🧪 International GCSE Chemistry (9202): Organic Chemistry
Obtaining Useful Substances from Crude Oil
Hello future Chemists! This chapter is incredibly important because it shows us how one single, black, sticky liquid—crude oil—is the foundation of most of our modern materials, fuels, and plastics. Don't worry if the names seem complicated; we are essentially learning how scientists use physical and chemical tricks to sort and restructure molecules!
By the end of these notes, you will understand:
- What crude oil is and why it's a limited resource.
- How we separate the useful parts using fractional distillation.
- How we chemically break down large molecules using cracking.
- How we build new materials using polymerization.
1. Crude Oil: The Basis of Organic Chemistry
1.1 Origin and Composition
Crude oil is a fossil fuel. It was formed millions of years ago from the remains of tiny sea creatures and plants buried under mud and rock. Intense heat and pressure converted this organic matter into the oily liquid we drill out of the ground today.
What is it made of?
Crude oil is a complex mixture of many different compounds, but almost all of them belong to one family: the hydrocarbons.
- A hydrocarbon is a molecule made up of only hydrogen (H) atoms and carbon (C) atoms.
- These molecules come in all different sizes, from very small ones (like methane, with 1 carbon atom) to very large ones (like bitumen, which has dozens of carbon atoms joined together).
1.2 Crude Oil as a Finite Resource
The term finite resource is crucial here. Crude oil takes millions of years to form, so we are using it up much faster than nature can replace it. This means:
It will eventually run out.
This is why scientists are constantly looking for alternative fuels (like hydrogen or biofuels) and working on processes like cracking to make sure we get the maximum usefulness out of the oil we still have.
Did you know? Crude oil is often nicknamed "Black Gold" because of its immense economic value and importance to industry.
Crude oil is a mixture of hydrocarbons. It is a finite resource, meaning its supply is limited and non-renewable over human timescales.
2. Fractional Distillation: Sorting the Mixture
Because crude oil is a mixture of many different substances (hydrocarbons), we can't use it efficiently in its crude state. We need to separate it into groups of molecules that have similar properties. These separated groups are called fractions.
2.1 The Principle of Separation
The different hydrocarbons in crude oil have different boiling points. The larger the molecule, the stronger the forces holding them together, and the higher the temperature needed to turn them into a gas.
- Small molecules = Low boiling point (turn into gas easily).
- Large molecules = High boiling point (harder to turn into gas).
2.2 The Process Step-by-Step
Separation happens in a giant structure called a fractionating column or distillation tower:
- Heating: Crude oil is heated to a very high temperature (around 350°C) until most of it turns into hot vapour (gas).
- Entering the Column: This hot vapour is pumped into the bottom of the fractionating column.
- Rising and Cooling: The column is hottest at the bottom and gradually cooler towards the top. As the vapour rises, it starts to cool down.
- Condensing: When a specific hydrocarbon vapour reaches the temperature equal to its boiling point, it condenses (turns back into a liquid).
- Collecting the Fractions: The different liquids are collected on trays at different levels.
Analogy: Think of fractional distillation like a multi-story car park where the temperature decreases the higher up you go. Small, speedy cars (small molecules with low BP) can make it all the way to the top floor. Big, slow trucks (large molecules with high BP) condense and stop moving right near the bottom entrance.
3. Properties and Uses of Fractions
It is vital to understand the trends (how properties change) as we move from the small fractions at the top to the large fractions at the bottom of the column.
3.1 Trends in Properties
We can summarize the property changes based on the length of the carbon chain:
| Property | Short Chains (Top of Column) | Long Chains (Bottom of Column) |
|---|---|---|
| Boiling Point | Low | High |
| Viscosity (How runny it is) | Low (Very runny, like gas/petrol) | High (Very thick, like syrup/tar) |
| Volatility (How easily it evaporates) | High | Low |
| Flammability (How easily it catches fire) | High (Very flammable) | Low (Difficult to ignite) |
Memory Trick: Think of the large molecules at the bottom. They are BIG, THICK, and SLUGGISH (high BP, low flammability).
3.2 Main Fractions and Their Uses
Starting from the top (lowest boiling point) to the bottom (highest boiling point):
- Refinery Gases: (Very short chains, e.g., propane, butane). Used as bottled gas for heating and cooking (LPG).
- Gasoline (Petrol): Fuel for cars.
- Naphtha: Used to make chemicals for plastics (petrochemicals).
- Kerosene (Paraffin): Jet fuel, heating oil.
- Diesel/Gas Oil: Fuel for diesel engines, lorries, and trains.
- Fuel Oil: Fuel for ships and power stations.
- Lubricating Oils & Waxes: Used for machine oil, greases, and candles.
- Bitumen: (Longest chains, high BP). Used for road surfacing (tarmac) and roofing.
4. Combustion of Hydrocarbons
The main reason we use crude oil fractions is for energy. Burning hydrocarbons releases heat, which can be harnessed to power vehicles and electricity generators.
4.1 Complete Combustion
When there is plenty of oxygen available, hydrocarbons burn completely. The only products are carbon dioxide and water:
$$ \text{Hydrocarbon} + \text{Oxygen} \rightarrow \text{Carbon Dioxide} + \text{Water} $$ Example (Methane): $$ CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O $$
4.2 Incomplete Combustion
When oxygen supply is limited (poor ventilation, badly maintained engine), the combustion is incomplete. This produces dangerous products:
$$ \text{Hydrocarbon} + \text{Limited Oxygen} \rightarrow \mathbf{\text{Carbon Monoxide}} + \mathbf{\text{Carbon}} + \text{Water} $$
The Danger: Carbon Monoxide (CO) is a highly toxic, colourless, and odourless gas. It prevents blood from carrying oxygen effectively, which can be fatal. The fine particles of unburnt carbon (soot) cause respiratory problems and dark smoke.
5. Cracking: Breaking Down the Big Guys
When crude oil is distilled, we get too much of the heavy, long-chain fractions (like fuel oil) and not enough of the highly demanded, short-chain fractions (like petrol/gasoline). This is where cracking comes in.
5.1 What is Cracking?
Cracking is a process that breaks down large, less useful hydrocarbon molecules into smaller, more useful molecules.
The products of cracking are extremely important:
- They include smaller alkanes (fuels like petrol).
- They include alkenes (molecules with a C=C double bond), which are highly reactive and essential for making plastics.
General Equation for Cracking: $$ \text{Long Alkane} \rightarrow \text{Shorter Alkane} + \text{Alkene} (+ \text{other small molecules}) $$
Example: A large molecule of \(C_{10}H_{22}\) might crack into a useful fuel like butane (\(C_4H_{10}\)) and ethene (\(C_2H_4\)) and propene (\(C_3H_6\)).
Crucial Point: Alkenes (\(C_nH_{2n}\)) are unsaturated because they contain a carbon-carbon double bond (\(C=C\)). This double bond makes them much more reactive than alkanes.
5.2 Conditions for Cracking
Cracking can be done in two main ways, both requiring significant energy:
- Thermal Cracking: Requires very high temperatures (up to 1000°C) and high pressure. This mainly produces alkenes.
- Catalytic Cracking: Uses a lower temperature (around 500°C) but requires a catalyst (often zeolite). This mainly produces hydrocarbons suitable for petrol (gasoline).
Do not confuse fractional distillation and cracking. Distillation is a physical separation process. Cracking is a chemical reaction (breaking bonds).
6. Addition Polymerization
Alkenes produced by cracking are the starting materials for manufacturing plastics through a process called polymerization.
6.1 Monomers and Polymers
The term polymer means 'many units'. It is a very long molecule made up of many identical small molecules joined together.
- The small, reactive starting molecules are called monomers (meaning 'one unit').
- The large product molecule is called the polymer.
Analogy: A monomer is like a single paperclip. A polymer is like a long chain made by linking thousands of paperclips together.
6.2 Addition Polymerization Process
Alkenes undergo a reaction called addition polymerization because of their reactive \(C=C\) double bond.
During the reaction, the double bond in the alkene monomer breaks open, allowing the molecule to link up with neighbouring monomers, forming a very long chain.
Example: Making Poly(ethene)
The monomer is ethene (\(C_2H_4\)). The polymer is poly(ethene), commonly known as polythene or polyethylene (a common plastic used for plastic bags and containers).
We represent the repeating unit structure using a large bracket:
$$ n \times \text{Ethene monomer} \rightarrow \text{Poly(ethene) polymer} $$
(Where 'n' is a large number representing the many units joined together.)
Key Fact: The name of the polymer is always "poly" followed by the name of the monomer.
- Propene $\rightarrow$ Poly(propene)
- Chloroethene $\rightarrow$ Poly(chloroethene) (PVC)
✔ Crude oil is a finite mixture of hydrocarbons.
✔ Fractional distillation separates fractions based on boiling point (size).
✔ Small molecules are volatile, flammable, and have low boiling points.
✔ Cracking converts large alkanes into smaller alkanes and reactive alkenes.
✔ Alkenes (monomers) join together via addition polymerization to make polymers (plastics).
You’ve covered the crucial steps of turning raw crude oil into usable fuels and indispensable plastics. Well done!