Fractional Distillation and Petrochemicals, and Properties of Hydrocarbons

SPEC 8462 • LESSON 7.02 • PAPER 2

Fractional Distillation, Petrochemicals & Properties

How crude oil becomes petrol, kerosene, and bitumen — and why heavier fractions are more viscous. Spec 4.7.1.2 & 4.7.1.3. ⭐ Priority — appears nearly every Paper 2.

What This Lesson Covers

Three connected topics that account for a 4-mark explain question almost every year. Strong technique here adds up fast on Paper 2.

4

Marking points · full FD answer

~10%

Scored full marks · Jun24 Q05.5

1 in 7

Scored 3/3 · propane bp Q08.1

Why This Matters

Fractional distillation has appeared on three of the last four Paper 2 sittings (Jun22, Jun24 — and feedstock/petrochemicals on Jun22 too). The Jun24 examiner report on kerosene noted only ~10% scored all the marks. The bottleneck is always the same: students miss the heating step or forget that vapours condense, not just "collect" or "accumulate". Today we fix that.

Key Definitions

Three terms you must hold in working memory — every exam question on this topic uses at least one of them as a marking point.

Fractional Distillation

The separation of crude oil into fractions of different boiling points, using a temperature gradient in a fractionating column.

Fraction

A mixture of hydrocarbons with similar boiling points, collected at a specific level in the column. Each fraction is itself a mixture — not a single pure compound.

Feedstock

A raw material used to produce other useful substances. Crude oil fractions act as feedstock for the petrochemical industry.

The Process · 4 Steps

Memorise this exact sequence — the AQA mark scheme awards one mark per step on the standard 4-mark FD explanation question (P2 Jun22 Q04.1).

1Heat. Crude oil is heated at the base of the fractionating column to vaporise the hydrocarbons.

2Gradient. The column has a temperature gradient — hot at the bottom, cool at the top — and vapours rise.

3Condense. Each hydrocarbon condenses when the temperature drops to its boiling point.

4Separate. Long-chain hydrocarbons (high b.p.) condense lower; short-chain (low b.p.) rise higher or stay as gases at the top.

The Four Marking Points

(1) Heated → vaporise. (2) Temperature gradient (in context — vapours rising up the column). (3) Vapours condense. (4) Different boiling points → fractions separate. Miss any one and you cap at 3/4.

Fractionating Column

The column is hottest at the base and coolest at the top. Each fraction condenses at the level where the temperature equals its boiling point.

Cool top (~25 °C) → hot base (~350 °C) · short chains rise; long chains condense low

Examiner Alert · Process

P2 Jun 2022 Q04.1 was a 4-mark explain question on fractional distillation. The examiner report identifies exactly which marks were lost — and how.

"About 1 in 10 students scored all four marks... Students often neglected to state the crude oil must be heated to begin the separation process. Some students failed to score... because their description of a temperature gradient was not in the context of gases rising through a fractionating column... many described the collection of fractions without mentioning condensing and so could not access this mark." — AQA Examiner Report, P2 Jun 2022 Q04.1

Mark lost on…Fix

Forgetting "heated"Always start the explanation: " Crude oil is heated to vaporise the hydrocarbons."

Temperature gradient out of contextLink explicitly: "vapours rise through a column with a temperature gradient (hot bottom, cool top)."

"Fractions accumulate / collect at levels""Vapours condense at their boiling point" — the key verb is condense .

"Petroleum gases condense at the top"Wrong — gases stay as gases (their bp is below room temp, so they don't condense).

Fractions & Their Uses

The spec lists 5 named fuels (verbatim 4.7.1.2): petrol, diesel oil, kerosene, heavy fuel oil, and liquefied petroleum gases. The spec also says: "Knowledge of the names of other specific fractions or fuels is not required." Bitumen is shown for completeness but not required.

FractionIn spec?Position in columnMain uses

Bitumennot requiredBottom — longest chains, highest b.p.Roads, roofs

Heavy fuel oil✓ named in spec↓Fuel for ships, heating

Diesel oil✓ named in spec↓Fuel for cars, lorries, trains

Kerosene✓ named in spec↓Jet fuel, domestic heating

Petrol✓ named in spec↓Fuel for cars (spark-ignition engines)

LPG (liquefied petroleum gases)✓ named in specTop — shortest chains, lowest b.p.Bottled gas: cooking, heating

EXAMINER ALERT (P2 Jun24 Q05.5)

"A few students thought that kerosene was either a product of cracking, or a polymer." Wrong. Kerosene is a fraction obtained directly from fractional distillation. Cracking products and polymers come from fractions — they are not fractions themselves.

Petrochemicals & Feedstock

Crude oil is more than fuel. Fractions act as feedstock — raw materials that the petrochemical industry processes into solvents, lubricants, polymers, and detergents.

Spec 4.7.1.2 — Four Petrochemical Categories (verbatim)

The spec lists four uses of petrochemicals other than fuels: solvents · lubricants · polymers · detergents. AQA Q04.2 (Jun22) gave students 5 tick-box options (Alloys / Ceramics / Detergents / Fertilisers / Solvents) — the correct answers were detergents and solvents.

"Just over a third of students were able to score both of the marks available in this multiple-choice question. This suggests that the feedstock concept in the specification is not well known." — AQA Examiner Report, P2 Jun 2022 Q04.2

Memory Hook

"Solvents · Lubricants · Polymers · Detergents" — four non-fuel uses. Anything you'd burn (petrol, kerosene, diesel) is a fuel; these four are products made from fractions, not the fractions themselves.

Property Trends · Spec 4.7.1.3

The spec is explicit: "Knowledge of trends in properties of hydrocarbons is limited to : boiling points, viscosity, flammability." These three are examinable; volatility is commonly taught alongside but follows from boiling point.

As chain length increases…In spec?Reason

Boiling point increases✓ 4.7.1.3Larger molecules → stronger intermolecular forces → more energy needed to overcome them

Viscosity increases✓ 4.7.1.3Larger molecules → more tangled → flow less easily

Flammability decreases✓ 4.7.1.3Larger molecules harder to ignite

Volatility decreasesderivedFollows from higher b.p. — not separately listed in spec

Why It Matters

These trends explain why fractions separate at different column heights, why kerosene flows more easily than heavy fuel oil, and why petrol is preferred for car engines (high flammability). Each trend follows from one cause: larger molecules → stronger intermolecular forces between molecules.

Boiling Point Deep-Dive

The 3-mark "explain why propane has a low boiling point" question (P1 Jun24 Q08.1) discriminated very strongly — only 1 in 7 scored all 3 marks. The mark scheme demands a precise mechanism.

The Mark-Scoring Chain · 3 Linked Steps

(1) Propane is a small molecule. (2) Small molecules have weak intermolecular forces (between molecules). (3) Little energy is needed to overcome those forces, so the boiling point is low.

✗ Weak (1/3)

"Propane is a hydrocarbon with single bonds, so it has a low boiling point."

✓ Strong (3/3)

"Propane is a small molecule with weak intermolecular forces between molecules. Little energy is needed to overcome these forces, so its boiling point is low."

Critical Distinction

Intermolecular forces are between molecules, NOT "between atoms" or "between bonds". Examiner reports flag this annually as a fatal error — the phrase loses both the second and third marks.

Examiner Alert · Propane B.P.

The Jun 2024 examiner report on the propane boiling-point question is unusually pointed about the language errors that lose marks.

"Few students recognised that it is the structure of propane as small molecules that is important rather than it being a hydrocarbon with single bonds... Although many students used the term intermolecular forces, it was clear that many did not know what this phrase means as they referred to 'intermolecular forces between atoms' or 'between bonds.' These students did not gain credit... Some, who correctly referred to intermolecular forces being weak, then described little energy being needed to break bonds rather than to break the intermolecular forces." — AQA Examiner Report, P1 Jun 2024 Q08.1

Common errorFix

"Hydrocarbon with single bonds → low b.p."Wrong angle. The mark is for small molecules , not single bonds.

"Intermolecular forces between atoms / between bonds"Intermolecular forces are between molecules . Different scale entirely.

"Little energy to break bonds"You don't break covalent bonds when boiling. You overcome intermolecular forces .

Viscosity · Two Linked Marks

P2 Jun 2022 Q04.5 asked students to justify why heavy fuel oil is more viscous than kerosene. The mark scheme is two linked statements — and "more viscous" alone scores zero.

AQA Mark Scheme · Q04.5 (verbatim)

(1) "as molecular size increases, viscosity increases" — 1 mark. (2) "(and) heavy fuel oil has larger molecules (than kerosene)" — 1 mark. Allow converse statements.

✗ 0/2

"Heavy fuel oil is more viscous than kerosene because it's heavier."

Just repeats the question. No structural mechanism.

✓ 2/2

"Heavy fuel oil has larger molecules (11–15 vs 20–40 carbons). As molecular size increases, viscosity increases because larger molecules are more tangled and flow less easily."

EXAMINER ALERT (P2 Jun22 Q04.5)

Students who wrote "heavy fuel oil is more viscous than kerosene" — restating the question — scored zero. The mark is for linking molecular size to viscosity, not for restating which fraction is more viscous.

Practice · Fractional Distillation

Explain

4 MARKS

Adapted from P2 Jun 2022 Q04.1.

MP1

Heat. Crude oil is heated at the base of the fractionating column to vaporise the hydrocarbons.

MP2

Gradient. Vapours rise up the column through a temperature gradient — hot at the bottom, cool at the top.

MP3

Condense. Each hydrocarbon condenses at its own boiling point, at a specific level in the column.

MP4

Boiling points. Fractions separate because they have different boiling points — long chains condense low (hot), short chains condense high (cool) or stay as gases.

Practice · Viscosity Comparison

Justify

2 MARKS

Adapted from P2 Jun 2022 Q04.5.

MP1

Identify. Heavy fuel oil has larger molecules than kerosene (20–40 carbons vs 11–15 carbons).

MP2

Link.As molecular size increases, viscosity increases — larger molecules are more tangled and flow less easily.

✗

Common error to avoid: writing "heavy fuel oil is more viscous than kerosene" (just restating the question — 0/2). The marks come from the structural reason, not the comparison itself.

Practice · Fraction Production

Explain

3 MARKS

Adapted from P2 Jun 2024 Q05.5 — only ~10% scored full marks.

MP1

Gradient. There is a temperature gradient in the (fractionating) column — hot at the bottom, cool going up.

MP2

Condenses. (So) kerosene condenses. (MS note: must specifically reference kerosene — generic "vapours condense" alone won't score MP2.)

MP3

Boiling point. At the level (in the column) corresponding to kerosene's boiling point (range). (MS note: kerosene must be named for MP3.)

⚠

MS rule (verbatim): "For the award of 2 marks for MP2 and MP3, a reference to kerosene must be made." Generic "the vapour condenses at its boiling point" caps at 1/3 (gradient mark only).

✗

Common error: "kerosene comes from cracking" or "kerosene is a polymer" — wrong. Kerosene is a fraction obtained directly from FD. Some students also said vapours condense below their bp — they condense at their bp.

Practice · Column Position

condense lower in a fractionating column

Explain why.

2 MARKS

Adapted from P2 Nov 2021 Q02.6. Examiner: "Many students stated that nonane has a higher boiling point... but far fewer could use this to explain the condensing location in the column."

MP1

Boiling point. Nonane has the highest boiling point of the alkanes listed (because it has the largest molecules / longest chain → strongest intermolecular forces).

MP2

Link to column. The column has a temperature gradient — hot at the bottom, cool going up. Nonane condenses where the column temperature equals its boiling point, which is lower (hotter) in the column than the others.

⚠

The trap (per examiner report): Stating "nonane has a higher boiling point" alone scores only MP1. The second mark requires linking bp to condensing position — i.e. higher bp → condenses where temperature is high → lower in column.

Mark-Scheme Language

The vocabulary AQA accepts vs the loose phrasing that loses marks. Memorise the column on the left.

ConceptMark-scoring phraseDo not accept

FD process"crude oil is heated ; vapours rise ; condense at boiling point""accumulates at levels"; "collects without heating"

Temperature gradient"hot at bottom, cool at top — in context of vapours rising ""different temperatures" (no gradient or context)

Why low b.p." small molecules ; weak intermolecular forces ; little energy to overcome""hydrocarbon"; "single bonds"; "between atoms"

Viscosity"as molecular size increases, viscosity increases""more viscous" alone (restates question)

Petrochemical usesolvents · lubricants · polymers · detergents"fuels" (these are the petrochemical uses other than fuel )

Kerosene"a fraction obtained from fractional distillation""product of cracking"; "a polymer"

Common Errors

The five mistakes that account for most lost marks across recent past papers. Pattern-match these in your own answers before you write.

✗ Common error

"Fractions accumulate at different levels"

✓ Mark-scheme

"Vapours condense at their boiling point"

✗ Common error

Starting with "vapours rise" (no heating step)

✓ Mark-scheme

"Crude oil is heated to vaporise the hydrocarbons"

✗ Common error

"Intermolecular forces between atoms"

✓ Mark-scheme

"Intermolecular forces between molecules"

✗ Common error

"Heavy fuel oil is more viscous than kerosene"

✓ Mark-scheme

"Larger molecules → more tangled → more viscous"

✗ Common error

"Kerosene is a product of cracking" / "a polymer"

✓ Mark-scheme

"Kerosene is a fraction from fractional distillation"

Lesson Summary

Key Definitions

Fractional distillation — separation of crude oil into fractions of different boiling points using a temperature gradient.

Fraction — a mixture of hydrocarbons with similar boiling points.

Feedstock — raw material used to produce other useful substances.

FD · 4 Marking Points

(1) Heat → vaporise. (2) Gradient in column. (3) Vapours condense at b.p. (4) Different b.p. → fractions separate.

Petrochemicals · 4 Uses

Solvents · Lubricants · Polymers · Detergents. Other than fuels.

Top Exam Tip

For any FD question: drill the verb "condense". Most marks lost come from "fractions collect / accumulate" — that wording fails. For viscosity and boiling point, always link to molecular size and intermolecular forces between molecules — never "between atoms" or "between bonds".