Alkanes: Fractional Distillation of Crude Oil

Question 1

What are alkanes?

Answer 1

• Alkanes are saturated hydrocarbons

• Saturated - all carbon-carbon bonds are single
• Hydrocarbon - compound containing only hydrogen and carbon atoms

Question 2

What is petroleum (crude oil)?

Answer 2

• Petroleum is a mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation

Question 3

What is the general formula of an alkane?

Answer 3

• CnH2n+2

Question 4

What happens during the fractional distillation of crude oil?

Answer 4

• Crude oil is vaporised at 350°C
• The vaporised crude oil goes into the bottom of the fractionating column and rises through the trays
• The largest hydrocarbons don’t vaporise, their boiling points are too high - they run to bottom
• As crude oil vapor rises, it gets cooler
• Fractions are then separated by the fact that they have different boiling points so they condense at different levels and are then removed
• Low boiling point fractions (smaller molecules - weaker Van der Waals forces) are collected at the top and higher boiling point fractions at the bottom
• The hydrocarbons with the lowest boiling points don’t condense - drawn off as gases at top of column

Question 5

Name the fractions from lowest boiling point to highest boiling point and their uses

Answer 5

• Gases - gaseous fuel
• Gasoline/petrol/naphtha - cars
• Kerosene - jet fuel, lighting
• Diesel oil - lorries/taxis
• Lubricating oil/waxes - candles, engine oil
• Fuel oil - ships, power stations
• Paraffin wax -
• Tar/bitumen - roads/roofing

Question 6

Name the fractions and the number of carbon atoms in their hydrocarbon chains

Answer 6

• Gases - 1-4
• Gasoline/petrol - 4-12
• Naphtha - 7-14
• Kerosene - 11-15
• Gas oil (diesel) - 15-19
• Lubricating oil - 20-30
• Fuel oil - 30-40
• Paraffin waxes - 40-50
• Bitumen - 50+

Question 7

Name the fractions and their boiling points

Answer 7

• Gases - Below 40
• Gasoline/petrol - 40
• Naphtha - 110
• Kerosene - 180
• Gas oil (diesel) - 250
• Lubricating oil - Above 350
• Fuel oil - Above 350
• Paraffin waxes - Above 350
• Bitumen - Above 350

Question 8

Outline the essential features of the fractional distillation of crude oil that enable the crude oil to be separated into fractions

Answer 8

• Boiling point depends on size of molecules
• Separation depends on boiling point
• Fractions with higher boiling points are collected lower down the column
• Fractionating column has to be hotter at bottom and cooler at top