Fractional distillation, cracking and biofuels explained: HSC Chemistry Module 7

Syllabus dot point

“Investigate the industrial sourcing and separation of hydrocarbons, including fractional distillation of petroleum, catalytic and thermal cracking of larger fractions, and the production of biofuels as an alternative source of hydrocarbon-derived fuels”

11 min readUpdated 28 Jun 2026

Quick answer

Fractional distillation is a physical separation of crude oil into fractions by boiling point (which rises with chain length as dispersion forces strengthen), condensing shorter chains near the cool top of the column and longer chains near the hot base. Cracking is a chemical process that breaks C-C bonds in surplus long-chain fractions to make more of the shorter-chain alkanes and alkenes the market actually wants, using either a zeolite catalyst at moderate temperature (catalytic) or very high temperature with no catalyst (thermal). Biofuels, such as bioethanol from fermentation of glucose (then purified by fractional distillation) and biodiesel from transesterification of oils/fats, offer a renewable, more carbon-neutral alternative source of hydrocarbon-derived fuels, at the cost of land/water competition with food crops and lower energy density.

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In plain English

Crude oil is a jumbled mixture of hydrocarbon chains of all different lengths, like a box containing everything from tiny paperclips to long pieces of string. Fractional distillation sorts that mixture the same way boiling a pot of mixed liquids would: heat it up, and the shortest, lightest molecules boil off and rise to the top of a tall column first, while the longest, heaviest molecules barely rise at all and are drawn off near the bottom. The trouble is that what comes out of the ground does not match what people actually want to buy, there is too much of the heavy stuff and not enough of the light stuff like petrol. Cracking fixes that mismatch by taking those long, low-demand chains and snapping them into shorter, higher-demand pieces, a bit like cutting a long piece of string into several short ones you can actually use. Biofuels sidestep crude oil altogether: instead of drilling for chains that took millions of years to form, you grow a crop, ferment its sugars into ethanol (or convert its oils into biodiesel), and burn that instead, trading a finite, ancient carbon source for a renewable, recently grown one.

What this dot point is asking

NESA wants you to explain how the three big real-world sourcing and separation processes for hydrocarbon fuels actually work: fractional distillation (a physical separation of crude oil by boiling point, no bonds broken), cracking (a chemical conversion that breaks C-C bonds to turn unwanted long-chain fractions into wanted shorter ones), and biofuel production (an alternative, renewable route to hydrocarbon-derived fuels that avoids crude oil entirely). You need the chemistry of each process, the reagents/catalysts/conditions involved, and the ability to justify why each process is used in industry.

The answer

Fractional distillation of petroleum

Crude oil is a complex mixture of hydrocarbons (mostly alkanes, with some cycloalkanes and aromatics) of widely varying chain length. Because the only significant intermolecular force between hydrocarbon molecules is dispersion (London) forces, and dispersion forces strengthen as chain length (and surface area) increases, boiling point rises smoothly with chain length across the mixture. Fractional distillation exploits this: crude oil is heated until it vaporises, and the vapour rises through a tall fractionating column that has a temperature gradient, hottest at the base and coolest at the top. As vapour rises and cools, each group of hydrocarbons ("fraction") condenses back to liquid once the column temperature drops below its boiling range, and is drawn off on a collection tray at that height. No chemical bonds are broken; this is a purely physical separation.

Fractional distillation checkpoints

Separation basis: boiling point, which increases with chain length because dispersion forces between molecules strengthen as surface area increases.
Physical, not chemical: no covalent bonds are broken or formed; only intermolecular forces are overcome.
Column arrangement: hottest at the base (crude oil enters here, heated to about 370 degrees C), coolest at the top. Short-chain, low-boiling fractions (refinery gas, petrol) condense and are drawn off high in the column; long-chain, high-boiling fractions (diesel, lubricating oil, bitumen) condense and are drawn off low in the column.
Common fraction order (low to high boiling point): refinery gas, petrol (gasoline), kerosene, diesel, lubricating oil, bitumen/residue.

Cracking

Fractional distillation alone produces a fixed distribution of fractions set by what is naturally present in crude oil, but market demand favours shorter-chain products (especially petrol and the small alkenes used as petrochemical feedstocks) more than the heavier fractions crude oil actually supplies in abundance. Cracking is a chemical process that breaks carbon-to-carbon bonds in longer-chain alkanes to produce a mixture of shorter-chain alkanes and alkenes, converting a less useful surplus fraction into more valuable, saleable products.

C_{10}H_{22} \rightarrow C_8H_{18} + C_2H_4

There are two industrial variants:

Type	Conditions	Typical products
Catalytic	zeolite (aluminosilicate) catalyst, about 450 to 500 degrees C, moderate pressure	mostly branched and aromatic alkanes/alkenes, high-octane petrol
Thermal	no catalyst, high temperature (700 to 900 degrees C) and high pressure	a higher proportion of small, unbranched alkenes such as ethene

Both processes must exclude air; in the presence of oxygen the hydrocarbon feedstock would combust rather than crack.

Cracking calculation and product identification

Worked example: cracking hexadecane.

Hexadecane, $C_{16}H_{34}$ , is catalytically cracked into one molecule of decane and a mixture of shorter alkenes. Write a balanced equation if the other products are propene and ethene, and identify the type of reaction.

Decane accounts for 10 carbons and 22 hydrogens ( $C_{10}H_{22}$ ).
Remaining carbons: $16 - 10 = 6$ . Remaining hydrogens: $34 - 22 = 12$ .
Propene ( $C_3H_6$ ) and ethene ( $C_2H_4$ ) together give $3 + 2 = 5$ carbons and $6 + 4 = 10$ hydrogens, one short on both counts, so a second propene is needed instead of a single ethene: try decane + 2 propene:
Carbons: $10 + 2(3) = 16$ . Hydrogens: $22 + 2(6) = 34$ . This balances exactly.

C_{16}H_{34} \rightarrow C_{10}H_{22} + 2C_3H_6

Reaction type. This is cracking: a carbon-to-carbon bond-breaking reaction. Catalytic cracking (zeolite catalyst, about 450 to 500 degrees C) favours exactly this kind of branched, mixed-product outcome; a simple thermal crack at very high temperature would more often give a larger share of ethene.

Marker's note: when a cracking equation "does not balance" with the first guess of products, check both carbon AND hydrogen counts systematically rather than guessing again; the coefficient on a smaller alkene product is the most common place an extra multiple is needed.

Biofuels as an alternative source

Because petroleum is a finite, non-renewable resource, biofuels offer an alternative route to hydrocarbon-derived fuels using recently grown biomass instead of crude oil.

Bioethanol is produced by fermentation: yeast (providing the enzyme zymase) converts glucose from sugar or starch crops into ethanol and carbon dioxide, under anaerobic conditions at 25 to 37 degrees C:

C_6H_{12}O_6 \xrightarrow{\text{zymase}} 2C_2H_5OH + 2CO_2

Fermentation alone only produces a dilute aqueous solution (ethanol becomes toxic to yeast above roughly 12 to 15 percent v/v, halting fermentation), so fractional distillation is used afterwards to concentrate the ethanol to a fuel-usable purity (commonly about 95 percent, the ethanol-water azeotrope).

Biodiesel is produced by transesterification: a vegetable oil or animal fat (a triglyceride ester) reacts with methanol, catalysed by a base such as NaOH, to give methyl esters of the fatty acids (biodiesel) plus glycerol as a by-product.

Biofuel checkpoints

Bioethanol source reaction: fermentation of glucose by yeast (zymase), anaerobic, 25 to 37 degrees C; $C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2$ .
Why anaerobic: aerobic respiration would oxidise the glucose fully to $CO_2$ and $H_2O$ instead of yielding ethanol.
Concentration limit: fermentation broth plateaus at roughly 12 to 15 percent v/v ethanol because higher concentrations poison the yeast; fractional distillation concentrates it further.
Renewability argument: biofuels are considered closer to carbon-neutral because the $CO_2$ released on combustion was fixed from the atmosphere by the crop within the last growing season, unlike fossil carbon locked away for millions of years.
Key limitation: land and water competition with food crops, and a lower energy density than petroleum fuels (more volume needed for the same energy output).

Exam technique for sourcing and separation questions

When asked to EXPLAIN fractional distillation, always connect the observation (fractions condense at different heights) back to the cause (dispersion forces strengthening with chain length, hence boiling point rising with chain length) - do not just describe the column without the intermolecular-force reasoning. When asked to justify WHY cracking is used, name the specific commercial mismatch (surplus of long-chain fractions, shortfall of short-chain/petrochemical-feedstock fractions) rather than a vague "to make more useful products". For a cracking equation, balance BOTH carbon and hydrogen counts explicitly as working, not just the final equation.

Common traps

Calling fractional distillation a chemical reaction: It is purely physical: only intermolecular forces are overcome, no covalent bonds break.
Forgetting to exclude oxygen during cracking: Without stating "in the absence of air", a cracking description risks describing combustion instead.
Confusing catalytic and thermal cracking conditions: Catalytic cracking uses a zeolite catalyst at a LOWER temperature (about 450 to 500 degrees C); thermal cracking uses NO catalyst at a much HIGHER temperature (700 to 900 degrees C). Swapping these loses marks.
Writing fermentation as aerobic: Fermentation to ethanol must be anaerobic; aerobic conditions give full respiration to $CO_2$ and $H_2O$ instead.
Treating fermentation as sufficient to produce fuel-grade ethanol: Fermentation alone caps out at roughly 12 to 15 percent v/v; fractional distillation is still required to concentrate it.

Examples in context

Example 1. Fractional distillation and cracking at the former Kurnell refinery. Until its 2014 conversion to an import terminal, Caltex's Kurnell refinery separated Bass Strait and imported crude oil by fractional distillation, then cracked surplus heavy fractions to boost petrol and petrochemical feedstock output. Steam (thermal) cracking of ethane gave ethene plus hydrogen, $C_2H_6 \rightarrow C_2H_4 + H_2$ , with the ethene feeding an adjacent ethanol and polyethene unit. The same boiling-point-based separation and bond-breaking chemistry taught for the HSC ran at industrial scale on this site for decades.

Example 2. Bioethanol blending under the NSW E10 mandate. Under the NSW biofuels mandate, some standard unleaded petrol sold in NSW service stations is blended with up to 10 percent ethanol (E10), produced by fermenting sugar cane molasses or grain starch and then purifying by fractional distillation to fuel-grade purity. This is a direct real-world application of the Module 7 sourcing dot point: a renewable, fermentation-derived hydrocarbon-oxygenate fuel blended alongside petroleum-derived petrol from fractional distillation and cracking.

Try this

Q1. Explain why the petrol fraction condenses higher up a fractionating column than the diesel fraction. [3 marks]

Cue. Petrol is shorter-chain, weaker dispersion forces, lower boiling point, so it stays as vapour until it reaches a cooler (higher) part of the column before condensing; diesel's stronger dispersion forces give it a higher boiling point, so it condenses lower down where it is still hot.

Q2. Write a balanced equation for the thermal cracking of octane ( $C_8H_{18}$ ) into butane and two alkene fragments of your choosing, showing that both carbon and hydrogen balance. [3 marks]

Cue. E.g. $C_8H_{18} \rightarrow C_4H_{10} + 2C_2H_4$ ; carbon check $4 + 2(2) = 8$ , hydrogen check $10 + 2(4) = 18$ .

Q3. A fermentation vat converts 900 g of glucose ( $M = 180.2\ \text{g mol}^{-1}$ ) completely to ethanol ( $M = 46.08\ \text{g mol}^{-1}$ ). Calculate the theoretical mass of ethanol produced, to 3 significant figures. [4 marks]

Cue. $n(\text{glucose}) = 900 / 180.2 = 4.995$ mol; $n(\text{ethanol}) = 2 \times 4.995 = 9.989$ mol (2:1 ratio from $C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2$ ); $m(\text{ethanol}) = 9.989 \times 46.08 = 460$ g.

Practice questions

Original practice questions graded from foundation to exam level, each with a full worked solution. Try them before revealing the solution.

foundation3 marks

Explain, in terms of intermolecular forces, why crude oil can be separated into fractions by fractional distillation.

Show worked solution →

A 3-mark EXPLAIN needs the cause (bonding/forces) linked to the observed effect (separation by boiling point).

Crude oil is a mixture of hydrocarbons of different chain lengths. The only intermolecular forces between hydrocarbon molecules are dispersion (London) forces, which strengthen as the number of carbon atoms (and hence the surface area of the molecule) increases. Longer-chain hydrocarbons therefore need more energy to separate their molecules and boil at higher temperatures than shorter-chain hydrocarbons.

In the fractionating column, crude oil vapour rises and cools. Each fraction condenses back to a liquid once the column temperature falls below that fraction's boiling point range, so different chain-length fractions condense out and are drawn off at different heights, from short-chain gases near the top to long-chain bitumen at the bottom.

Marking criteria: 1 mark for identifying dispersion forces as the relevant intermolecular force, 1 mark for linking chain length to force strength and boiling point, 1 mark for explaining that condensation at different heights (temperatures) is what achieves the separation.

foundation3 marks

Write a balanced equation for the catalytic cracking of dodecane (

C_{12}H_{26}

) into hexane and two molecules of ethene, and state the catalyst and approximate temperature used.

Show worked solution →

Balancing the equation. Dodecane has 12 carbons. Hexane accounts for 6 carbons, and two ethene molecules account for a further 4 carbons (2 x 2), so an additional 2-carbon fragment is needed to conserve mass; check the hydrogen count to find the third product.

Dodecane $C_{12}H_{26}$ has $2(12) + 2 = 26$ H, consistent with a saturated alkane. Hexane $C_6H_{14}$ has $6$ carbons and $14$ hydrogens. Two ethene molecules $2 \times C_2H_4$ contribute $4$ carbons and $8$ hydrogens. Remaining: $12 - 6 - 4 = 2$ carbons and $26 - 14 - 8 = 4$ hydrogens, which is a further ethene molecule ( $C_2H_4$ ).

C_{12}H_{26} \rightarrow C_6H_{14} + 3C_2H_4

Carbon check: $6 + 3(2) = 12$ . Hydrogen check: $14 + 3(4) = 26$ . Balanced.

Catalyst and conditions. A zeolite (aluminosilicate) catalyst, at approximately 450 to 500 degrees C and moderate pressure (catalytic cracking).

Marking criteria: 1 mark for a correctly balanced equation (any consistent set of shorter alkane/alkene products with matching atom counts is acceptable), 1 mark for naming a zeolite/aluminosilicate catalyst, 1 mark for a temperature in the correct catalytic-cracking range.

core6 marks

A refinery cracks 500 kg of a heavy naphtha fraction, modelled as pure hexadecane (

C_{16}H_{34}

M = 226.4\ \text{g mol}^{-1}

), completely into octane (

C_8H_{18}

M = 114.2\ \text{g mol}^{-1}

) and octene (

C_8H_{16}

M = 112.2\ \text{g mol}^{-1}

) in a 1:1 mole ratio. Calculate the theoretical mass of octene produced, to 3 significant figures, and comment on one reason the actual industrial yield would be lower.

Show worked solution →

Step 1: write the balanced cracking equation.

C_{16}H_{34} \rightarrow C_8H_{18} + C_8H_{16}

Carbon check: $8 + 8 = 16$ . Hydrogen check: $18 + 16 = 34$ . Balanced, 1:1:1 mole ratio.

Step 2: convert the feedstock mass to moles.

n(C_{16}H_{34}) = \frac{m}{M} = \frac{500\,000\ \text{g}}{226.4\ \text{g mol}^{-1}} = 2208.5\ \text{mol}

Step 3: apply the 1:1 mole ratio to octene.

n(C_8H_{16}) = n(C_{16}H_{34}) = 2208.5\ \text{mol}

Step 4: convert moles of octene to mass.

m(C_8H_{16}) = n \times M = 2208.5\ \text{mol} \times 112.2\ \text{g mol}^{-1} = 247\,795\ \text{g}

Step 5: round to 3 significant figures and convert to kg (matching the 500 kg data, 3 s.f.).

m(C_8H_{16}) = 2.48 \times 10^{2}\ \text{kg}

Comment on yield. This is a THEORETICAL (100 percent) yield assuming complete, single-product cracking. Actual industrial cracking produces a distribution of many different chain-length alkanes and alkenes (not just one clean pair of products), and side reactions (over-cracking to smaller fragments, or coke/carbon deposition on the catalyst) reduce the real mass of any single named product obtained.

Marking criteria: 1 mark for the correctly balanced 1:1:1 equation, 1 mark for correct moles of feedstock, 1 mark for correctly applying the 1:1 ratio, 1 mark for the mass calculation, 1 mark for the final answer to 3 significant figures with correct units, 1 mark for a valid, specific reason the real yield would be lower.

core5 marks

The graph below is an illustrative fractional distillation temperature profile for a crude oil column, showing column temperature against height above the base. Describe the relationship shown, identify the two labelled fractions, and explain why the gasoline fraction is drawn off higher in the column than the diesel fraction.

Show worked solution →

Reading the graph: Temperature falls steadily from a high value at the base of the column to a low value at the top. Two points are labelled on the curve: one at high temperature, low height (diesel fraction) and one at lower temperature, greater height (gasoline fraction).
Description of the relationship: Column temperature decreases continuously with increasing height above the base, consistent with the column being hottest where the heated crude oil vapour enters at the bottom and coolest at the top where vapour exits to be condensed or continues to distillation columns downstream.
Identifying the fractions: The point at high temperature and low height corresponds to the diesel fraction (longer-chain hydrocarbons, higher boiling point, condenses low in the column). The point at lower temperature and greater height corresponds to the gasoline (petrol) fraction (shorter-chain hydrocarbons, lower boiling point).
Explanation: Gasoline-range hydrocarbons (roughly C5 to C10) have weaker dispersion forces between molecules than the longer diesel-range hydrocarbons (roughly C12 to C20), so gasoline has a lower boiling point. Vapour must rise further up the column, cooling as it goes, before the temperature falls low enough for the gasoline fraction to condense; the diesel fraction reaches its (higher) condensation temperature much sooner, closer to the hot base of the column.

Marking criteria: 1 mark for describing the temperature-height trend, 1 mark for correctly identifying each labelled fraction, 1 mark for linking boiling point to chain length/dispersion forces, 1 mark for explaining why the weaker-force fraction condenses higher up, 1 mark for a coherent overall explanation connecting all of the above.

core4 marks

Outline the industrial production of bioethanol from a starch or sugar crop, including the role of enzymes, the conditions required, and one step needed to obtain a higher-purity fuel-grade product.

Show worked solution →

Step 1: source the sugar. A crop such as sugar cane (sucrose) or corn/wheat (starch) is processed; starch is first hydrolysed by the enzyme amylase into glucose.

Step 2: fermentation. Yeast is added, providing the enzyme zymase, which converts glucose to ethanol and carbon dioxide under anaerobic conditions at 25 to 37 degrees C:

C_6H_{12}O_6 \xrightarrow{\text{zymase}} 2C_2H_5OH + 2CO_2

Step 3: limitation. Fermentation alone typically yields only a dilute aqueous ethanol solution (up to roughly 12 to 15 percent v/v) because higher ethanol concentrations are toxic to the yeast and halt fermentation.

Step 4: purification. Fractional distillation is used to concentrate and purify the ethanol from the fermentation broth (typically to about 95 percent, the ethanol-water azeotrope, for fuel-grade use).

Marking criteria: 1 mark for identifying the correct sugar source and enzyme(s) involved, 1 mark for a correct fermentation equation with anaerobic conditions and temperature range, 1 mark for stating the concentration limitation and its cause, 1 mark for naming fractional distillation as the purification step.

exam7 marks

Evaluate the effectiveness of biofuels, compared with petroleum-derived fuels obtained by fractional distillation and cracking, as a long-term strategy for supplying Australia's liquid transport fuel needs.

Show worked solution →

This is a 7-mark EVALUATE: markers reward a balanced judgement with specific evidence on both sides, not a one-sided list.

Band 6 PLAN.

Thesis: biofuels offer meaningful advantages in renewability and carbon accounting but cannot, on current technology and land availability, fully replace petroleum-derived fuels as Australia's primary liquid transport fuel source in the near term; they are most effective as a blended supplement rather than a full substitute.
Petroleum route: crude oil, separated by fractional distillation into a fixed natural distribution of fractions, does not match market demand (surplus of heavy fractions, shortfall of petrol/petrochemical feedstock); cracking (catalytic or thermal) converts long-chain alkanes into shorter, more valuable alkanes and alkenes, giving refineries flexibility to meet demand from a finite, non-renewable resource.
Biofuel route: bioethanol from fermentation of sugar/starch crops, and biodiesel from transesterification of vegetable oils/fats, are renewable on a human timescale and closer to carbon-neutral because the CO2 released on combustion was fixed from the atmosphere by the growing crop in the recent past, unlike fossil carbon that has been sequestered for millions of years.
Advantages of biofuels: renewability, reduced net fossil CO2 contribution, domestic production reducing import reliance, use of agricultural by-products/waste feedstocks.
Limitations of biofuels: competition with food crops for arable land and water (the food-versus-fuel debate), lower energy density than petrol/diesel meaning more fuel volume needed for the same energy output and reduced vehicle range, fermentation yields require a further fractional distillation purification step (added energy/processing cost), and current biofuel production volumes are far too small relative to national transport fuel demand to replace petroleum fuels outright.
Judgement: biofuels are effective as a partial, blended solution (e.g. E10 fuel) that reduces net fossil fuel use, but a full transition would require land, water and yield improvements not currently available at the scale of national demand; petroleum-derived fuels, refined and upgraded by fractional distillation and cracking, remain the dominant practical source in the medium term despite being non-renewable.

Model paragraph (excerpt). Fractional distillation and cracking give the petroleum industry a reliable and flexible way to convert a finite crude oil resource into the specific mix of fuels the market demands, but the resource itself is non-renewable and its combustion releases fossil carbon that has been locked away for millions of years. Biofuels solve the renewability and carbon-accounting problem by using recently fixed atmospheric carbon, yet fermentation-based bioethanol production is constrained by yeast's tolerance for ethanol (requiring a further distillation step to reach fuel-grade purity) and by the land and water needed to grow feedstock crops at a scale that could challenge food production. On balance, biofuels are best understood as a valuable blending component that reduces net fossil fuel demand rather than a wholesale replacement for petroleum-derived fuels in the short to medium term.

Marker's note: top-band answers (1) correctly describe BOTH processes (fractional distillation/cracking AND biofuel production) with accurate chemistry, (2) give specific, evidence-based advantages and limitations rather than vague claims, (3) reach an explicit, reasoned judgement rather than a neutral "it depends", and (4) keep the judgement grounded in Australia's specific context (land, water, import reliance) as the question asks.

Practise this

Sources & how we know this

Chemistry Stage 6 Syllabus (2017) — NESA
Chemistry HSC exam papers — NESA