Comprehensive Study Notes: Crude Oil (Organic Chemistry 9202)
Hello future chemist! This chapter is all about Crude Oil—the messy, black gold that powers much of our modern world. Understanding crude oil is vital because it introduces us to the fundamental ideas of Organic Chemistry (the study of carbon compounds) and one of the most important industrial separation techniques: Fractional Distillation.
Don't worry if separating black goo sounds complicated; we'll break it down step-by-step! Let's get started.
1. What is Crude Oil? The Raw Material
A. Definition and Composition
Crude oil is a thick, black, sticky liquid found deep underground, trapped in porous rocks. When we first drill it out, it’s not immediately useful because it is a complex mixture.
- It is a Mixture: This is the most important idea. Crude oil is not one chemical compound; it is a mixture of hundreds of different substances, primarily molecules called hydrocarbons.
- Hydrocarbons: These are molecules made up only of Hydrogen (H) atoms and Carbon (C) atoms. Since they contain carbon, they form the foundation of our study of Organic Chemistry.
- Alkanes: The majority of the hydrocarbons in crude oil belong to a chemical family called the alkanes.
Did you know? Crude oil is often called a "fossil fuel" because it formed over millions of years from the remains of ancient sea creatures and plants.
B. Finite Resource
Crude oil is a finite resource. This means there is a limited amount of it on Earth, and once it is used up, it cannot be replaced naturally in a useful timescale. This fact drives efforts to find alternative, renewable energy sources.
Quick Takeaway: Crude oil is a valuable but finite mixture of hydrocarbons, mostly alkanes.
2. Separating the Mixture: Fractional Distillation
Since we cannot use crude oil in its raw form, we need to separate it into its useful components, called fractions. We use a process called fractional distillation.
Analogy: Imagine crude oil is a giant box of mixed-up pasta—some tiny rice shapes, some medium tubes, and some giant lasagna sheets. Fractional distillation is the process of sorting them based on their size.
A. The Principle of Separation
Fractional distillation separates hydrocarbons based on their boiling points. The size of the molecule dictates its boiling point:
- Small Molecules: Have weak forces holding them together, so they have low boiling points (they turn into gas easily).
- Large Molecules: Have strong forces holding them together, so they have high boiling points (they need a lot of heat to turn into gas).
B. The Process: Step-by-Step
The separation happens inside a tall column called a fractionating column (or tower). The column is hot at the bottom and cool at the top.
Step 1: Heating the Oil
The crude oil is heated to a very high temperature (around 350°C to 400°C) until most of the hydrocarbons turn into hot gas (vapour).
Step 2: Entering the Column
These hot vapours are pumped into the bottom of the fractionating column.
Step 3: Rising and Cooling
As the hot vapours rise up the column, they begin to cool down.
Step 4: Condensation (Turning back into liquid)
- High Boiling Point Fractions (Large Molecules): These cool down and condense back into liquid almost immediately at the very hot bottom of the column.
- Low Boiling Point Fractions (Small Molecules): These remain as vapour, rise much higher up the column, and only condense into liquid when they reach the cooler temperatures near the top.
The resulting liquid fractions are then collected at different levels (trays) of the column.
Key Takeaway: Fractional distillation separates crude oil based on boiling point. Smaller molecules rise to the cool top; larger molecules condense quickly at the hot bottom.
3. The Fractions and Their Properties
The different fractions collected have distinct uses, which relate directly to their physical properties (size, boiling point, and thickness).
A. Trends in Properties
As we move from the top of the column to the bottom, the properties change in a predictable way:
| Property | Top (Small Molecules) | Bottom (Large Molecules) |
|---|---|---|
| Number of C Atoms | Very Low (e.g., C1 to C4) | Very High (e.g., C20+) |
| Boiling Point | LOW | HIGH |
| Volatility (How easily it evaporates) | HIGH (Very runny) | LOW (Hard to evaporate) |
| Viscosity (Thickness/Stickiness) | LOW (Very runny, like water) | HIGH (Very sticky, like tar) |
| Flammability (How easily it catches fire) | HIGH (Burns easily) | LOW (Harder to ignite) |
Memory Trick: Think about honey versus water. Water (low viscosity, volatile, low BP) is like the top fractions. Honey (high viscosity, non-volatile, high BP) is like the bottom fractions.
The Rule: The larger the molecule, the higher the boiling point, and the thicker the liquid.
B. Major Fractions and Uses (From Top to Bottom)
Here are the key fractions, starting from the smallest molecules collected at the coolest top of the column:
1. Refinery Gases (Very Top)
Size: Very small (C1–C4)
Uses: Bottled gas for heating and cooking (e.g., LPG – Liquefied Petroleum Gas).
2. Gasoline (Petrol)
Size: Small (C5–C10)
Uses: Fuel for cars and motor vehicles. It is very volatile and flammable.
3. Naphtha
Uses: Used in the chemical industry to make other chemicals (a feedstock).
4. Kerosene (Paraffin)
Uses: Aircraft fuel (jet fuel) and domestic heating oil.
5. Diesel Oil (Gas Oil)
Uses: Fuel for diesel engines (trains, lorries, cars).
6. Fuel Oil (Heavy Oil)
Uses: Fuel for ships and large power stations.
7. Bitumen (Residue – Very Bottom)
Size: Very large (C70+)
Uses: Road surfacing (tarmac) and roofing felt. It is so viscous and has such a high boiling point that it often never vaporizes fully and is drawn off as a thick liquid residue at the very bottom of the column.
Common Mistake Alert: Students sometimes mix up volatility and flammability. Volatility is about how easily it evaporates; flammability is about how easily it burns once it is evaporated or heated. In general, high volatility usually means high flammability, but they are technically different properties!
Quick Review: The Chain of Separation
Refinery Gas → Gasoline → Kerosene → Diesel → Fuel Oil → Bitumen
(Smallest & Fastest Burning) → → → → (Largest & Stickiest)
Chapter Summary
You have successfully mastered the basics of crude oil!
- Crude oil is a finite mixture of hydrocarbons.
- We separate it using fractional distillation based on differences in their boiling points.
- Small molecules (low BP) rise to the top; large molecules (high BP) stay at the bottom.
- As molecule size increases (from top to bottom), viscosity increases, volatility decreases, and flammability decreases.
Keep practising the order of the fractions and their corresponding uses—you're doing great!