Chemistry (0620) | Fuels

Welcome to Section 11.3: Fuels!

Hello! This chapter is all about where we get the energy that powers our world—fuels. Since this topic is part of Organic Chemistry, we will focus on the most important types of fuels: those derived from crude oil (petroleum).

Understanding fuels is not just about passing the exam; it helps you understand global energy issues, car engines, and the chemistry behind everyday life. Don't worry if the names sound complicated; we’ll break down how these crucial substances are separated and what makes them different!

Key Takeaway from the Introduction

Fuels are substances that release energy when burned. We will focus primarily on fossil fuels derived from organic sources.

1. What are Fossil Fuels and Hydrocarbons? (11.3.1, 11.3.3, 11.3.4)

Fossil Fuels

The major non-renewable energy sources that power much of the modern world are the fossil fuels. These were formed millions of years ago from the remains of dead plants and animals.

The three main fossil fuels are:

• Coal (a solid)
• Natural Gas (a gas)
• Petroleum (Crude Oil) (a liquid)

The Chemistry of Hydrocarbons

Most of these fuels, especially natural gas and petroleum, are made up of compounds called hydrocarbons.

Definition: A hydrocarbon is a compound containing hydrogen and carbon atoms only.

Example: Methane (\(CH_4\)), Octane (\(C_8H_{18}\))

Natural Gas and Methane (11.3.2)

When we talk about natural gas, we are mostly talking about one simple hydrocarbon:

The main constituent of natural gas is methane (\(CH_4\)).

Methane is the smallest hydrocarbon molecule, making it very effective as a gas fuel for heating and cooking.

2. Petroleum (Crude Oil) and its Separation (11.3.4, 11.3.5)

Petroleum is a Mixture

Unlike methane, which is a pure compound, petroleum (or crude oil) is a thick, black, sticky liquid that is a complex mixture of many different hydrocarbons.

These hydrocarbons vary greatly in size, from very small molecules (like those in gas) to very large molecules (like those in tar).

Fractional Distillation (11.3.5)

Since crude oil is a mixture, we can't use it effectively as is. We need to separate it into groups of hydrocarbons that have similar properties and boiling points. These groups are called fractions.

This separation is achieved using a process called fractional distillation in a tall column called a fractionating column.

Step-by-step Process:

1. Vaporisation: Crude oil is heated strongly (around \(350^\circ C\)) in a furnace until most of it turns into hot vapour (gas).
2. Entering the Column: These hot vapours are pumped into the bottom of the tall fractionating column.
3. Temperature Gradient: The column is hotter at the bottom (near the furnace) and cooler at the top. This creates a temperature gradient.
4. Condensation: As the vapours rise, they cool down. When a hydrocarbon vapour cools to its specific boiling point, it condenses back into a liquid (a fraction) and is collected on trays at that level.
5. Collection:
   • Hydrocarbons with high boiling points condense lower down the column (closer to the heat).
   • Hydrocarbons with low boiling points travel higher up the column before they cool enough to condense, or they exit as gas at the top.

Analogy: Imagine the fractionating column is a multi-story car park. The cars (hydrocarbons) that melt easily (low boiling point) can float up to the very top. The heavy, sticky trucks (high boiling point) can only make it to the ground floor.

3. Properties of Fractions and Key Trends (11.3.6)

The physical properties of the fractions change predictably as you move either up or down the fractionating column.

The key factor determining these properties is the size of the hydrocarbon molecule (the chain length).

A. Trends from Bottom to Top of the Column

As you move up the column (from bitumen to refinery gas), the molecules condense at lower temperatures, meaning:

1. Decreasing Chain Length: The hydrocarbon molecules get smaller (fewer carbon atoms).
2. Lower Boiling Points: They need less heat to vaporise, so they condense high up where it is cooler.
3. Higher Volatility: They turn into gas more easily. (Volatile means how easily a substance evaporates).
4. Lower Viscosity: They are thinner and flow more easily (less sticky). Think of water (low viscosity) versus treacle (high viscosity).

Memory Aid: Think of the Top Fraction (Refinery Gas): Small, Light, Floats High, Easy to Burn, Runny (low viscosity).

Conversely, the fractions collected at the bottom (like Bitumen) have long chain lengths, high boiling points, low volatility, and high viscosity (they are thick and sticky).

4. Uses of Petroleum Fractions (11.3.7)

Each fraction collected from the column has distinct uses based on its properties (especially its chain length and boiling point):

1. Refinery Gas Fraction
Properties: Very short chains (1-4 carbon atoms), lowest boiling points.
Use: Used as a fuel for heating and cooking (bottled gas).

2. Gasoline / Petrol Fraction
Properties: Small chains, highly volatile.
Use: Fuel used in cars (petrol engines).

3. Naphtha Fraction
Properties: Medium chains.
Use: Used as a chemical feedstock (raw material) for making plastics and other chemicals.

4. Kerosene / Paraffin Fraction
Properties: Medium chains.
Use: Fuel for jet aircraft (jet fuel) and domestic heating/paraffin lamps.

5. Diesel Oil / Gas Oil Fraction
Properties: Longer chains, less volatile than petrol.
Use: Fuel used in diesel engines (trucks, buses).

6. Fuel Oil Fraction
Properties: Long chains, high boiling point.
Use: Fuel used in ships and for home heating systems (boilers).

7. Lubricating Oil Fraction
Properties: Very long chains, thick (high viscosity).
Use: Used as lubricants (to reduce friction in engines), waxes, and polishes.

8. Bitumen Fraction
Properties: Longest chains, highest boiling point (often remaining liquid at the bottom of the column), very thick and sticky.
Use: Used for making roads (asphalt) and roofing materials.