Welcome to the World of Crude Oil!
Hello future Chemists! This chapter takes us into the fascinating world of crude oil. Why should we care about this dark, sticky liquid? Because it is the foundation of almost everything we use—from the fuels that power transport to the plastics in our homes!
In these notes, we will break down exactly what crude oil is, how it is separated by fractional distillation, the trends and uses of its fractions, combustion reactions, atmospheric pollution, and how catalytic cracking converts large molecules into more useful products.
1. What Exactly is Crude Oil?
1.1 Origin and Composition
Crude oil (petroleum) is a thick, dark liquid found underground. It was formed over millions of years from the remains of microscopic sea organisms buried under high pressure and temperature.
- Key Concept: Crude oil is not a pure substance; it is a complex mixture of hydrocarbons.
- Definition of Hydrocarbon: A compound that contains hydrogen and carbon only.
Analogy: Think of crude oil like a bowl of mixed spaghetti noodles. Some molecules are very short, while others are extremely long. We need a way to separate the short chains from the long chains.
2. Separating Crude Oil: Fractional Distillation
Because crude oil is a mixture, it cannot be used directly as a fuel. It is separated into groups of similar hydrocarbons called fractions using industrial fractional distillation.
Principle: Fractional distillation works because different hydrocarbons have different boiling points depending on their chain length.
2.1 The Process in a Fractionating Column
- Heating and Vaporisation: Crude oil is heated until most of it vaporises and enters the bottom of the fractionating column.
- Temperature Gradient: The column is hot at the bottom and becomes gradually cooler near the top.
- Rising and Condensation: Vapours rise up the column. When a fraction reaches the level where the temperature is equal to or below its boiling point, it condenses into a liquid.
- Collection: Fractions are collected from different heights of the column. The very smallest hydrocarbons do not condense and leave as gases from the top, while the heaviest hydrocarbons do not vaporise and leave as a thick residue from the bottom.
Summary of Separation:
- Small molecules: Low boiling points → rise to the cool top before condensing.
- Large molecules: High boiling points → condense near the hot bottom.
3. Properties and Uses of Fractions
3.1 Key Trends (From Bottom to Top of Column)
As you move up the column (from larger to smaller molecules):
| Property | Trend (Bottom → Top) | Reason |
|---|---|---|
| Boiling Point | Decreases | Smaller molecules have weaker intermolecular forces, needing less energy to separate. |
| Viscosity (Thickness/flow) | Decreases (Flows more easily / less viscous) | Smaller molecules slide past each other more easily. |
| Colour | Becomes Lighter (Dark brown/black → Pale yellow → Colourless) | Large hydrocarbons absorb more light; small hydrocarbons are clear. |
| Flammability | Increases (Ignites more easily) | Volatile liquids with low boiling points vaporise and catch fire easily. |
3.2 The Prescribed Fractions and Their Uses
You need to know the specific uses for the six main fractions (listed from top to bottom):
- Refinery gases: Used for domestic heating and cooking (bottled gas).
- Gasoline (petrol): Used as fuel for cars.
- Kerosene: Used as fuel for aircraft (jet fuel) and domestic heating.
- Diesel: Used as fuel for diesel engines (cars, lorries, buses, and trains).
- Fuel oil: Used as fuel for ships and power stations.
- Bitumen: Used for surfacing roads and roofing.
4. Burning Fuels: Combustion and Atmospheric Pollution
Hydrocarbons are useful fuels because they release heat energy when burned (exothermic reaction).
4.1 Complete Combustion
Occurs when there is a plentiful supply of oxygen:
\[ \text{Hydrocarbon} + \text{Oxygen} \longrightarrow \text{Carbon Dioxide} + \text{Water} \]
Example for methane:
\[ \text{CH}_4 + 2\text{O}_2 \longrightarrow \text{CO}_2 + 2\text{H}_2\text{O} \]
4.2 Incomplete Combustion
Occurs when the supply of oxygen is limited or insufficient:
\[ \text{Hydrocarbon} + \text{Limited Oxygen} \longrightarrow \text{Carbon Monoxide} + \text{Carbon (soot)} + \text{Water} \]
Dangers of Products of Incomplete Combustion
- Carbon Monoxide (\(\text{CO}\)): A colourless, odourless, and poisonous gas. It is toxic because it reduces the capacity of the blood to carry oxygen by binding strongly to haemoglobin.
- Carbon (\(\text{C}\), soot): Tiny solid particles that cause respiratory problems, make buildings dirty, and can cause global dimming.
4.3 Atmospheric Pollutants and Acid Rain
Burning fossil fuels produces two other major gases that contribute to acid rain:
- Sulfur Dioxide (\(\text{SO}_2\)): Most crude oil contains small amounts of sulfur impurities. When the fuel is burned, sulfur reacts with oxygen to form sulfur dioxide (\(\text{S} + \text{O}_2 \longrightarrow \text{SO}_2\)). \(\text{SO}_2\) dissolves in rainwater to form sulfurous/sulfuric acid.
- Oxides of Nitrogen (\(\text{NO}_x\), such as \(\text{NO}\) and \(\text{NO}_2\)): Inside high-temperature car engines, nitrogen and oxygen from the air react together (\(\text{N}_2 + \text{O}_2 \longrightarrow 2\text{NO}\)). In the atmosphere, nitrogen oxides react with water and oxygen to form nitric acid.
Consequences of Acid Rain: Acidifies lakes and rivers (killing fish and aquatic life), damages trees and forests, and corrodes limestone buildings and metal structures.
5. Adapting Fractions: Catalytic Cracking
5.1 Why is Cracking Needed?
Fractional distillation produces more long-chain hydrocarbons than the market needs, but not enough short-chain hydrocarbons (like gasoline). There is a surplus of heavy fractions and a high demand for lighter fractions. In addition, there is a high demand for alkenes to make polymers (plastics).
5.2 What is Cracking?
Cracking is the thermal decomposition of long-chain alkanes into smaller, more useful short-chain alkanes and alkenes.
5.3 Conditions for Catalytic Cracking
In industry, catalytic cracking requires the following specific conditions:
- Catalyst: Silica (\(\text{SiO}_2\)) or alumina (\(\text{Al}_2\text{O}_3\)).
- Temperature: 600–700 °C.
5.4 Cracking Reactions
Cracking converts a long-chain alkane into a mixture of shorter alkanes and alkenes (or an alkene and hydrogen):
\[ \text{Long-chain alkane} \longrightarrow \text{Shorter alkane} + \text{Alkene} \]
Example:
\[ \text{C}_{10}\text{H}_{22} \longrightarrow \text{C}_8\text{H}_{18} + \text{C}_2\text{H}_4 \]
\[ \text{Decane} \longrightarrow \text{Octane (useful fuel)} + \text{Ethene (monomer for plastics)} \]
Chapter Summary Review
- Crude oil: A mixture of hydrocarbons.
- Hydrocarbon: Contains hydrogen and carbon only.
- Fractional distillation: Separates crude oil by boiling point using a fractionating column (hot at bottom, cool at top).
- Fractions & uses (top to bottom): Refinery gases (domestic cooking/heating) → Gasoline (petrol for cars) → Kerosene (jet fuel) → Diesel (diesel engines) → Fuel oil (ships/power stations) → Bitumen (roads/roofing).
- Trends up the column: Lower boiling point, lower viscosity (more runny), lighter colour, higher flammability.
- Carbon monoxide toxicity: Reduces the blood's capacity to carry oxygen.
- Acid rain contributors: Sulfur dioxide (from sulfur impurities in fuels) and oxides of nitrogen (from high temperatures in car engines).
- Catalytic cracking: Heated over silica or alumina catalyst at 600–700 °C to convert long alkanes into short alkanes and alkenes.