Fuels

Fractional distillation of petroleum appears on nearly every 0620 paper in one form: name the fraction, match the use, or explain why one fraction leaves the column above another. The marks are pure recall plus one idea (molecular size controls boiling point), yet examiner reports repeatedly note candidates who reverse the column or invent uses. Here is the whole subtopic, organised the way questions ask it.

Fossil fuels

The syllabus names three: coal, natural gas and petroleum (also called crude oil). Natural gas is mainly methane, CH4. That two-word fact is a recurring 1-mark question. All three formed from the remains of dead organisms over millions of years, which is why they are finite and why their CO2 contributes to the enhanced greenhouse effect covered in chemistry of the environment.

Petroleum itself is a mixture of hydrocarbons, meaning compounds of hydrogen and carbon only. Because it is a mixture, the components keep their own properties and can be separated physically.

Fractional distillation of petroleum

The separation works because different hydrocarbons have different boiling points. Crude oil is heated until it vaporises and fed into a fractionating column that is hot at the bottom and cooler towards the top. Vapours rise and each fraction condenses at the level where the column temperature falls below its boiling point. A fraction is a group of hydrocarbons with similar boiling points, not a single compound.

Fraction (top → bottom)	Use
Refinery gas	bottled gas for heating and cooking
Gasoline (petrol)	fuel for cars
Naphtha	feedstock for the chemical industry
Kerosene (paraffin)	jet fuel
Diesel oil (gas oil)	fuel for diesel engines
Fuel oil	fuel for ships and home heating systems
Lubricating fraction	lubricants, waxes, polishes
Bitumen	road surfaces

Learn the order top-to-bottom and the uses as fixed pairs. “Naphtha: chemical feedstock” is the one students blank on most.

The size–property pattern

Everything explainable in this subtopic comes from one trend. As molecules get smaller:

• boiling point decreases (weaker attractions between smaller molecules)
• volatility increases (the liquid evaporates more easily)
• flammability increases
• viscosity decreases (the liquid flows more easily)

So refinery gas, at the top, is the most volatile and most flammable; bitumen, at the bottom, is so viscous and involatile it is spread on roads. An “explain” question wants the chain of logic: small molecules → low boiling point → condenses high in the column. Demand for the small, light fractions outstrips their natural supply, which is why long alkanes are cracked into shorter ones. The chemistry of that sits with alkenes.

Burning any of these fuels is combustion of alkanes: complete combustion gives carbon dioxide and water; limited oxygen gives toxic carbon monoxide.

Worked exam question

The diagram shows a fractionating column used to separate petroleum. Fraction A leaves at the top of the column; fraction F leaves near the bottom. (a) Describe how fractional distillation separates petroleum. [3] (b) Compare the molecules in fraction A with those in fraction F. Give two differences. [2]

Model answer, mark by mark:

• (a) M1: the petroleum is heated and vaporised (evaporated).
• (a) M2: the column is hotter at the bottom than at the top / there is a temperature gradient.
• (a) M3: fractions condense at different heights according to their boiling points, and hydrocarbons with similar boiling points collect together.
• (b) M4: molecules in A are smaller / have fewer carbon atoms than those in F.
• (b) M5: molecules in A have a lower boiling point / are more volatile / are more flammable / are less viscous than those in F.

Note what (a) does not need: fraction names, uses, or cracking. Answer the question asked.

The mistakes that cost marks

1. Reversing the column. Writing “hot at the top” inverts the entire answer. The vapour rises and cools; small molecules condense last, at the top.
2. “Petroleum is a compound.” It is a mixture of hydrocarbons. Mixture is a marking point; compound contradicts it.
3. Confusing evaporating and condensing. The separation happens when fractions condense at their boiling points in the column. “The fractions evaporate at different levels” muddles the mechanism.
4. Invented uses. “Kerosene for cars” or “fuel oil for planes” scores zero. The pairs are fixed: learn the table, not a vague impression of it.

How examiners want it phrased

Student wording	Mark-scheme wording
”The oil gets split up by heat"	"Petroleum is vaporised and the fractions condense at different temperatures"
"Light stuff floats to the top"	"Smaller molecules have lower boiling points, so they condense higher up the column"
"Gas at the top burns easily"	"The fraction is more volatile and more flammable because the molecules are smaller"
"Crude oil is full of chemicals"	"Petroleum is a mixture of hydrocarbons”

This is one of the highest mark-per-minute subtopics in organic chemistry: fifteen facts, asked the same way every series. If you want the whole fraction table locked in with a retrieval drill rather than rereading, that is exactly the kind of thing we set up in a free trial lesson.

Test yourself

Do these three without looking back at the table. Answers are hidden until you click.

Q1 (1 mark). Name the main compound in natural gas.

Show answer

• methane (CH4 accepted) [1]

Q2 (3 marks). Give one use of each fraction: kerosene, naphtha, bitumen.

Show answer

• kerosene: jet fuel [1] • naphtha: feedstock for the chemical industry [1] • bitumen: road surfaces [1]

Q3 (3 marks). Explain why bitumen is collected at the bottom of the fractionating column.

Show answer

• bitumen contains the largest molecules / longest chains [1] • so it has the highest boiling point [1] • it condenses (or remains liquid) low in the column, where the temperature is highest [1]