Crude oil (AQA GCSE Chemistry): Revision Notes
Crude oil
What is crude oil?
Crude oil forms when sea creatures die and get buried under layers of mud for millions of years. Over this very long time, they slowly turn into the thick, dark liquid we call crude oil.
Crude oil is a mixture containing thousands of different compounds. Most of these are hydrocarbons - molecules made of only carbon and hydrogen atoms.
Where does crude oil come from?
Crude oil is found deep underground in rocks. We get it out by drilling holes down into these rocks. Once we pump it up, we can use it to make many useful products.
Fractional distillation
Crude oil by itself isn't very useful. We need to separate it into different parts called fractions. We do this using a process called fractional distillation.
How fractional distillation works
How the Fractionating Column Works:
Step 1: Crude oil gets heated until it becomes vapour
Step 2: The hot vapour rises up the column
Step 3: As it goes higher, it gets cooler
Step 4: Different compounds condense (turn back to liquid) at different heights
Step 5: Each fraction has molecules with similar numbers of carbon atoms
The process happens in a tall tower called a fractionating column.
The fractions we get
From top to bottom of the column, we collect:
- Liquefied petroleum gases (top - smallest molecules)
- Petrol
- Kerosene
- Diesel oil
- Heavy fuel oil (bottom - largest molecules)
Key Rule: Smaller molecules have lower boiling points and come out at the top. Larger molecules have higher boiling points and come out at the bottom.
Uses of crude oil products
Crude oil fractions have two main uses:
1. Fuels
Different fractions burn to release energy:
- Petrol - for cars
- Kerosene - for jet engines
- Diesel - for trucks and some cars
- Heavy fuel oil - for ships and power stations
2. Feedstock for making other products
Some fractions become raw materials for the petrochemical industry. This industry makes:
- Solvents - for cleaning and dissolving things
- Lubricants - oils and greases
- Polymers - plastics
- Detergents - washing powders and liquids
Why we need to conserve crude oil
Critical Fact: Crude oil is a finite resource. This means once we use it all up, there won't be any more. It took millions of years to form, so we can't make new supplies quickly.
That's why it's important to:
- Use crude oil wisely
- Recycle plastic products made from it
- Find alternative energy sources
Key facts about molecules and boiling points
Molecules with more carbon atoms have higher boiling points. For example: boils at -89°C, but boils at 316°C.
This difference in boiling points is what makes fractional distillation work.
Key Points to Remember:
- Crude oil formed from dead sea life over millions of years
- Fractional distillation separates crude oil into useful fractions
- Smaller molecules = lower boiling points = collected higher up the column
- Fractions are used as fuels or to make other products
- Crude oil is finite - once it's gone, it's gone forever