9.1–9.2 Hydrocarbons and Fractional Distillation of Oil

A free GCSE Chemistry lesson for AQA (8462) from OBA School: what crude oil is, the alkanes, how fractional distillation separates crude oil, and how properties change with chain length. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe crude oil as a finite resource made mainly of hydrocarbons
• Recall the general formula of the alkanes and name and draw the first four
• Describe and explain how crude oil is separated by fractional distillation
• Explain how boiling point, viscosity and flammability change with chain length
• Name the main fractions and give a use for each, including as feedstock for the petrochemical industry

Key words

• Crude oil: A finite resource found in rocks. It is a mixture of a very large number of compounds, mostly hydrocarbons.
• Hydrocarbon: A compound made of hydrogen and carbon atoms only.
• Alkane: A hydrocarbon with only single C–C bonds. General formula CnH₂n₊₂.
• Saturated: Has only single covalent bonds between carbon atoms, so no more hydrogen can be added.
• Fraction: A mixture of hydrocarbons with similar numbers of carbon atoms and similar boiling points.
• Viscosity: How thick a liquid is, or how hard it is to pour.
• Volatility: How easily a liquid turns into a gas (evaporates).
• Flammability: How easily a substance catches fire.
• Displayed formula: A drawing of a molecule showing every atom and every covalent bond as a line.
• Feedstock: A raw material used to make other substances, for example in the petrochemical industry.

Crude oil, a finite resource

Crude oil formed over millions of years from the remains of tiny sea creatures, mainly plankton, that were buried in mud. There was no oxygen in the mud, so the remains did not fully decay. Over time more layers of rock built up on top, and the high temperature and pressure, in the absence of oxygen, slowly changed them into oil. It is being used up far faster than it forms, so it is a finite (non-renewable) resource.

Crude oil is a mixture of a very large number of compounds. Most of them are hydrocarbons, and most of those are alkanes. The compounds are not chemically joined, so they can be separated by a physical process.

Key fact: A hydrocarbon contains hydrogen and carbon only. Ethanol, C₂H₅OH, is not a hydrocarbon because it contains oxygen.

The alkanes

The alkanes are a family (a homologous series) of hydrocarbons. Each member has one more CH₂ than the one before. Their general formula is CnH₂n₊₂. They are saturated: every carbon atom forms four single covalent bonds, so each molecule holds as many hydrogen atoms as possible. No more hydrogen can be added.

You can show an alkane in two ways. The molecular formula (e.g. C₃H₈) gives the number of each type of atom. The displayed formula shows every atom and every bond. To draw one, put the carbon atoms in a row joined by single lines, then add hydrogen atoms until every C has four bonds and every H has one.

• Name; 1 methane; 2 ethane; 3 propane; 4 butane; 5 pentane; 8 octane
• Molecular formula; 1 CH₄; 2 C₂H₆; 3 C₃H₈; 4 C₄H₁₀; 5 C₅H₁₂; 8 C₈H₁₈

Worked example: using the general formula

An alkane has 12 carbon atoms. Find its formula and its relative formula mass (C = 12, H = 1).
H atoms = 2n + 2 = 2 × 12 + 2 = 26, so the formula is C₁₂H₂₆.
Mr = (12 × 12) + (26 × 1) = 144 + 26 = 170

Answer: C₁₂H₂₆, Mr = 170

Worked example: working backwards from the hydrogen atoms

An alkane molecule has 30 hydrogen atoms. How many carbon atoms does it have?
2n + 2 = 30, so 2n = 28 and n = 14.

Answer: C₁₄H₃₀ (14 carbon atoms)

Exam tip: Memory aid for the first four: Monkeys Eat Peanut Butter (meth-, eth-, prop-, but-). The “-ane” ending means alkane.

Fractional distillation

Crude oil straight from the ground is not much use: it contains too many substances with different boiling points. Mixing does not change the properties of the substances, so the hydrocarbons keep their own boiling points and can be separated by distillation.

In a refinery this is done in a fractional distillation column, which runs non-stop (a continuous process):

• Refinery gases (LPG); C atoms per molecule (approx.) 1 to 4; Boiling range (approx.) below 25 °C; Main use bottled gas for heating and cooking
• Petrol; C atoms per molecule (approx.) 5 to 10; Boiling range (approx.) 25 to 175 °C; Main use fuel for cars
• Kerosene; C atoms per molecule (approx.) 10 to 16; Boiling range (approx.) 175 to 250 °C; Main use fuel for aircraft (jet fuel)
• Diesel oil; C atoms per molecule (approx.) 14 to 20; Boiling range (approx.) 250 to 350 °C; Main use fuel for lorries, trains and some cars
• Heavy fuel oil; C atoms per molecule (approx.) 20 to 40; Boiling range (approx.) 350 to 400 °C; Main use fuel for ships and some power stations
• Bitumen (residue); C atoms per molecule (approx.) more than 40; Boiling range (approx.) above 400 °C; Main use surfacing roads and roofs

1. Crude oil is heated in a furnace until most of it evaporates. 2. The hot vapour goes into the bottom of a tall fractionating column. The column is hot at the bottom and cooler towards the top. 3. The vapours rise. Each hydrocarbon condenses when it reaches a level where the temperature is below its boiling point. 4. Liquids are drawn off at different levels as fractions. Small molecules with low boiling points leave near the top as gases. Very large molecules do not evaporate and leave at the bottom as a thick residue. Fractions from low down are very thick liquids or even solids at room temperature.

Key fact: A fraction is still a mixture, but its molecules have a similar number of carbon atoms and so similar boiling points.

Modelling distillation in the lab

You can model the refinery on a small scale. A crude oil mixture soaked into mineral wool is heated, gently at first and then more strongly. Vapour leaves through the side arm, condenses in a tube cooled by cold water, and the tube is changed each time the thermometer reading passes the end of a temperature range (for example 20–100 °C, 100–200 °C, 200–300 °C).

Comparing the fractions (typical results)

1st (lowest temperature): almost colourless, very runny, catches fire at once with a clean flame and little smoke.

2nd: pale yellow, runny, lights quite easily, some smoke.

3rd (highest temperature): dark yellow to orange, thick and slow to pour, hard to light, very smoky flame.

Properties change with chain length

As the hydrocarbon chain gets longer (more carbon atoms):

• Boiling point; Longer chain higher; Why it matters bigger molecules have stronger intermolecular forces, so more energy is needed to separate them
• Viscosity; Longer chain more viscous (thicker); Why it matters long chains get tangled and do not flow easily
• Volatility; Longer chain less volatile; Why it matters evaporates less easily (stronger intermolecular forces)
• Flammability; Longer chain less flammable; Why it matters harder to ignite and burns with a smokier flame, so long-chain fractions make poor fuels for engines

Exam tip: In “explain” questions, link chain length → size of molecule → strength of intermolecular forces → boiling point. Covalent bonds inside the molecules do not break when an alkane boils.

Fuels and feedstock

Many useful materials come from crude oil. Most fractions are used as fuels, because burning hydrocarbons releases a lot of energy. The short-chain fractions (refinery gases, petrol) make the best fuels: they are volatile, ignite easily and burn with a cleaner, less smoky flame. Some are used as feedstock by the petrochemical industry to make solvents, lubricants, detergents and polymers (plastics).

Carbon atoms can bond to each other to form chains and rings. This is why there are so many different hydrocarbons, and so many families (homologous series) of carbon compounds.

Common mistakes

• Calling a fraction a pure substance. Each fraction is a mixture of hydrocarbons with similar boiling points.
• Saying large molecules collect at the top of the column. Large molecules have high boiling points and condense low down, where it is hot.
• Saying covalent bonds break when an alkane boils. Only the weak intermolecular forces are overcome.
• Writing alkane formulae such as C₃H₆. Use 2n + 2: propane is C₃H₈.
• Mixing up viscosity (thickness) with volatility (how easily it evaporates). A long-chain hydrocarbon is more viscous but less volatile.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the general formula of the alkanes? (Choose from: CnH₂n, CnH₂n₊₂, CnH₂n−₂, CnHn₊₂.)

Answer: CnH₂n₊₂

Working: CnH₂n₊₂. For n = 1 this gives CH₄, for n = 2 it gives C₂H₆. CnH₂n is the formula of the alkenes.

Question 2. Calculate the relative formula mass (Mr) of heptane, C₇H₁₆. (C = 12, H = 1)

Answer: 100

Working: (7 × 12) + (16 × 1) = 84 + 16 = 100.

Question 3. True or false? Each fraction collected from a fractionating column is a pure compound.

Answer: False

Working: False. A fraction is a mixture of hydrocarbons with similar numbers of carbon atoms and so similar boiling points.

Related lessons

• Previous lesson: 8.10 Required Practicals: Rates and Equilibrium
• Next lesson: 9.3 Burning Hydrocarbon Fuels
• In this chapter: 9.4 Cracking Hydrocarbons

Continue in the full course

This lesson is section 9.1–9.2 (AGCCH0901) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.