Carbon families and crude oil: homologous series and fractional distillation
By the end of this lesson you will be able to
- Identify an alkane, alkene, alcohol or carboxylic acid from its name, formula or structure, and state the general formula of each family.
- Outline what a homologous series is and deduce the formula of any member from its number of carbon atoms or its molar mass.
- Describe how crude oil formed and explain why it is a finite resource.
- Explain how fractional distillation separates crude oil, using the boiling points of the fractions and the temperature gradient in the column.
- Explain the trend in boiling point, viscosity and flammability with chain length using forces between molecules, and analyse boiling-point data with values.
You met this before
- A carbon atom has four outer electrons (2,4), so it forms four covalent bonds — four shared pairs of electrons; hydrogen forms one, oxygen two.
- Substances made of small molecules (simple molecular substances) have low boiling points: boiling separates whole molecules from each other and breaks no covalent bonds.
- Distillation separates liquids with different boiling points: the liquid with the lower boiling point evaporates first, then condenses in a cooler place.
- Burning a fuel is an exothermic reaction; the energy released warms water by E = m × c × ΔT, with c = 4.18 J g⁻¹ °C⁻¹.
- Names of organic molecules (Unit 6 of this course, Chemical nomenclature): the stem counts the carbons — meth- 1, eth- 2, prop- 3, but- 4, pent- 5, hex- 6, hept- 7, oct- 8 — and the ending gives the family: -ane, -ene, -anol, -anoic acid.
Draw the dot-and-cross structure of methane, CH₄, in words: how many shared pairs does the carbon atom have, and why does methane boil at −162 °C although its C–H bonds are strong?
Reveal
Four shared pairs — one to each hydrogen — giving carbon eight outer electrons. The C–H bonds are strong, but boiling does not break them: it only separates one CH₄ molecule from the next.
The forces between methane molecules are very weak, so little energy is needed and methane boils far below room temperature. That distinction — bonds inside molecules, forces between them — runs through this whole lesson.
Revise it in full: Year 3 · Atomic structure and bonding · Lesson 4 — Covalent bonding: shared pairs, small molecules and giant lattices · Year 1 · Everyday materials · Lesson 4 — Separating mixtures · Year 3 · Energy and chemical change · Lesson 5 — Change that people use
The puzzle
A single barrel of crude oil — 159 litres of a thick, black, foul-smelling liquid — can end up as the gas in a cooking cylinder in Francistown, the petrol in a taxi in Lagos, the jet fuel of a flight out of Johannesburg, the diesel of a mine truck at Jwaneng, and the black tar under the A1 highway.
Nobody makes these in a reaction: they are all in the barrel already, mixed together. How can one tower of steel pull them apart, sending jet fuel out of one pipe and petrol out of another, using nothing but heat?
Crude oil contains molecules with 1 to more than 40 carbon atoms. Predict which ones come out at the top of a tall, hot tower and which at the bottom, and give a reason in terms of boiling point.
Compare your answer
The smallest molecules (1–4 carbons) leave at the top as gases; the largest (over 40 carbons) never boil and leave at the bottom as a thick residue. Small molecules have low boiling points, so they stay as vapour even where the tower is coolest.
Big molecules condense as soon as they rise into a region cooler than their boiling point. If your reason used the word “lighter” rather than “lower boiling point”, keep reading: mass is not the cause — the forces between molecules are.
The science — structured notes
1.1 Why carbon builds families
Organic chemistry is the chemistry of carbon compounds (apart from a few simple ones such as CO₂ and carbonates). More than ten million are known, far more than all other elements’ compounds together. Two facts about carbon explain the number.
- Carbon forms four strong covalent bonds, so each atom can join to up to four others.
- Carbon atoms bond strongly to each other, forming chains, branches and rings of almost any length, with single, double or triple bonds.
A hydrocarbon contains hydrogen and carbon only. A hydrocarbon with only single C–C bonds is saturated: every carbon holds as many hydrogen atoms as it can. One with a C=C double bond is unsaturated: it could take on more atoms by opening the double bond (Lesson 3).
1.2 Homologous series and functional groups
Organic compounds fall into families called homologous series. The members of one series:
- have the same general formula;
- differ from the next member by a CH₂ unit (14 g mol⁻¹);
- contain the same functional group — the atom or group of atoms that decides how the molecule reacts — so they have similar chemical properties;
- show a gradual trend in physical properties, such as boiling point rising with chain length.
The IB content for this unit names four series. Learn the functional group, the ending of the name and the general formula of each, and learn to spot the group in a structure you have never seen, because that is exactly how the on-screen papers test it.
| Series | Functional group | Name ending | General formula | First members |
|---|---|---|---|---|
| Alkanes | none (C–C single bonds only) | -ane | CₙH₂ₙ₊₂ | methane CH₄, ethane C₂H₆, propane C₃H₈ |
| Alkenes | C=C double bond | -ene | CₙH₂ₙ (n ≥ 2) | ethene C₂H₄, propene C₃H₆, butene C₄H₈ |
| Alcohols | –OH (hydroxyl) | -anol | CₙH₂ₙ₊₁OH | methanol CH₃OH, ethanol C₂H₅OH, propanol C₃H₇OH |
| Carboxylic acids | –COOH (carboxyl) | -anoic acid | CₙH₂ₙ₊₁COOH (n ≥ 0) | methanoic acid HCOOH, ethanoic acid CH₃COOH, propanoic acid C₂H₅COOH |
Two traps in the table. The carbon of –COOH counts in the name: CH₃COOH has two carbons, so it is ethanoic acid, not methanoic acid. And an alcohol is not an alkali: the –OH is covalently bonded to carbon, not a hydroxide ion, so ethanol solution is neutral.
State the name and the homologous series of each: (i) C₅H₁₂; (ii) C₃H₆; (iii) C₄H₉OH; (iv) C₂H₅COOH.
(i) Test the alkane formula CₙH₂ₙ₊₂ with n = 5: 2 × 5 + 2 = 12 hydrogens. It fits, so C₅H₁₂ is pentane, an alkane.
(ii) Three carbons and six hydrogens is CₙH₂ₙ, so propene, an alkene.
(iii) –OH on a C₄H₉ chain is CₙH₂ₙ₊₁OH with n = 4: butanol, an alcohol.
(iv) Count every carbon, including the one in –COOH: 2 + 1 = 3, so propanoic acid, a carboxylic acid.
Part (i) is the May 2021 Chemistry question almost exactly; its markscheme gave one mark for “pentane” and one for “alkane”.
A leak detector on a gas pipeline identifies an alkane of molar mass 58.1 g mol⁻¹, and a sample from a fuel drum is an alcohol of molar mass 60.1 g mol⁻¹. Deduce both formulae. Use the relative atomic masses C 12.0, H 1.0 and O 16.0.
Alkane. Molar mass of CₙH₂ₙ₊₂ = 12.0n + 1.0(2n + 2)
Molar mass = 14.0n + 2.0
14.0n + 2.0 = 58.1, so 14.0n = 56.1
n = 4.0, so the alkane is C₄H₁₀, butane (the gas in a lighter).
Alcohol. Molar mass of CₙH₂ₙ₊₁OH = 12.0n + 1.0(2n + 1) + 16.0 + 1.0
Molar mass = 14.0n + 18.0
14.0n + 18.0 = 60.1, so 14.0n = 42.1
n = 3.0, so the alcohol is C₃H₇OH, propanol.
The 14.0 in both is the CH₂ step of the homologous series: every member is 14 g mol⁻¹ heavier than the one before.
Identify the homologous series of each: (i) CH₃CH₂CH₂OH; (ii) HCOOH; (iii) C₆H₁₂ with one double bond; (iv) CH₃CH₂CH₃. Then give the molecular formula of the alkane with 20 carbon atoms.
Reveal answer
(i) alcohol (propanol); (ii) carboxylic acid (methanoic acid — the ant-sting acid, and the “molecule B” of the May 2023 paper); (iii) alkene (hexene); (iv) alkane (propane).
C₂₀: H = 2 × 20 + 2
H = 42, so the alkane is C₂₀H₄₂ (icosane, part of the diesel fraction).
“Ethanol, C₂H₅OH, contains an –OH group, so its solution in water is alkaline, like sodium hydroxide.”
Reveal answer
False. In sodium hydroxide the OH is a hydroxide ion, OH⁻, held by ionic bonding and released into water. In ethanol the O–H is covalently bonded to a carbon atom and stays in the molecule, so ethanol solution is neutral (pH 7).
The functional group is called hydroxyl, not hydroxide, precisely to keep them apart.
1.3 Crude oil: a finite store of ancient sunlight
Crude oil (petroleum) is a mixture of hundreds of hydrocarbons, mostly alkanes, with some rings and small amounts of sulfur and nitrogen compounds. It formed over tens to hundreds of millions of years from the remains of tiny marine organisms — plankton — buried in mud on the sea floor where there was too little oxygen for them to decay.
Layers of sediment piled on top; heat and pressure slowly cracked the dead material into hydrocarbons. The oil seeped upwards through porous rock until an impermeable cap rock trapped it.
That is why crude oil, natural gas and coal are called fossil fuels and why they are finite (non-renewable): we are burning in about two centuries what took hundreds of millions of years to form, and none of it is being replaced on a human timescale. The energy they release when burned was first captured from sunlight by photosynthesis in organisms long dead.
Crude oil and the ethanol made from sugar cane both release energy when burned. Explain why one is called finite and the other renewable, although both contain carbon captured from the air by photosynthesis.
Reveal answer
Crude oil formed from organisms that captured carbon hundreds of millions of years ago; it takes millions of years to form, so once burned it is not replaced on any human timescale — finite.
Sugar cane captures carbon from today’s air and grows again in about a year, so the fuel can be replaced as fast as it is used — renewable. (Renewable does not automatically mean carbon-neutral: fertiliser, transport and processing still use energy, as Lesson 5 shows.)
1.4 Boiling point and chain length: forces between molecules
Every molecule in crude oil is held together inside by strong covalent bonds. But the molecules are attracted to each other only by weak intermolecular forces. Boiling separates the molecules; it breaks no C–C or C–H bonds.
The longer the chain, the more electrons it has and the more surface it presses against its neighbours, so the intermolecular forces are stronger. More energy — a higher temperature — is needed to pull molecules apart.
Read the graph with numbers, not just “it goes up”. From methane to ethane the boiling point rises by 73 °C; from nonane to decane by only 23 °C. The rise is steady in direction but shrinking in size, so the relationship is not proportional (decane has ten times the carbons of methane but its boiling point is not ten times anything).
The same forces explain three other trends down the fractions: longer chains are more viscous (they tangle and hold on to each other, so they flow slowly), less volatile (fewer molecules escape as vapour at room temperature) and harder to ignite (too little vapour mixes with the air near a flame).
Butane, C₄H₁₀, is sold as a liquid in lighter refills but comes out as a gas. Using the graph, explain why butane is a gas at 20 °C, and why the liquid in the refill is under pressure.
Reveal answer
Butane boils at −1 °C at 101 kPa. At 20 °C, which is above its boiling point, its molecules have enough energy to overcome the weak forces between them, so at atmospheric pressure it is a gas.
Squeezing the molecules together (about 2 bar at room temperature) lets the intermolecular forces hold them as a liquid; when the valve opens, the pressure drops and the liquid boils. That is why a lighter refill feels cold — the evaporating butane takes energy from its surroundings.
“The graph shows that boiling point is directly proportional to the number of carbon atoms, because as one goes up the other goes up.”
Reveal answer
Two errors. First, a rise together is a positive correlation, not proportionality. Directly proportional needs a straight line through the origin, with doubling one doubling the other: in °C the values even pass through zero near butane and are negative before it, so the test fails immediately.
Second, even in kelvin (112 K for methane, 447 K for decane) ten times the carbons gives four times the boiling point. Better: “Boiling point increases with chain length, from −162 °C for CH₄ to 174 °C for C₁₀H₂₂, with each extra CH₂ adding less than the one before.”
1.5 Fractional distillation: separating by boiling point
Crude oil is heated in a furnace to about 350–400 °C, so most of it vaporises, and the hot vapour enters near the bottom of a fractionating column up to 60 m tall. The column is hot at the bottom and cool at the top — a temperature gradient.
- The vapour rises through holes in horizontal trays, cooling as it climbs.
- Each substance condenses on the tray where the temperature falls below its boiling point, and is piped off from that level.
- Molecules with the lowest boiling points (the shortest chains) never condense and leave from the top as gases.
- The longest chains never vaporise: they flow out of the bottom as a thick residue.
Each product is a fraction — a mixture of hydrocarbons with similar numbers of carbon atoms and a range of boiling points, not a pure substance. The ranges in the table are typical; refineries adjust them.
| Fraction (top to bottom) | Typical chain length | Approximate boiling range / °C | Main uses |
|---|---|---|---|
| Refinery gases | C₁–C₄ | below 25 | LPG for cooking and heating; feedstock for ethene and propene |
| Petrol (gasoline) | C₅–C₁₀ | 40–175 | fuel for cars |
| Naphtha | C₆–C₁₁ | 70–200 | feedstock for cracking: chemicals and plastics |
| Kerosene (paraffin) | C₁₀–C₁₆ | 150–275 | jet fuel; lamps and stoves |
| Diesel (gas oil) | C₁₄–C₂₀ | 250–350 | trucks, buses, trains, generators |
| Fuel oil | C₂₀–C₄₀ | above 350 | ships, industrial boilers, power stations |
| Bitumen (residue) | above C₄₀ | does not vaporise | road surfaces, roofing |
Dodecane, C₁₂H₂₆, boils at 216 °C. Identify the fraction it will be collected in and explain why it does not leave the column with the petrol.
Match the data. 216 °C lies in the kerosene range (150–275 °C), and 12 carbons lies in C₁₀–C₁₆: dodecane is in kerosene.
Explain with the gradient. As dodecane vapour rises it reaches trays cooler than 216 °C and condenses there, so it cannot climb to the petrol trays, which are cooler still.
Explain with particles. Its 12-carbon chains have stronger intermolecular forces than the C₅–C₁₀ molecules of petrol, so they need a higher temperature to stay as vapour.
Use the table to name the fraction that would contain: (i) heptane, C₇H₁₆; (ii) C₃₀H₆₂; (iii) a hydrocarbon mixture boiling between 260 and 330 °C; (iv) propane.
Reveal answer
(i) petrol (C₅–C₁₀; heptane boils at 98 °C); (ii) fuel oil (C₂₀–C₄₀); (iii) diesel (250–350 °C); (iv) refinery gases — propane is sold as LPG.
The residue from the bottom of the column still contains useful fuel oil and lubricating oil, but heating it above about 400 °C starts to crack and char the molecules. Refineries send it to a second column run at a much lower pressure. What if the pressure is lowered: predict the effect on the boiling points, and explain why this helps.
Reveal answer
Lower pressure lowers every boiling point: the molecules need less energy to escape because fewer gas molecules press back on the liquid surface (the same reason water boils at about 97 °C in Gaborone).
So the heavy molecules vaporise and separate at temperatures below those at which they would decompose. This is vacuum distillation, and it is how bitumen and lubricating oils are made.
1.6 Fuels: what makes a good one
Most fractions are burned as fuels. A good fuel releases a lot of energy per kilogram (for a vehicle) or per cubic metre (for a tank), ignites easily but safely, burns cleanly, is easy to store and transport, and is cheap. No fuel wins on all of these, which is why the column makes several.
LPG (mainly propane and butane) stores as a liquid under modest pressure, so a 9 kg cylinder cooks for weeks. Kerosene has a high energy per kilogram and is much less volatile than petrol, so it does not boil off at the low pressures of high altitude and is safer to store in large tanks, which is why jet engines burn it.
Complete combustion releases 2 220 kJ per mole of propane (C₃H₈) and 5 470 kJ per mole of octane (C₈H₁₈). Which releases more energy per gram? Use C = 12.0, H = 1.0.
Molar masses. M(C₃H₈) = 3 × 12.0 + 8 × 1.0
M(C₃H₈) = 44.0 g mol⁻¹
M(C₈H₁₈) = 8 × 12.0 + 18 × 1.0
M(C₈H₁₈) = 114.0 g mol⁻¹
Energy per gram. Propane: 2 220 kJ mol⁻¹ ÷ 44.0 g mol⁻¹
Propane: 50.5 kJ g⁻¹
Octane: 5 470 kJ mol⁻¹ ÷ 114.0 g mol⁻¹
Octane: 48.0 kJ g⁻¹
Answer. Octane releases more per mole (it is a bigger molecule) but propane slightly more per gram, because a larger share of its mass is hydrogen, and hydrogen releases far more energy per gram when it burns than carbon does. The difference is small: all alkanes release about 46–56 kJ g⁻¹.
A family in Maun cooks on a 9.0 kg LPG cylinder. Assume LPG is pure propane releasing 50.5 kJ g⁻¹ and that 40 % of the energy reaches the food. Each day they boil 6.0 kg of water from 25 °C to 100 °C. Calculate how many days the cylinder lasts, and suggest one reason why the real figure is shorter.
Reveal answer
Energy needed per day: E = m × c × ΔT
E = 6 000 g × 4.18 J g⁻¹ °C⁻¹ × 75 °C
E = 1 881 000 J, which is 1 881 kJ
Energy from the fuel per day = energy needed ÷ efficiency
Energy from the fuel per day = 1 881 kJ ÷ 0.40
Energy from the fuel per day = 4 703 kJ
Mass of propane per day = 4 703 kJ ÷ 50.5 kJ g⁻¹
Mass of propane per day = 93.1 g
Days = 9 000 g ÷ 93.1 g per day
Days = 97 days (about 3 months)
Real cylinders run out sooner: the family also cooks food, which needs energy for the food itself and for the water that boils away as steam (a large latent heat), and wind round an open burner lowers the 40 %.
Try it — how does chain length set the boiling point?
The lab warms a tube of each alkane from methane (1 carbon) to decane (10 carbons) with a temperature probe held just above the liquid, and reads the steady boiling temperature in kelvin (K = °C + 273). You can switch between the straight chain and its branched isomer (the 2-methyl version, same formula), and between sea-level pressure (101 kPa) and Gaborone (90 kPa).
The probe reads to 0.1 K; the model adds a small random scatter, as a real probe would.
Your means for the straight chains at 101 kPa were: C₅ 309.2 K, C₆ 341.8 K, C₇ 371.3 K, C₈ 398.7 K. The branched isomer with 8 carbons gave 391.1 K.
(a) Calculate the three increases and describe the pattern.
(b) Explain why the branched C₈ isomer boils lower although it has the same molecular formula, C₈H₁₈.
Reveal answer
(a) 341.8 − 309.2 = 32.6 K
371.3 − 341.8 = 29.5 K
398.7 − 371.3 = 27.4 K
Each extra CH₂ raises the boiling point, but by less each time, so the curve rises and flattens.
(b) Same formula, same number of electrons — but the branched molecule is more compact, so it cannot lie as closely along its neighbours as a straight chain. There is less contact, the intermolecular forces are weaker, and less energy (a lower temperature, 7.6 K lower) is needed to separate the molecules.
Common mistakes to avoid
- “Boiling breaks the bonds” — boiling separates molecules; the covalent bonds inside them are untouched. Say “intermolecular forces”.
- Counting the carbons of –COOH wrongly — CH₃COOH is ethanoic acid (two carbons), not methanoic acid.
- Calling a fraction a compound — each fraction is a mixture with a range of boiling points.
- “Heavier molecules sink to the bottom of the column” — the column separates by boiling point, not by weight; the long chains condense low down because that is where it is hot enough for them to condense and no higher.
- Mixing up hydroxyl and hydroxide — the –OH of an alcohol is covalent; alcohols are neutral.
- Correlation called proportion — “boiling point increases with chain length” is a trend; proportional needs a straight line through the origin.
- Using “amount” for a measured quantity — say “mass of fuel” or “volume of fraction”; examiners accept “amount” only for time or number of moles.
Study smart: how this is examined
Study routine. Today: make one card with the four families (group, ending, general formula, first member) and one with the seven fractions; redo both EXAMPLES in section 1.2 from memory. In two days: explain fractional distillation aloud in three sentences using “temperature gradient” and “condenses”, then do the Spot-the-errors check without looking.
In a week: run the lab as a full investigation, then the Think task and the quiz.
How we found out — from “vital force” to carbon chains
Until the early 1800s many chemists believed that the compounds of living things could be made only by living things, through a “vital force”. In 1828 Friedrich Wöhler in Germany heated ammonium cyanate, an inorganic salt, and obtained urea, a compound found in urine.
It did not kill vitalism overnight — the idea faded over decades as more organic compounds were made in the laboratory — but it showed that the chemistry of life follows the same rules as the rest of chemistry.
What those rules were came in 1858, when August Kekulé in Heidelberg and, independently, Archibald Scott Couper in Paris proposed that carbon is tetravalent (forms four bonds) and that carbon atoms join to each other in chains. Suddenly formulas such as C₄H₁₀ made sense as structures, and chemists could predict families of compounds differing by CH₂ — the homologous series.
The oil industry grew at the same time. In the 1850s the Polish pharmacist Ignacy Łukasiewicz distilled kerosene from seep oil to light a hospital in Lviv, and in 1859 Edwin Drake drilled the first commercial oil well at Titusville, Pennsylvania.
Early refineries wanted only the kerosene, for lamps, and burned or dumped the petrol fraction as a nuisance — until the motor car, around 1900, turned the “waste” into the most valuable fraction of all. What counts as useful in a barrel has always depended on what society needs.
Where it is used now
- Africa’s refineries. Nigeria is one of Africa’s largest oil producers, but for years exported crude and imported petrol. The Dangote refinery near Lagos, built to process about 650 000 barrels a day, began producing fuels in 2024 — a single fractionating complex on a scale that changes a continent’s fuel trade.
- Botswana’s fuel. Botswana has no crude oil refinery; its petrol, diesel, paraffin and LPG arrive already refined, mostly by road and rail through South Africa. Every litre in Gaborone was separated in a column like the one in section 1.5, hundreds of kilometres away.
- Not only crude oil. In South Africa, Sasol’s plant at Secunda makes petrol and diesel from coal: the coal is turned into carbon monoxide and hydrogen, which are rebuilt into hydrocarbon chains (the Fischer–Tropsch process) and then fractionally distilled like crude.
- Cooking. Across southern Africa LPG cylinders replace firewood and paraffin in many kitchens, cutting smoke indoors; the same C₃–C₄ fraction fills lighters and camping stoves.
- Roads and chemicals. Bitumen from the residue surfaces almost every tarred road; naphtha is cracked into ethene and propene, the starting point of most plastics (Lessons 2 and 4).
Science now — and the scientist you need to be
What is happening in 2026
The world still uses about 100 million barrels of crude oil a day, and the International Energy Agency has projected in recent reports that demand will level off around the end of this decade as electric vehicles spread. Refineries are adapting. Some have been converted to “biorefineries” that feed used cooking oil and animal fats, instead of crude, through the same kinds of units to make renewable diesel and jet fuel (Lesson 5).
Refinery control rooms now use machine-learning software trained on years of sensor data to adjust furnace temperatures and tray draw-offs, squeezing a little more of the most valuable fractions from each barrel — the same boiling-point physics, steered by data.
And chemists are asking a longer question: which carbon compounds must still come from a well once we stop burning oil? Plastics, medicines, lubricants and fertilisers need carbon atoms, not energy, and that carbon could come from plants, waste or captured CO₂ (Lessons 4 and 5).
The mindshift
What you need to be able to do
Spot the functional group in a molecule you have never seen and name its family; move between name, formula and general formula; place any alkane in a fraction from its chain length or boiling point; and explain boiling, viscosity and volatility with forces between molecules, quoting data. The on-screen papers ask for these in their first questions — fast marks you should never drop.
See an example
Petrol for a car: the petrol fraction, C₅–C₁₀. A polythene bag: poly(ethene), made from ethene cracked from naphtha or ethane (Lessons 2 and 4). Candle wax: paraffin wax, long alkanes (about C₂₀–C₄₀) from the heavy fractions — the C₂₅H₅₂ of the May 2021 paper.
A correct assistant answer calls kerosene a mixture of hydrocarbons, mostly C₁₀–C₁₆, boiling over a range of about 150–275 °C. A single boiling point for kerosene is a sign the answer treated a fraction as a compound.
Key terms
| Organic compound | A compound of carbon (except a few simple ones such as CO₂ and carbonates), usually containing hydrogen. |
| Hydrocarbon | A compound of hydrogen and carbon only. |
| Saturated / unsaturated | Saturated: only C–C single bonds. Unsaturated: at least one C=C double (or triple) bond. |
| Homologous series | A family with the same general formula and functional group, similar chemical properties, a trend in physical properties, and successive members differing by CH₂. |
| Functional group | The atom or group of atoms that gives a family its characteristic reactions: C=C, –OH, –COOH. |
| General formula | The formula that fits every member of a series, such as CₙH₂ₙ₊₂ for the alkanes. |
| Crude oil | A finite fossil fuel: a mixture of hydrocarbons formed from buried marine organisms over millions of years. |
| Fraction | A mixture of hydrocarbons with similar chain lengths and boiling points, collected together from a fractionating column. |
| Fractional distillation | Separating a mixture of liquids by boiling point in a column with a temperature gradient. |
| Intermolecular forces | Weak attractions between molecules; stronger for longer molecules, giving higher boiling points and viscosity. |
| Viscosity / volatility | How thick and slow-flowing a liquid is / how easily it evaporates. Longer chains: more viscous, less volatile. |
Think like a scientist
Criterion C A student measured the boiling points of six straight-chain alcohols at 101 kPa with a temperature probe, three trials each. Her results, in °C:
| Alcohol | Carbon atoms | Trial 1 / °C | Trial 2 / °C | Trial 3 / °C |
|---|---|---|---|---|
| methanol | 1 | 64.6 | 64.9 | 64.7 |
| ethanol | 2 | 78.2 | 78.5 | 78.4 |
| propan-1-ol | 3 | 97.1 | 97.4 | 97.2 |
| butan-1-ol | 4 | 117.6 | 108.9 | 117.8 |
| pentan-1-ol | 5 | 137.8 | 138.2 | 138.1 |
| hexan-1-ol | 6 | 157.2 | 156.8 | 157.0 |
(a) Identify the anomalous result and suggest a specific cause. [2]
(b) Calculate the mean boiling point of each alcohol to an appropriate precision. [2]
(c) Describe the relationship shown, quoting values, and determine whether boiling point is proportional to the number of carbon atoms. [3]
(d) Pentane (C₅H₁₂, molar mass 72) boils at 36 °C and butan-1-ol (C₄H₉OH, molar mass 74) at 118 °C. Explain, at the molecular level, why the alcohol boils so much higher although the molar masses are almost equal. [2]
(e) Predict the boiling point of heptan-1-ol with a justification from the data. [2]
(f) Evaluate the reliability of the data and suggest one improvement that would increase confidence in the conclusion. [2]
Model answer
(a) Butan-1-ol, trial 2: 108.9 °C, about 9 °C below the other two trials (117.6 and 117.8 °C).
Specific cause: the liquid was contaminated with water or a lower-boiling solvent left in the tube from the previous test (a mixture begins to boil below the pure alcohol), or the reading was taken before the probe had reached a steady value.
(b) Means, to one decimal place like the data: methanol 64.7; ethanol 78.4; propan-1-ol 97.2; butan-1-ol 117.7 (mean of 117.6 and 117.8 only, anomaly excluded); pentan-1-ol 138.0; hexan-1-ol 157.0 °C.
(c) Boiling point increases steadily with chain length, from 64.7 °C with 1 carbon to 157.0 °C with 6 carbons.
The increases are 13.7, 18.8, 20.5, 20.3 and 19.0 °C — about 20 °C per extra CH₂ after the first step.
Not proportional: doubling from 3 to 6 carbons raises the boiling point from 97.2 to 157.0 °C, a factor of 1.6, not 2, and the line would not pass through the origin.
(d) Pentane molecules attract each other only by weak intermolecular forces between the chains.
Butan-1-ol molecules have similar forces between their chains plus much stronger attractions between the –OH groups of neighbouring molecules (hydrogen bonds), so far more energy is needed to separate them — about 82 °C more.
(e) About 176–177 °C: the last three increases are close to 20 °C (20.5, 20.3, 19.0), and slightly shrinking, so add about 19–20 °C to 157.0 °C. (The data-book value is 176 °C.)
(f) Reliable: apart from the anomaly, the three trials agree within 0.5 °C, and the pattern is smooth. Limitation: only one tube of each alcohol and one probe were used, so a systematic error (a probe reading 1 °C high) would not show up.
Improvement: calibrate the probe in boiling water at the same pressure (it should read 100.0 °C at 101 kPa) and correct every reading; also measure the pressure each day, since boiling points shift with it.
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth. Relative atomic masses: C 12.0, H 1.0, O 16.0.
State the name and the homologous series of C₇H₁₆.
Reveal answer
Heptane [1]; alkane — it fits CₙH₂ₙ₊₂ with n = 7 [1].
A molecule contains the groups shown: a chain of three carbons, one C=C double bond, and a –COOH group on the end carbon. Identify the two functional groups present and the two families they belong to.
Reveal answer
C=C: alkene [1]; –COOH: carboxylic acid [1]. (A molecule can contain more than one functional group; the markscheme wants each group named separately.)
Outline how fractional distillation separates crude oil into fractions.
Reveal answer
Crude oil is heated so that most of it vaporises, and the vapour enters the bottom of the column [1].
The column is hot at the bottom and cooler towards the top (a temperature gradient), and the vapour cools as it rises [1]. Each fraction condenses at the level where the temperature is below its boiling point and is collected there; short-chain, low-boiling molecules reach the top as gases, and the longest chains leave at the bottom [1].
Explain why diesel is more viscous and less flammable than petrol.
Reveal answer
Diesel molecules (C₁₄–C₂₀) are longer than petrol molecules (C₅–C₁₀) [1]; the intermolecular forces between longer chains are stronger, so the molecules slide past each other less easily — more viscous [1]; fewer molecules escape as vapour at room temperature, so less vapour mixes with air near a flame and the liquid is harder to ignite [1].
An alcohol has a molar mass of 88.0 g mol⁻¹. Deduce its molecular formula and name.
Reveal answer
14.0n + 18.0 = 88.0
14.0n = 70.0, so n = 5 [1]
C₅H₁₁OH, pentanol [1].
Explain why crude oil is described as a finite resource.
Reveal answer
It formed from buried marine organisms over millions of years under heat and pressure [1]; it is being used far faster than it forms, and none is being replaced on a human timescale, so the supply will run out / is non-renewable [1].
7. The boiling points of the first alkanes increase by 73, 47, 41, 37 and 33 °C from methane to hexane.
A student claims: “Each CH₂ adds the same amount of intermolecular attraction, so the steps should be equal; the data must be wrong.” Evaluate the claim, and use the data to estimate the boiling point of dodecane, C₁₂H₂₆, given that decane boils at 174 °C and the step from nonane to decane is 23 °C.
Reveal answer
The first part is roughly right — each CH₂ adds a similar extra attraction — but the conclusion does not follow [1]. Boiling point depends on the total attraction compared with the energy of the molecules: each extra CH₂ is a smaller fraction of an already long molecule, so it raises the boiling point by less. The data show a smooth, consistent pattern, which is evidence against random error [1].
Estimate: the steps shrink slowly (…, 25, 23), so perhaps about 21 and 20 °C for the next two [1].
Dodecane ≈ 174 + 21 + 20
Dodecane ≈ 215 °C (the data-book value is 216 °C) [1].
8. A refinery’s crude contains, by mass, 30 % molecules in the C₅–C₁₀ range and 45 % above C₂₀. Demand in its market is 45 % petrol and 15 % heavy oils. Explain why fractional distillation alone cannot meet this demand, and suggest, with a reason, what the refinery could do with the surplus. (This leads into Lesson 2.)
Reveal answer
Distillation only separates what is already in the crude; it cannot change the size of any molecule [1]. The refinery would get only 30 % petrol against a demand of 45 %, and 45 % heavy material against a demand of 15 % — a shortage of one and a surplus of the other [1].
Suggestion: crack the surplus long-chain molecules [1] — heat them with a catalyst so their C–C bonds break into shorter alkanes for petrol and alkenes such as ethene for plastics, which turns low-value heavy oil into high-value products [1].
Summary
- Carbon forms four covalent bonds and bonds to itself in chains, so it builds homologous series: same general formula and functional group, similar reactions, a trend in physical properties, members differing by CH₂.
- Alkanes CₙH₂ₙ₊₂ (no functional group), alkenes CₙH₂ₙ (C=C), alcohols CₙH₂ₙ₊₁OH (–OH, neutral), carboxylic acids CₙH₂ₙ₊₁COOH (–COOH; count its carbon in the name).
- Crude oil is a finite fossil fuel: a mixture of hydrocarbons formed from buried plankton over millions of years.
- Fractional distillation separates it by boiling point in a column hot at the bottom and cool at the top; each fraction condenses where the column is cooler than its boiling point and is still a mixture.
- Longer chains have stronger intermolecular forces: higher boiling point, more viscous, less volatile, harder to ignite — a trend that rises in shrinking steps, not a proportion.
Next time → Alkanes at work: burning them completely and incompletely, what carbon monoxide and soot do, how chlorine swaps into an alkane in sunlight, and cracking — the refinery trick that turns surplus heavy oil into petrol and ethene.