Fractional Distillation and Cracking

This lesson covers two key industrial processes in the AQA GCSE Combined Science Trilogy (8464) specification: fractional distillation of crude oil and cracking of long-chain hydrocarbons. You need to understand how crude oil is separated into useful fractions and why cracking is necessary.

---

Fractional Distillation of Crude Oil

Fractional distillation is the process used to separate crude oil into useful fractions — groups of hydrocarbons with similar boiling points and chain lengths.

How It Works

1. Crude oil is heated in a furnace until most of it vaporises.
2. The vapour enters a fractionating column — a tall tower that is hot at the bottom and cool at the top.
3. Vapours rise up the column. Each hydrocarbon condenses when it reaches the level where the temperature equals its boiling point.
4. Short-chain hydrocarbons (low boiling points) rise to the top before condensing.
5. Long-chain hydrocarbons (high boiling points) condense near the bottom.
6. Fractions are collected at different levels.

graph TB
    subgraph "Fractionating Column"
        direction TB
        TOP["🔝 Cool — ~25°C"]
        R["Refinery gases (C1–C4) — gases"]
        P["Petrol/Gasoline (C5–C10) — liquid"]
        N["Naphtha — liquid"]
        K["Kerosene (C11–C15) — liquid"]
        DI["Diesel (C15–C20) — liquid"]
        FO["Fuel oil (C20–C40) — liquid"]
        BIT["Bitumen (C40+) — solid/semi-solid"]
        BOT["🔥 Hot — ~400°C"]
    end
    TOP --- R --- P --- N --- K --- DI --- FO --- BIT --- BOT

The Fractions and Their Uses

Fraction	Carbon Chain Length	Boiling Range	Uses
Refinery gases	C₁–C₄	Below 25°C	Bottled gas (LPG), heating
Petrol (gasoline)	C₅–C₁₀	25–75°C	Fuel for cars
Naphtha	C₅–C₁₀	75–150°C	Feedstock for chemicals
Kerosene	C₁₁–C₁₅	150–240°C	Jet fuel
Diesel	C₁₅–C₂₀	240–350°C	Fuel for lorries, buses, trains
Fuel oil	C₂₀–C₄₀	350–500°C	Fuel for ships, power stations
Bitumen	C₄₀+	Above 500°C	Road surfaces, roofing

Exam Tip: You do not need to memorise the exact boiling ranges, but you must know the order of fractions from top to bottom and understand that boiling point increases down the column.

---

Why Fractional Distillation Works

Fractional distillation works because the different hydrocarbons in crude oil have different boiling points due to their different chain lengths. This is a physical process — no chemical bonds are broken. The hydrocarbons are separated based on their intermolecular forces.

Chain Length	Intermolecular Forces	Boiling Point	Position in Column
Short	Weak	Low	Top
Long	Strong	High	Bottom

---

Supply and Demand

The fractions produced by fractional distillation do not always match the demand for each fraction:

• There is high demand for short-chain hydrocarbons (petrol, gases) and alkenes (for making plastics).
• There is low demand for long-chain hydrocarbons (fuel oil, bitumen).
• Crude oil produces too many long-chain hydrocarbons and not enough short-chain ones.

This mismatch is solved by cracking.

---

Cracking

Cracking is the process of breaking down long-chain hydrocarbons into shorter, more useful molecules. The products include shorter alkanes (for fuels) and alkenes (for making polymers/plastics).

Types of Cracking

Type	Conditions	Details
Catalytic cracking	High temperature (~600°C), catalyst (zeolite or alumina)	Produces mainly alkenes and branched alkanes
Steam cracking	Very high temperature (~850°C), steam	Produces mainly alkenes

What Happens During Cracking?

A long-chain alkane is broken into a shorter alkane and one or more alkenes.

Example:

$\text{C}_{10}\text{H}_{22} \rightarrow \text{C}_{8}\text{H}_{18} + \text{C}_2\text{H}_4$C10​H22​→C8​H18​+C2​H4​

(decane → octane + ethene)

Key Point: Cracking always produces at least one alkene. Alkenes contain a carbon-carbon double bond (C=C) and are described as unsaturated.

Testing for Alkenes

Alkenes can be distinguished from alkanes using bromine water:

Test	Alkane	Alkene
Bromine water	Stays orange	Turns colourless (decolourised)

Exam Tip: If a question mentions a hydrocarbon that decolourises bromine water, it is an alkene (unsaturated). Alkanes do not react with bromine water under normal conditions.

---

Why Is Cracking Important?

1. Matches supply to demand — converts excess long-chain hydrocarbons into short-chain fuels.
2. Produces alkenes — essential feedstock for making polymers (plastics), such as poly(ethene) from ethene.
3. Economic value — short-chain hydrocarbons and alkenes are more valuable than long-chain hydrocarbons.

---

Common Mistakes

• Confusing fractional distillation (physical separation) with cracking (chemical reaction).
• Forgetting that cracking produces alkenes, not just shorter alkanes.
• Not balancing cracking equations — the total number of carbon and hydrogen atoms must be the same on both sides.

---

Summary

• Fractional distillation separates crude oil into fractions based on boiling point
• Short-chain fractions collected at the top (low boiling point), long-chain at the bottom
• Cracking breaks long-chain alkanes into shorter alkanes and alkenes
• Catalytic cracking uses a catalyst at ~600°C; steam cracking uses steam at ~850°C
• Alkenes are unsaturated (contain C=C) and decolourise bromine water
• Cracking is necessary to match supply with demand and produce useful alkenes

Exam Tip: Practise balancing cracking equations. Remember: total carbons on left = total carbons on right, and total hydrogens on left = total hydrogens on right.

---

Extended Worked Examples and Industrial Context

Worked Example 1: Balancing a Cracking Equation

Question: Decane (C₁₀H₂₂) is cracked to produce octane (C₈H₁₈) and one other product. Identify the other product and write the balanced equation.

Solution: Carbon balance: 10 − 8 = 2 C. Hydrogen balance: 22 − 18 = 4 H. The remaining fragment is C₂H₄, which is ethene — an alkene.

$\text{C}_{10}\text{H}_{22} \rightarrow \text{C}_8\text{H}_{18} + \text{C}_2\text{H}_4$C10​H22​→C8​H18​+C2​H4​

Worked Example 2: Multiple Products

Question: Suggest a possible cracking of dodecane (C₁₂H₂₆) to give one alkane and two molecules of ethene.

Solution: Ethene is C₂H₄; two ethene molecules = C₄H₈. The alkane must contain the remaining atoms: C = 12 − 4 = 8; H = 26 − 8 = 18. That is octane, C₈H₁₈.