from petrochemicals; solvents, lubricants and detergents.
fuels for transport, e.g. petrol, diesel oil or kerosene for aircraft
fuels for heating, cooking or power, e.g. liquefied petroleum gases or heavy fuel oil