Crude Oil, Hydrocarbons & Alkanes
Crude oil is a finite resource found in rocks. It is the remains of an ancient biomass — mostly plankton — that was buried in mud and compressed over millions of years. “Finite” matters: it is being used far faster than it forms, so on any human timescale it cannot be replaced.
- Crude oil — a finite resource found in rocks; the remains of an ancient biomass, consisting mainly of plankton, that was buried in mud.
- Hydrocarbon — a molecule made up of hydrogen and carbon atoms only. The word only is usually a mark on its own: ethanol (C2H5OH) contains carbon and hydrogen but is not a hydrocarbon, because it also contains oxygen.
- Mixture — crude oil is a mixture of a very large number of compounds, most of which are hydrocarbons. Because it’s a mixture, its components keep their own properties and can be separated physically (section 2).
Alkanes — the main hydrocarbons in crude oil
Most of the hydrocarbons in crude oil belong to one family: the alkanes. Alkanes are saturated hydrocarbons — every bond is a single covalent bond, so each molecule holds as many hydrogen atoms as it possibly can. They form a homologous series with the general formula:
CnH2n+2
Put in the number of carbon atoms (n) and the formula tells you the number of hydrogens. The first four alkanes — the only ones you need to know by name — are methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10). The prefixes meth- (1C), eth- (2C), prop- (3C) and but- (4C) repeat across every family in this topic — alkenes, alcohols, carboxylic acids — so learn them once with a memory hook (Monkeys Eat Peanut Butter) and they pay out four times.
The first four alkanes. Every bond is a single covalent bond — the molecules are saturated — and each one fits CnH2n+2.
Alkanes are a homologous series — a family of compounds that:
- share the same general formula (CnH2n+2 for alkanes),
- differ from the next member by CH2 (count up the table: CH4, C2H6, C3H8…),
- have similar chemical reactions, and
- show gradually changing physical properties (boiling point rises with size — section 3).
Exam questions exploit the pattern: given four successive members with one formula missing, fill the gap by adding CH2 — if C4H10 and C6H14 are members, the one between them is C5H12.
Recognising alkanes from formulae
You can be shown a molecule three ways — a name, a molecular formula (C2H6) or a displayed formula (every atom and every bond drawn out, as in the figure above) — and you must recognise alkanes in all of them. With a formula, test it against CnH2n+2:
- C6H14: n = 6, so 2n+2 = 14. The hydrogens match — alkane.
- C7H14: n = 7, so an alkane would need 16 hydrogens. This molecule has two fewer — it fits CnH2n instead, so it’s an alkene (section 4).
- In a displayed formula, look for the giveaway instead: all single bonds means alkane; a C=C double bond means alkene.
- Writing C4H12 for the alkane after propane. The general formula gives C4H10 — always calculate 2n+2 rather than guessing the pattern.
- Saying crude oil is “made of dinosaurs”. The mark scheme wants ancient biomass / plankton, buried in mud.
🧪 Exam-style questions
Which of these compounds is an alkane? Tick (✓) one box.
Ethane, propane and butane are successive members of the alkane homologous series. The next member is pentane. What is the formula of pentane? Tick (✓) one box.
What is the simplest whole-number ratio of carbon : hydrogen atoms in a molecule of butane (C4H10)? Complete the ratio 2 : …
Show answer
- C : H = 4 : 10. Divide both by 2 to get the simplest form: 2 : 5. 1 mark
Octane (C8H18) is found in petrol. Which statement explains why octane is a hydrocarbon? Tick (✓) one box.
Fractional Distillation
Crude oil straight out of the ground is nearly useless — a thick black mixture of thousands of different hydrocarbons. It becomes useful when it is separated into fractions: groups of hydrocarbons with a similar number of carbon atoms, and therefore similar boiling points. The separation happens in a fractionating column, and the whole process runs on just two physical changes: evaporation and condensation.
How the column works — the answer examiners want
This process is one of the most reliable extended-answer questions on Paper 2 (anything from 2 to 6 marks), and the mark scheme is built from a fixed chain of ideas. Learn it as a sequence:
- The crude oil is heated until it evaporates (vaporises) — around 350 °C in the furnace.
- The vapours enter a column that is hot at the bottom and cool at the top (a temperature gradient).
- Vapours rise up the column and cool down.
- Each fraction condenses when it reaches the level where the temperature equals its boiling point, and is piped off.
- Hydrocarbons with high boiling points (large molecules) condense near the bottom; those with low boiling points (small molecules) rise higher before condensing.
- The smallest molecules — the refinery gases — never condense: they leave the top of the column still as gases. Bitumen, with the highest boiling point, drains from the very bottom.
Notice what makes the separation possible in the first place: the fractions have different boiling points because boiling point depends on molecular size — bigger molecules are held together by stronger intermolecular forces, a structure-and-bonding idea from C2 that section 3 recaps and links back to. And keep the vocabulary physical, not chemical — no bonds inside molecules are broken anywhere in this process. Fractional distillation is a physical separation of a mixture, which is exactly why it works: the hydrocarbons in crude oil are not chemically combined.
One more linking idea: each fraction is still a mixture — its molecules have similar boiling points, not identical ones — so a fraction boils over a range of temperatures. A pure substance boils at one sharp temperature (the purity test you’ll meet in C8), which is why data tables always give fractions a boiling point range.
Worked example — placing a fraction
A question gives you boiling point ranges — kerosene 180–260 °C, diesel oil 260–320 °C, heavy fuel oil 320–400 °C — and asks: explain why diesel oil collects above heavy fuel oil but below kerosene (2 marks).
- Diesel oil’s boiling point is lower than heavy fuel oil’s, so its vapour rises further up the column before it condenses — it collects above heavy fuel oil. 1 mark
- Diesel oil’s boiling point is higher than kerosene’s, so it condenses at a hotter level, lower down than kerosene. 1 mark
- The pattern for any “where does it collect” question: match the boiling point to the temperature of the level in the column.
Fuels and petrochemical feedstock
Most fractions are processed into fuels — much of modern life runs on them: petrol for cars, kerosene for aircraft, diesel oil for lorries and trains, heavy fuel oil for ships and power stations, and liquefied petroleum gases (LPG) for camping stoves and heating. You don’t need to memorise other fraction names — the exam will supply any table it wants you to use.
Crude oil is also the feedstock (raw material) for the petrochemical industry, which makes solvents, lubricants, polymers and detergents. That short list of four is worth learning — “which two are produced from petrochemical feedstock?” is a recurring multiple-choice question, with alloys, ceramics and fertilisers as the distractors.
The vast array of natural and synthetic carbon compounds exists because carbon atoms can form families of similar compounds — chains and rings linked by C–C bonds, sorted into homologous series like the alkanes, alkenes and alcohols in this topic.
🧪 Exam-style questions
Three fractions have these boiling point ranges: fraction A 200–300 °C, fraction B 100–150 °C, fraction C below 30 °C. The column is 30 °C at the top and 400 °C at the bottom. Where is each fraction collected? Tick (✓) one box.
Which two of these are produced by the petrochemical industry from crude oil feedstock? Tick (✓) two boxes, then press Check.
A sample of crude oil is 16.5% petrol, 11.5% diesel oil and 72% other fractions by mass. Petrol and diesel oil are both used as car fuel. Calculate the total mass of car fuel that can be produced from 2000 kg of this crude oil.
Show answer
- Total percentage used as car fuel = 16.5 + 11.5 = 28%. 1 mark
- Mass = 0.28 × 2000 1 mark
- = 560 kg. 1 mark
Most of the compounds in crude oil are hydrocarbons; those with the smallest molecules are the most volatile. Using the fractionating column above, describe and explain how petrol is separated from the mixture of hydrocarbons in crude oil. This is a levels-of-response question — you are also assessed on clear written communication, so organise your answer logically and use specialist terms.
Show a model answer
How it is marked — marks depend on the quality of written communication as well as the science:
- Level 3 (5–6): a reasonable explanation of how petrol / the fractions are separated from crude oil using evaporating AND condensing.
- Level 2 (3–4): some description of heating / evaporating crude oil, AND either that fractions have different boiling points OR that there is a temperature difference up the column.
- Level 1 (1–2): a statement that crude oil is heated OR that substances are cooled, with little detail.
Example chemistry points:
- the crude oil / mixture of hydrocarbons is heated (to about 350 °C) so that most of the hydrocarbons evaporate / form vapours
- the column has a temperature gradient — hot at the bottom, cooler at the top
- the vapours rise up the column and cool as they rise
- a fraction condenses when it cools to its boiling point; the condensed fraction (petrol) separates from the vapours and flows out through a pipe
- hydrocarbons with low boiling points (like petrol) are collected near the top; those with high boiling points near the bottom
- the process is fractional distillation
Note: if cracking / a catalyst is given as part of the separation process, the answer is capped at Level 2.
Source: AQA GCSE Chemistry.
Properties of Hydrocarbons & Combustion
Why is petrol runny and easy to light, while heavy fuel oil is gloopy and reluctant to burn? Same family of molecules — different sizes. Exactly three properties change with molecular size, and all three move in a predictable direction.
As molecules get bigger: boiling point and viscosity increase, flammability decreases. Small molecules make the best fuels.
- Boiling point increases — bigger molecules are harder to boil, which is exactly what fractional distillation exploits.
- Viscosity increases — bigger molecules flow less easily (compare runny petrol with sticky bitumen).
- Flammability decreases — bigger molecules are harder to ignite.
These properties decide how each fraction is used as a fuel: the small, runny, easy-to-light molecules (petrol, kerosene) power engines, while a fraction like bitumen — very viscous and very hard to ignite — isn’t used as a fuel at all.
Complete combustion
Hydrocarbons are fuels because their combustion releases energy — strongly exothermic reactions. Burn a hydrocarbon in plenty of air and you get complete combustion: both elements in the fuel are fully oxidised. The carbon becomes carbon dioxide, the hydrogen becomes water — and the oxygen comes from the air.
hydrocarbon + oxygen → carbon dioxide + water
You already know how to balance an equation, and complete combustion just applies it. One ordering trick makes it painless: balance carbon first, then hydrogen, then oxygen last. Because the oxygen on the left is on its own (as O2), it can soak up whatever number the carbon dioxide and water end up needing. If that oxygen total comes out odd, balance with a half — C2H6 + 3½O2 → 2CO2 + 3H2O — then double everything for whole numbers.
Balance the equation for the complete combustion of propane.💡 Hint: balance carbon first, then hydrogen, then oxygen last. Type a number in each box (leave it as 1 if no number is needed), then press Check — any correct set of numbers is accepted.
Show answer
- Carbons: 3 → 3CO2. Hydrogens: 8 → 4H2O.
- Right-hand oxygens: (3×2) + (4×1) = 10, so 5O2: C3H8 + 5O2 → 3CO2 + 4H2O.
In a limited supply of air, combustion is incomplete: the products can include carbon monoxide (a toxic gas) and carbon itself — the soot that blackens a beaker held over a smoky flame. A smoky, sooty flame is the visual signature of incomplete combustion, and it’s why alkenes (which burn with smoky flames) make poor fuels (section 6). The pollutant chemistry — carbon monoxide, particulates, sulfur dioxide and oxides of nitrogen — is examined fully in C9 Chemistry of the Atmosphere, but fuels questions in C7 borrow it freely: particulates cause global dimming, sulfur dioxide causes acid rain, and the fix examiners accept for city air pollution is fuel-efficient engines and electric vehicles.
🧪 Exam-style questions
The hydrocarbon C18H38 is cracked to produce C6H14, C4H8 and C2H4. Compared with the three products, C18H38 has… Tick (✓) one box.
Which two substances are produced by the complete combustion of any hydrocarbon? Tick (✓) two boxes, then press Check.
Balance the equation for the complete combustion of methane.Type a balancing number in each box (leave it as 1 if no number is needed), then press Check. Any correct set of numbers is accepted.
Show answer
- 1 carbon → 1CO2; 4 hydrogens → 2H2O. 1 mark
- Right-hand oxygens: 2 + 2 = 4, so 2O2: CH4 + 2O2 → CO2 + 2H2O. 1 mark
Petrol contains the hydrocarbon C9H20. Balance the equation for its complete combustion.Type a balancing number in each box (leave it as 1 if no number is needed), then press Check.
Show answer
- 9 carbons → 9CO2; 20 hydrogens → 10H2O. 1 mark
- Right-hand oxygens: 18 + 10 = 28, so 14O2: C9H20 + 14O2 → 9CO2 + 10H2O. 1 mark
When a hydrocarbon was burned, the bottom of the beaker above the flame turned black. Why? Tick (✓) one box.
Propene (C3H6) burns in a limited supply of air and incomplete combustion occurs. Which equation correctly represents this reaction? Tick (✓) one box.
Successive alkanes have these boiling points: butane 0 °C, pentane ?, hexane 69 °C, heptane 98 °C. Which is the best estimate of the boiling point of pentane? Tick (✓) one box.
Cracking & the Test for Alkenes
Fractional distillation has an economics problem: the mix of fractions a barrel of crude oil contains is not the mix the world wants to buy. There is high demand for fuels with small molecules — petrol above all — and limited demand for the large, viscous, hard-to-ignite fractions. The fix is cracking: breaking large hydrocarbon molecules down into smaller, more useful ones. Cracking is a chemical reaction — a thermal decomposition — unlike the physical separation in section 2.
Cracking, drawn as displayed formulae: one large, low-demand alkane breaks into a smaller alkane (fuel) plus an alkene (the C=C, in red, makes it reactive — used for polymers). Atoms are conserved — count the carbons: 10 = 8 + 2.
You only need the conditions “in general terms”:
- Catalytic cracking — the fraction is heated to vaporise it, and the vapours are passed over a hot catalyst, which speeds the reaction up without being used up (the rate idea from C6).
- Steam cracking — the vaporised fraction is mixed with steam and heated to a very high temperature.
Asked for two conditions for cracking? High temperature + catalyst (or high temperature + steam) are the safe pairings. And a nice “suggest” question: why must air be kept out of the reactor? Because the hot hydrocarbon vapour would combust.
The products — and balancing cracking equations
Cracking produces two kinds of hydrocarbon: smaller alkanes, useful as fuels because small molecules are in high demand, and alkenes — a new homologous series with general formula CnH2n and a C=C double bond. Alkenes are more reactive than alkanes, and are used to make polymers (section 9) and as starting materials for many other chemicals.
The diagram above names one alkene — ethene (C2H4) — as an example, but Combined Science students do not need the names or formulae of individual alkenes. What everyone needs from this section is that cracking produces alkenes, that alkenes contain a C=C double bond, and the bromine-water test. The names and formulae of specific alkenes — ethene, propene, butene, pentene — are Triple Chemistry only T and are covered in section 5.
Balancing a cracking equation is pure atom-counting — no new atoms appear and none vanish:
C15H32 → C12H26 + ?
- Carbons: 15 − 12 = 3. Hydrogens: 32 − 26 = 6.
- The missing product is C3H6 (propene) — and it fits CnH2n, an alkene, as expected: cracking one alkane into another alkane always leaves an alkene’s worth of atoms over.
- Watch for coefficients: in C10H22 → C4H10 + 3C2H4, the 3 in front of ethene multiplies it to C6H12-worth of atoms. Check: C 10 = 4+6 ✓, H 22 = 10+12 ✓.
A long-chain alkane is cracked as shown. Work out the formula of the alkene that forms.💡 Hint: atoms are conserved, so subtract the alkane on the right from the starting molecule. The leftover always fits CnH2n — an alkene. Type the two subscripts, then press Check.
Show answer
- Carbons: 14 − 10 = 4. Hydrogens: 30 − 22 = 8.
- The alkene is C4H8 (butene) — and it fits CnH2n, as every cracked-off alkene does.
The test for alkenes — bromine water
Alkanes and alkenes can have identical-looking formulae in a list, so chemists tell them apart with a one-step test tube reaction. Shake the hydrocarbon with bromine water (orange):
Bromine water: orange → colourless with an alkene; stays orange with an alkane.
The colour change is orange → colourless. “Clear” is rejected — orange bromine water is already clear (you can see through it); what changes is the colour. And it’s the bromine water that changes colour, not the alkene. Spot alkenes in formula lists by CnH2n: of C7H14, C7H16, C8H16 and C8H18, only C7H14 and C8H16 would turn bromine water colourless.
A common 6-mark question stitches sections 2 and 4 into one answer: “Explain how alkenes are produced from crude oil using fractional distillation followed by cracking.” A Level 3 response covers both processes in order, with the right vocabulary:
- Heat the crude oil until it evaporates; the vapours rise the column, which is hot at the bottom, cool at the top.
- Each fraction condenses at its boiling point and is collected — small molecules higher up, large molecules lower down.
- A large-molecule fraction is then heated to vaporise it and passed over a hot catalyst (or mixed with steam at very high temperature).
- The large alkanes break down (thermally decompose) into smaller alkanes and alkenes.
Six linked steps, in sequence, with the right vocabulary — that’s the whole game.
🧪 Exam-style questions
Which hydrocarbon could be cracked to produce octane (C8H18)? Tick (✓) one box.
Decane is cracked: C10H22 → C5H10 + C3H8 + ? What is the formula of the third product? Tick (✓) one box.
Balance the equation for cracking decane into butane and ethene.Type a balancing number in each box (leave it as 1 if no number is needed), then press Check.
Show answer
- Butane takes 4 carbons, leaving 6 for ethene molecules: 6 ÷ 2 = 3C2H4. Hydrogens check: 10 + (3×4) = 22 ✓. C10H22 → C4H10 + 3C2H4 1 mark
Cracking also produces hydrocarbons with five and six carbon atoms. Which pair would both turn bromine water colourless? Tick (✓) one box.
Which two conditions are used to crack large alkane molecules? Tick (✓) two boxes, then press Check.