Crude oil & the alkanes
By the end of this topic you'll know what crude oil is, what a hydrocarbon is, the first four alkanes, and how to use the general formula CnH2n+2.
Part 1What crude oil is
Crude oil is a finite resource found in rocks. It formed over millions of years from the remains of ancient plankton (tiny sea creatures) buried in mud, under high pressure and temperature. Because it takes that long to form, we are using it up far faster than it is replaced — that's what "finite" means.
Crude oil is a mixture of a very large number of different compounds. Most of them are hydrocarbons.
Two words to be precise about
- Hydrocarbon
- A compound made of hydrogen and carbon only — nothing else.
- Alkane
- The simplest type of hydrocarbon, where the carbon atoms are joined by single bonds. They are saturated.
Most of the hydrocarbons in crude oil are alkanes. An alkane is saturated, which means every carbon–carbon bond is a single bond — there are no spare bonds and no double bonds.
⚠ Watch out — "hydrocarbon" has a strict meaning
A hydrocarbon contains only hydrogen and carbon. If a molecule also contains oxygen, chlorine or anything else, it is not a hydrocarbon — even if it has lots of H and C in it. Examiners use this to catch you out, so check the formula has only H and C.
Part 2The first four alkanes
You need to recall the first four alkanes — their names, formulae and that they go up by one carbon each time. Notice the names all end in -ane.
The first four alkanes (learn these)
- Methane — CH4
- 1 carbon. The main gas in natural gas.
- Ethane — C2H6
- 2 carbons.
- Propane — C3H8
- 3 carbons. Bottled gas for camping/heating.
- Butane — C4H10
- 4 carbons. Lighter fuel.
Every bond in an alkane is a single bond, and every carbon has formed four bonds in total. That is what "saturated" looks like on paper.
General formula
- CnH2n+2 given
- For an alkane with n carbon atoms, the number of hydrogen atoms is (2 × n) + 2. This isn't on the AQA equation sheet, but you are given the formula in the question — you don't have to recall it from memory.
Worked example — using the general formula
An alkane has 5 carbon atoms. Work out its molecular formula.
⚠ Watch out — alkanes vs alkenes
It's easy to mix up the names. Alkanes (-ane) are saturated with only single bonds, formula CnH2n+2. Alkenes (-ene) — coming in Topic 4 — have a C=C double bond and a different formula. One letter, completely different molecules.
Which of these is the formula of an alkane with 3 carbon atoms?
- AC3H6
- BC3H8
- CC3H4
- DC3O8
Show answer
What is a hydrocarbon?
A compound made from hydrogen and carbon only (no other elements).Name the first four alkanes in order.
Methane, ethane, propane, butane (CH4, C2H6, C3H8, C4H10).What does it mean to say an alkane is "saturated"?
All of its carbon–carbon bonds are single bonds — there are no double bonds.An alkane has 8 carbon atoms. Use CnH2n+2 to find its formula.
H = (2 × 8) + 2 = 18, so the formula is C8H18 (octane).How did crude oil form, and why is it called finite?
From the remains of ancient plankton buried in mud over millions of years under heat and pressure. It's finite because it's being used up far faster than it forms.
Fractional distillation
How we split crude oil into useful fractions — using boiling point and a tall tower — and what each fraction is for.
Part 1Splitting the mixture
Crude oil straight from the ground isn't much use — it's a mixture of hundreds of hydrocarbons. We separate it by fractional distillation. The separation works because hydrocarbons of different chain lengths have different boiling points.
Here is the process, step by step:
The oil is heated until it evaporates (turns to vapour). The vapour rises up a tall fractionating column which is hot at the bottom and cool at the top. As each vapour rises and cools, it reaches a level where it is at its boiling point, so it condenses back to a liquid and is collected. We call each liquid that's collected a fraction.
⚠ Watch out — it's a physical separation
Fractional distillation does not make new substances — it only separates the molecules already in the oil. No bonds are broken or made, so it's a physical process. Each fraction is still a mixture, just of hydrocarbons with a similar number of carbon atoms and similar boiling points.
Part 2What the fractions are used for
The fractions are processed into the fuels and materials we use every day. Most of the oil ends up as fuels — petrol, diesel, kerosene and fuel oil — which is why oil matters so much to modern life.
Fractions and their uses
- Petrol
- Fuel for cars. Short chains, near the top.
- Kerosene
- Fuel for aircraft (jet fuel).
- Diesel oil
- Fuel for some cars, lorries and trains.
- Fuel oil
- Fuel for large ships and some power stations.
- Bitumen
- Surfacing roads and roofs. Very long chains, at the bottom.
Crude oil is also the main source of useful feedstock (raw materials) for the petrochemical industry. Many useful materials — such as solvents, lubricants, polymers and detergents — are made from compounds in crude oil.
It works as a feedstock because of the huge number of hydrocarbons it contains, and because carbon atoms can form families of similar compounds. The alkanes are one such family — a group of compounds with the same general formula whose properties change gradually as the chain grows.
A fraction condenses high up the column, where it is coolest. What can you say about it?
- AIt has long chains and a high boiling point
- BIt has short chains and a low boiling point
- CIt is bitumen, used for roads
- DIt is a pure compound, not a mixture
Show answer
What property of the hydrocarbons allows crude oil to be separated by fractional distillation?
Hydrocarbons of different chain length have different boiling points.Describe what happens to the crude oil in the fractionating column.
It is heated so it evaporates; the vapours rise up the column; each vapour condenses when it reaches the level at its boiling point (cool at the top, hot at the bottom) and is collected as a fraction.Give the use of (a) petrol and (b) bitumen.
(a) Fuel for cars. (b) Surfacing roads (and roofs).Other than fuels, give one type of useful material made from crude-oil feedstock.
Any one of: solvents, lubricants, polymers, detergents.
Properties & burning hydrocarbons
How the properties change as the chain gets longer, and the difference between complete and incomplete combustion.
Part 1Trends with chain length
As the hydrocarbon molecules get longer, four properties change in a predictable way. Learn the direction of each trend.
As chain length increases: the boiling point increases, the liquid becomes more viscous (thicker, flows less easily), it is less flammable (harder to ignite), and it is less volatile (evaporates less easily).
⚠ Watch out — "flammable" and "volatile" go DOWN
Boiling point and viscosity go up with chain length, but flammability and volatility go down. A common slip is to assume everything increases. Short chains are the runny, easily-ignited fuels (petrol); long chains are thick and hard to light (bitumen).
Part 2Combustion
Burning a hydrocarbon is an oxidation reaction — the hydrogen and carbon in the fuel are oxidised, and energy is released (it's exothermic). That released energy is why we use these as fuels.
In complete combustion there is plenty of oxygen. The only products are carbon dioxide and water.
Complete combustion (word equation)
- hydrocarbon + oxygen → carbon dioxide + water recall
- Both the carbon and the hydrogen are fully oxidised. e.g. methane: CH4 + 2O2 → CO2 + 2H2O.
Worked example — products of complete combustion
Propane is burned in plenty of oxygen. Name the two products and write the word equation.
If there is not enough oxygen, you get incomplete combustion. This is dangerous: as well as water, it can produce carbon monoxide (CO) — a toxic gas — and carbon (soot, particulates).
⚠ Watch out — carbon monoxide is the silent killer
Carbon monoxide is colourless and odourless, and it stops your blood carrying oxygen — that's why faulty gas appliances are deadly. Carbon dioxide (from complete combustion) is the greenhouse gas; carbon monoxide (from incomplete combustion) is the toxic one. Don't swap them.
A gas heater in a poorly ventilated room burns with a yellow, sooty flame. What does this tell you, and what dangerous gas may form?
- AComplete combustion; carbon dioxide
- BIncomplete combustion; carbon monoxide
- CComplete combustion; only water
- DIncomplete combustion; hydrogen
Show answer
As chain length increases, what happens to boiling point and viscosity?
Both increase — longer chains have higher boiling points and are more viscous (thicker).As chain length increases, what happens to flammability and volatility?
Both decrease — longer chains are harder to ignite and evaporate less easily.Name the two products of the complete combustion of a hydrocarbon.
Carbon dioxide and water.Why is incomplete combustion dangerous?
It can produce carbon monoxide (a toxic gas) and carbon (soot/particulates), because there isn't enough oxygen.Is combustion endothermic or exothermic, and why is that useful?
Exothermic — it releases energy, which is why hydrocarbons are used as fuels.
Cracking & the alkenes
Breaking long chains into the short, useful ones we want — and the bromine-water test that proves you've made an alkene.
Part 1Why we crack
Fractional distillation gives us too much of the long-chain fractions and not enough of the short-chain ones that people want (like petrol). Cracking fixes this mismatch: it breaks long-chain hydrocarbons into smaller, more useful molecules.
Cracking always produces a shorter alkane and an alkene. Both supply and demand drive it: there is high demand for short-chain hydrocarbons as fuels, and the alkenes produced are valuable raw materials too.
There are two ways to provide the energy to break the bonds. In catalytic cracking, the long-chain hydrocarbon vapour is passed over a hot catalyst. In steam cracking (a type of thermal cracking), the vapour is mixed with steam and heated to a very high temperature.
Two methods of cracking
- Catalytic cracking
- Heat the long-chain vapour and pass it over a hot catalyst.
- Steam cracking
- Mix the vapour with steam and heat to a very high temperature.
⚠ Watch out — cracking is the opposite of distillation
Fractional distillation only separates molecules — no bonds change. Cracking is a chemical reaction: it breaks covalent bonds to make new, smaller molecules. So cracking always needs heat (and often a catalyst), and the products are different substances from what you started with.
Part 2Alkenes & the bromine test
Alkenes are hydrocarbons that contain a carbon–carbon double bond (C=C). Because a double bond means two carbons are sharing more than a single bond — and could bond to more atoms — alkenes are described as unsaturated. That double bond makes them more reactive than alkanes.
We test for an alkene using bromine water. Add the substance to orange bromine water and shake:
Test result to learn
- Alkene + bromine water → orange goes colourless recall
- An alkene decolourises bromine water (orange → colourless). An alkane does not — it stays orange. This is how you tell them apart.
Worked example — identifying a gas
A student bubbles an unknown hydrocarbon gas through bromine water. The orange colour disappears. What is the gas, and what does this prove about its bonds?
⚠ Watch out — which way round the colour change goes
Bromine water starts orange and an alkene turns it colourless. Don't write "turns brown" or "goes orange" — and don't say the alkene "changes colour". It's the bromine water that loses its colour. The alkane leaves it unchanged.
Two test tubes of bromine water are tested with two gases. Tube 1 stays orange; tube 2 goes colourless. What are the gases?
- ATube 1 = alkene, Tube 2 = alkane
- BTube 1 = alkane, Tube 2 = alkene
- CBoth are alkenes
- DBoth are alkanes
Show answer
Why is cracking carried out?
To turn surplus long-chain hydrocarbons into shorter, more useful ones (high demand for short chains as fuels), and to make alkenes.Name the two products always formed when a long-chain alkane is cracked.
A shorter alkane and an alkene.Describe the two methods of cracking.
Catalytic cracking: pass the heated vapour over a hot catalyst. Steam cracking: mix the vapour with steam and heat to a very high temperature.What is the difference between an alkane and an alkene?
Alkanes are saturated (single bonds only); alkenes are unsaturated — they contain a carbon–carbon double bond (C=C) and are more reactive.State the test for an alkene and the result.
Add bromine water: an alkene turns it from orange to colourless (it decolourises it). An alkane leaves it orange.