Organic chemistry: fractional distillation and uses of fractions

What you'll learn

This revision guide covers the separation of crude oil through fractional distillation and the practical uses of different hydrocarbon fractions. You'll understand how physical properties determine where fractions are collected, and why different fractions suit specific applications in industry and everyday life.

Key terms and definitions

Crude oil — a finite resource found in rocks, consisting of a mixture of many different hydrocarbons, most of which are alkanes

Fraction — a mixture of hydrocarbons with similar boiling points, collected from fractional distillation

Fractional distillation — the separation process that divides crude oil into different fractions based on differences in boiling points

Hydrocarbon — a compound containing hydrogen and carbon atoms only

Viscosity — a measure of how easily a liquid flows; high viscosity means the liquid is thick and flows slowly

Volatility — the tendency of a substance to evaporate; volatile substances have low boiling points and evaporate easily

Alkane — a saturated hydrocarbon with the general formula CₙH₂ₙ₊₂, containing only single carbon-carbon bonds

Homologous series — a family of compounds with the same general formula and similar chemical properties

Core concepts

Crude oil composition and formation

Crude oil is a complex mixture of hydrocarbons formed over millions of years from the remains of ancient marine organisms. Under high temperature and pressure, these remains were converted into crude oil, which became trapped in porous rocks beneath impermeable rock layers.

The hydrocarbons in crude oil are mainly alkanes, which form a homologous series. Each member differs from the next by CH₂. Examples include:

• Methane (CH₄)
• Ethane (C₂H₆)
• Propane (C₃H₈)
• Butane (C₄H₁₀)

Crude oil also contains smaller amounts of other hydrocarbons and impurities such as sulfur compounds. The exact composition varies depending on where the oil was extracted.

The fractional distillation process

Fractional distillation separates crude oil into useful fractions because different hydrocarbons have different boiling points. The process occurs in a fractionating column, which is hotter at the bottom and cooler at the top.

Step-by-step process:

1. Crude oil is heated to approximately 350°C in a furnace, vaporising most of the hydrocarbons
2. The hot vapour enters the fractionating column at the bottom
3. Vapours rise up through the column, which has a temperature gradient
4. As vapours rise and cool, hydrocarbons condense when they reach their boiling point
5. Fractions are collected at different levels via trays or bubble caps
6. Shorter chain hydrocarbons with lower boiling points rise higher before condensing
7. Longer chain hydrocarbons with higher boiling points condense lower down
8. The shortest hydrocarbons (gases) exit at the top
9. Very long chain hydrocarbons with very high boiling points don't vaporise and are removed as residue at the bottom

The column operates continuously, with crude oil constantly being fed in and fractions constantly being removed.

Properties of hydrocarbon fractions

The physical properties of hydrocarbons depend on the length of their carbon chains. Understanding these relationships is essential for GCSE examinations.

As carbon chain length increases:

• Boiling point increases — longer molecules have stronger intermolecular forces (London dispersion forces), requiring more energy to separate molecules
• Viscosity increases — longer molecules become more tangled, making the liquid thicker and slower to flow
• Volatility decreases — higher boiling points mean less tendency to evaporate
• Flammability decreases — longer chains are harder to ignite and burn less easily
• Colour darkens — fractions change from colourless/pale yellow to dark brown/black

Summary table:

These property trends explain why fractions separate in the column and determine their uses.

Main fractions and their uses

The fractionating column produces several important fractions, each with specific applications based on their properties.

Refinery gases (1-4 carbon atoms):

• Collected at the top of the column
• Very volatile, low boiling points (below 40°C)
• Uses: domestic heating, cooking fuel (bottled gas), fuel for heating in oil refineries
• Examples: methane, ethane, propane, butane

Petrol/gasoline (4-12 carbon atoms):

• Collected near the top
• Volatile liquid, boiling point 40-110°C
• Uses: fuel for cars, motorbikes, and other vehicles with petrol engines
• Must ignite easily and burn cleanly in engines

Kerosene/paraffin (10-16 carbon atoms):

• Collected in the middle section
• Medium viscosity, boiling point 110-250°C
• Uses: aircraft fuel (jet fuel), heating fuel, fuel for some lamps
• Requires balance between energy content and safety

Diesel oil/gas oil (15-20 carbon atoms):

• Collected below the middle
• More viscous, boiling point 250-350°C
• Uses: fuel for diesel engines in cars, trains, lorries, buses, and ships; heating fuel
• Higher energy density than petrol

Fuel oil (20-30 carbon atoms):

• Collected near the bottom
• Very viscous, high boiling point
• Uses: fuel for large ships, power stations, industrial heating
• Needs to be heated before use in some applications

Bitumen (over 30 carbon atoms):

• Residue at the bottom
• Very thick, semi-solid at room temperature
• Uses: surfacing roads and roofs, making asphalt
• Waterproof and durable

The separation of crude oil into these fractions is essential because crude oil itself isn't useful. Each fraction's properties must match its intended use.

Economic importance and sustainability

Crude oil is a finite resource, meaning it cannot be replaced once used. It formed over millions of years, but current consumption rates are extremely high. Most crude oil is used as fuel, releasing carbon dioxide when burned, contributing to climate change.

Key considerations:

• Supply and demand — not all fractions are produced in quantities that match demand. For example, there's high demand for petrol but crude oil doesn't naturally contain enough short-chain hydrocarbons
• Cracking — this process (covered in separate revision notes) breaks longer chains into shorter, more useful molecules to meet demand
• Alternative resources — renewable energy sources and sustainable fuels are increasingly important as crude oil reserves decline
• Value — crude oil and its fractions are extremely valuable to the global economy, making oil-producing regions economically significant

Physical vs chemical properties

Understanding the difference between physical and chemical properties is important for GCSE examinations.

Physical properties (tested in fractional distillation):

• Boiling point
• Viscosity
• Volatility
• Colour
• Density

These properties don't involve breaking chemical bonds. Fractional distillation is a physical separation process — no chemical reactions occur, and the chemical identity of molecules remains unchanged.

Chemical properties:

• How hydrocarbons react (combustion, cracking)
• Types of products formed
• Energy released in reactions

All alkane fractions have similar chemical properties because they're part of the same homologous series, but their physical properties vary significantly with chain length.

Worked examples

Example 1: Explaining separation in the fractionating column

Question (4 marks): Crude oil is separated into fractions in a fractionating column. Explain why petrol is collected near the top of the column whilst bitumen is collected at the bottom.

Mark scheme answer:

• Petrol contains shorter chain hydrocarbons / molecules with fewer carbon atoms (1 mark)
• Shorter chains have lower boiling points / weaker intermolecular forces (1 mark)
• These vaporise and rise higher up the column before condensing (1 mark)
• Bitumen contains longer chains with higher boiling points, so condenses at the bottom / doesn't vaporise (1 mark)

Examiner tip: Link molecular size to boiling point, then to position in the column. Show the cause-and-effect relationship clearly.

Example 2: Matching fractions to uses

Question (3 marks): Draw one line from each fraction to its most appropriate use.

Answer:

• Kerosene → Fuel for aircraft (1 mark)
• Diesel → Fuel for lorries (1 mark)
• Bitumen → Surfacing roads (1 mark)

Examiner tip: Learn the specific uses for each major fraction. Don't just remember "fuel" — know which type of vehicle or application.

Example 3: Predicting properties

Question (6 marks): Fuel oil and petrol are both obtained from crude oil.

(a) Which fraction has the higher boiling point? (1 mark) (b) Explain your answer to part (a). (2 marks) (c) Which fraction is more viscous? (1 mark) (d) Which fraction is more flammable? (1 mark) (e) Suggest why petrol is more suitable than fuel oil for use in car engines. (1 mark)

Answers:

(a) Fuel oil (1 mark)

(b) Fuel oil contains longer chain hydrocarbons / molecules with more carbon atoms (1 mark), which have stronger intermolecular forces / require more energy to separate (1 mark)

(c) Fuel oil (1 mark)

(d) Petrol (1 mark)

(e) Petrol is more volatile / ignites more easily / has a lower boiling point / is less viscous so flows better in engines (1 mark)

Examiner tip: When comparing fractions, always identify which has longer chains first, then explain how this affects the specific property asked about.

Common mistakes and how to avoid them

• Confusing physical and chemical processes — fractional distillation is physical separation, not a chemical reaction. The molecules aren't changed, just separated. Don't write "crude oil is broken down" — it's separated into fractions.

• Reversing property trends — remember: as chain length increases, boiling point, viscosity, and colour all increase, whilst volatility and flammability decrease. Create a memory aid or draw a diagram showing these relationships.

• Vague answers about intermolecular forces — don't just say "stronger forces" in exams. Specify that longer molecules have stronger intermolecular forces between molecules (not within molecules). Better answers mention that these are weak forces between molecules, not covalent bonds.

• Misremembering fraction uses — don't guess fraction uses in exams. Petrol is for cars, kerosene for aircraft, diesel for lorries/trains, bitumen for roads. Each fraction name gives clues: bitumen sounds like "bitter" or "sticky," kerosene sounds like "paraffin" (alternative name).

• Confusion between crude oil and petrol — crude oil is the mixture extracted from the ground; petrol is one fraction obtained from it. They're not the same thing. Never write "crude oil is used in cars."

• Not explaining why properties matter for uses — when asked why a fraction suits a particular use, link its properties to the requirements. For example, "petrol is used in cars because it's volatile and ignites easily, which is needed for combustion engines."

Exam technique for "Organic chemistry: fractional distillation and uses of fractions"

• Command words matter — "Describe" requires you to state what happens; "Explain" requires reasons or mechanisms. For "Explain why petrol condenses near the top," you must link chain length → boiling point → position in column. Just stating "it has a low boiling point" won't earn full marks.

• Use comparative language — questions often compare two fractions. Use words like "shorter/longer chains," "higher/lower boiling point," "more/less viscous." Absolute statements like "petrol is volatile" are weaker than "petrol is more volatile than diesel."

• Structure extended answers logically — for 4-6 mark questions, follow the pattern: identify molecular difference (chain length) → explain effect on intermolecular forces → explain effect on property → link to position in column or use. This shows clear scientific reasoning.

• Learn specific examples — examiners reward precision. Writing "gases like propane" scores higher than just "gases." Similarly, "used for aircraft fuel" is better than "used as fuel."

Quick revision summary

Crude oil is separated by fractional distillation in a fractionating column using differences in boiling points. Shorter chain hydrocarbons have lower boiling points and rise higher before condensing, whilst longer chains condense lower down. As chain length increases, boiling point, viscosity, and colour increase, but volatility and flammability decrease. Different fractions suit specific uses: petrol for cars, kerosene for aircraft, diesel for lorries, bitumen for roads. Fractional distillation is a physical process that doesn't change the chemical identity of hydrocarbons.