Edexcel IGCSE Chemistry · Spec 4.7-4.11
Crude Oil & Fractional Distillation
How crude oil is separated into useful fractions by fractional distillation.
Chemistry revision video
Crude Oil & Fractional Distillation
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Explained
Separating crude oil, and why boiling point does all the work
Crude oil is a mixture of hydrocarbons, mostly alkanes, with chains of many different lengths. It is a fossil fuel, formed over millions of years from the remains of tiny sea creatures buried under mud without oxygen.
As a mixture it is close to useless. Separated into groups of similar chain length, called fractions, it becomes the source of most fuels and most plastics.
Why the separation works
The molecules in crude oil are held to each other by intermolecular forces. Longer chains have more contact between molecules, so the forces between them are stronger, so more energy is needed to separate them. Longer chains therefore have higher boiling points.
That single trend is the whole basis of the process, and it explains the other properties too. Long chain fractions are more viscous, less volatile, less easily ignited and burn with a smokier flame. Short chain fractions are runny, evaporate readily and ignite easily.
Inside the column
The oil is heated until most of it vaporises, and the vapour enters the fractionating column near the bottom. The column is hot at the bottom and cooler at the top.
The vapour rises. Each fraction cools as it rises, and when it reaches the level where the temperature has fallen to its boiling point, it condenses and is drawn off there. Long chains have high boiling points, so they condense low down. Short chains rise furthest before they condense.
Working from the bottom up: bitumen, fuel oil, diesel, kerosene, petrol, and refinery gases at the very top.
The uses
Refinery gases for bottled gas, petrol for cars, kerosene for aircraft, diesel for lorries and some cars, fuel oil for ships and heating, and bitumen for surfacing roads. Knowing the order gives you the uses, because they run from lightest to heaviest.
What examiners say about this topic
A principal examiner report for Edexcel International GCSE Chemistry describes a question showing this process. Learners were highly successful at identifying it as fractional distillation, and the most common incorrect response was cracking. Knowledge of the process was good, with the majority knowing the correct order of the fractions and where kerosene is collected.
The diagrams were the weakness. The report records that the standard of the diagrams produced was generally quite poor, with some very unusual towers, a frequent lack of care about where the crude oil enters the column, and unclear entrances and exits drawn as single lines. Fewer learners indicated the temperature gradient on their diagrams.
So if you are asked to draw the column, put the crude oil in near the bottom, mark hot at the bottom and cool at the top, and label the outlets clearly. Those are the marks that were being lost.
Distillation does not change the molecules
Fractional distillation is a physical separation. No bonds are broken and no new substances are made; the molecules that come out are the ones that went in.
Cracking is different. It breaks long chain molecules into shorter, more useful ones and is a chemical change. Confusing the two is the error the examiners recorded, and the distinction between physical separation and chemical change is what keeps them apart.
Spec 4.7-4.11
What you need to know
- Say what crude oil is
- Describe fractional distillation
- Name the fractions and their uses
Active recall
Quick check
Answer each question before opening the answer.
How does fractional distillation separate crude oil?
The oil is heated and the vapours rise up a column, condensing at different heights according to their boiling points.
How do the fractions change as you go down the column (larger molecules)?
Higher boiling point, more viscous, less flammable and darker in colour.
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