Fossil Fuels: From Crude Oil to Everyday Products (and the Pollution Problem)

Hey everyone! Ever wondered where the petrol for cars comes from, or the material used to make plastic bottles? The answer is often a thick, black, sticky liquid buried deep underground called crude oil. In this chapter, we're going to go on a journey with crude oil. We'll see how we can turn it into incredibly useful things using a process called fractional distillation, explore what those products are used for, and also take an honest look at the environmental problems caused by burning these fuels. It's a really important topic because it affects our daily lives and the future of our planet. Let's get started!


1. What is Crude Oil (Petroleum)?

First things first, what exactly is this stuff?

  • Crude Oil (also called petroleum) is a complex mixture of many different chemical compounds.
  • Most of these compounds are hydrocarbons.
  • It's a fossil fuel, which means it was formed from the remains of tiny sea plants and animals that died millions of years ago. Heat and pressure deep within the Earth turned them into the oil we use today.
  • Because it's a mixture, the different hydrocarbons in it are not chemically bonded to each other. This is great news, because it means we can separate them!
Quick Review: What's a Hydrocarbon?

Don't worry if you've forgotten! A hydrocarbon is simply a compound made of hydrogen (H) and carbon (C) atoms ONLY. They come in all shapes and sizes, from small, short chains to very long, complex chains.

Did you know?

The word petroleum literally means "rock oil" from the Latin words 'petra' (rock) and 'oleum' (oil). Crude oil is a finite resource, which means once we use it all up, it's gone for good!

Key Takeaway

Crude oil is a fossil fuel and a mixture of many different hydrocarbons of various sizes. Because it's a mixture, we can separate it into its useful parts.


2. The Great Separation: Fractional Distillation

So, we have this messy mixture of crude oil. How do we separate it into useful substances like petrol and diesel? We use a clever industrial process called fractional distillation.

The Main Idea

Imagine you have a jar full of different sized sand, pebbles, and rocks all mixed up. You could separate them using a set of sieves with different sized holes. Fractional distillation is similar, but instead of separating by size, it separates hydrocarbons based on their boiling points.

The Key Principle

The whole process works because of a simple rule:

The larger the hydrocarbon molecule (the longer its carbon chain), the higher its boiling point.

Why? Larger molecules have stronger intermolecular forces of attraction (van der Waals' forces) between them. It takes more energy (a higher temperature) to overcome these forces and turn the liquid into a gas.

The Process: Step-by-Step

The separation happens in a giant tower called a fractionating column.

  1. Heating: The crude oil is heated in a furnace to a very high temperature (about 400°C). This turns most of the hydrocarbons into gas (vapour). The ones with very high boiling points remain as a hot liquid.
  2. Entering the Column: This hot mixture of liquid and gas is pumped into the bottom of the fractionating column.
  3. Temperature Gradient: The column has a temperature gradient – it's very hot at the bottom and gradually gets cooler towards the top.
  4. Rising and Cooling: The hot hydrocarbon vapours rise up the column. As they rise, they cool down.
  5. Condensing: When a specific hydrocarbon vapour reaches a height in the column where the temperature is equal to or below its boiling point, it condenses (turns back into a liquid).
  6. Collecting: The condensed liquids are collected on trays at different levels. These collected liquids are called fractions.

Short-chain hydrocarbons have low boiling points. They keep rising high up the column before they condense where it's cool.
Long-chain hydrocarbons have high boiling points. They condense and are collected lower down the column where it's still hot.
The very longest chains with boiling points over 400°C never vaporised at all. They stay at the bottom as a thick, gooey liquid residue.

Key Takeaway

Fractional distillation separates crude oil into different groups (fractions) based on their boiling points. This works because hydrocarbons with different chain lengths have different boiling points. The process takes place in a fractionating column that is hot at the bottom and cool at the top.


3. The Fractions and Their Uses

Here are the main fractions obtained from crude oil, from the top of the column to the bottom.

Fraction Name Boiling Point Range (°C) Main Uses
Refinery Gas Below 25 Liquefied Petroleum Gas (LPG) for cooking and heating
Gasoline (Petrol) 25 - 75 Fuel for cars
Naphtha 75 - 150 Feedstock for making chemicals and plastics
Kerosene 150 - 240 Jet fuel for aircraft, paraffin for lamps and heaters
Diesel Oil (Gas Oil) 240 - 350 Fuel for diesel engines (lorries, buses, trains)
Fuel Oil Above 350 Fuel for ships, power stations, and industrial furnaces
Bitumen (Residue) Above 400 Making roads (asphalt), waterproofing roofs

Gradation in Properties

As you go DOWN the fractionating column (from top to bottom):

  • Size of molecules: Increases (longer carbon chains)
  • Boiling point: Increases
  • Viscosity (thickness): Increases (the liquid gets gooier)
  • Volatility (evaporates easily): Decreases
  • Flammability (catches fire easily): Decreases
  • Colour: Becomes darker
Memory Aid!

Think about the fractions at the top (like petrol) vs. the bottom (bitumen). Petrol is runny, evaporates quickly (you can smell it!), is colourless, and very flammable. Bitumen is a thick, black, non-volatile solid that is hard to set on fire. This helps you remember all the trends!


4. Getting What We Need: Cracking

The Problem of Supply and Demand

Fractional distillation gives us lots of different fractions, but not always in the amounts we need. We have a very high demand for smaller-chain hydrocarbons like petrol. However, fractional distillation produces a large amount of long-chain fractions like fuel oil, which are in lower demand. What can we do?

The Solution: Cracking!

Cracking is a process that breaks down large, less useful hydrocarbon molecules into smaller, more useful and valuable ones.

  • What it does: It's like taking a long piece of string and cutting it into smaller pieces.
  • How it works: It involves heating the long-chain hydrocarbons to a high temperature (around 600-700°C) and passing them over a catalyst (like aluminium oxide or silicon dioxide).
  • The Products: Cracking usually produces a smaller alkane (useful for petrol) and an alkene. Alkenes are super important as they are used to make polymers (plastics).

Example of cracking:

Decane (a long-chain alkane) can be cracked into octane (a smaller alkane for petrol) and ethene (an alkene for making poly(ethene)).

Decane → Octane + Ethene

$$C_{10}H_{22}(l) \rightarrow C_8H_{18}(l) + C_2H_4(g)$$
Key Takeaway

Cracking is essential for the oil industry. It converts surplus long-chain fractions into high-demand petrol and provides the building blocks (alkenes) for the plastics industry.


5. The Environmental Consequences of Burning Fossil Fuels

While fossil fuels power our world, burning them releases harmful substances into the atmosphere. This is a major cause of air pollution.

The Main Pollutants

  • Carbon Dioxide (CO₂): Produced from the complete combustion of any fossil fuel. CO₂ is a greenhouse gas. While it's naturally present, adding more traps extra heat in the atmosphere, leading to the enhanced greenhouse effect and contributing to global warming and climate change.

  • Sulphur Dioxide (SO₂): Crude oil contains sulphur impurities. When the fuel is burned, this sulphur reacts with oxygen to form sulphur dioxide.
    Sulphur + Oxygen → Sulphur Dioxide
    SO₂ dissolves in rainwater to form sulphurous acid, which falls as acid rain. Acid rain damages buildings, harms forests, and makes lakes too acidic for fish to survive.

  • Nitrogen Oxides (NOₓ): The air is mostly nitrogen. The very high temperatures inside a car engine can cause nitrogen and oxygen from the air to react, forming various nitrogen oxides. These gases also contribute to acid rain and can cause respiratory problems.

  • Carbon Monoxide (CO) and Unburnt Hydrocarbons: If there isn't enough oxygen when fuel burns (incomplete combustion), poisonous carbon monoxide (CO) is produced instead of CO₂. Unburnt hydrocarbons can also be released. CO is toxic because it reduces the ability of your blood to carry oxygen.

Common Mistakes to Avoid

Don't confuse the greenhouse effect with acid rain!
Greenhouse effect is caused by CO₂ trapping heat.
Acid rain is caused by SO₂ and NOₓ dissolving in water.


6. Solutions to Reduce Air Pollution

Scientists and engineers have developed ways to reduce the amount of pollution from burning fossil fuels.

  • Catalytic Converters: These are fitted to car exhausts. They contain catalysts (like platinum and rhodium) that convert harmful gases into less harmful ones.
    • Carbon monoxide is oxidised to carbon dioxide ($$2CO + O_2 \rightarrow 2CO_2$$).
    • Nitrogen oxides are reduced to harmless nitrogen gas ($$2NO \rightarrow N_2 + O_2$$).

  • Flue Gas Desulphurisation: Power stations can remove sulphur dioxide from their waste gases (flue gases) before they enter the atmosphere. The gases are passed through a slurry of a base, like calcium carbonate, which neutralises the acidic SO₂.
Key Takeaway

Burning fossil fuels releases pollutants like CO₂, SO₂, and NOₓ, which cause global warming and acid rain. Technologies like catalytic converters and flue gas desulphurisation help to reduce these harmful emissions.