What you'll learn
Organic chemistry is the unit of AQA GCSE Combined Science: Trilogy that studies compounds of carbon, and in particular the hydrocarbons that make up crude oil. Carbon is unusual among the elements because its atoms can form four covalent bonds and can join to one another in chains and rings, which is why the number of carbon compounds runs into millions. This unit explains what crude oil is and how it formed, how fractional distillation separates it into useful fractions, how the properties of hydrocarbons change with molecular size, what happens when they burn, why long molecules are cracked into shorter ones, and how alkenes differ from alkanes. By the end you should be able to name the first four alkanes, write and interpret their formulae, explain the trends in boiling point, viscosity and flammability, describe complete and incomplete combustion, explain the economics of cracking, and describe the bromine water test. This unit is assessed on Chemistry Paper 2.
Key terms and definitions
Hydrocarbon — a compound containing hydrogen and carbon only
Crude oil — a finite resource formed over millions of years from the remains of ancient biomass, mainly plankton, buried in mud
Alkane — a saturated hydrocarbon in which all the carbon–carbon bonds are single bonds, with the general formula carbon n hydrogen two n plus two
Alkene — an unsaturated hydrocarbon containing at least one carbon–carbon double bond
Saturated — containing only single carbon–carbon bonds, so no more atoms can be added
Unsaturated — containing at least one carbon–carbon double bond, so more atoms can be added across it
Homologous series — a family of compounds with the same general formula and similar chemical properties, whose physical properties change gradually along the series
Fractional distillation — the separation of a mixture into fractions with similar boiling points, using a column with a temperature gradient
Fraction — a group of hydrocarbons with similar numbers of carbon atoms and therefore similar boiling points
Cracking — breaking large hydrocarbon molecules into smaller, more useful ones, producing alkanes and alkenes
Viscosity — how easily a liquid flows; a more viscous liquid flows less easily
Core concepts
Crude oil and its formation
Crude oil is a finite resource, formed from the buried remains of ancient biomass, chiefly plankton, over millions of years. Because it forms far more slowly than it is used, it cannot be replaced within any human timescale, which is the whole meaning of the word finite.
Crude oil is a mixture of a very large number of compounds, and most of these compounds are hydrocarbons: molecules made of hydrogen and carbon only. Many of them are alkanes.
Alkanes
The alkanes are a homologous series. They share the general formula in which the number of hydrogen atoms is twice the number of carbon atoms plus two, so a molecule with three carbons has eight hydrogens.
The first four alkanes must be known by name and formula. Methane has one carbon and four hydrogens. Ethane has two carbons and six hydrogens. Propane has three carbons and eight hydrogens. Butane has four carbons and ten hydrogens.
Alkanes are saturated, meaning every carbon–carbon bond is a single bond and every remaining bond on each carbon is taken up by hydrogen. No further atoms can be added without first removing something.
Fractional distillation
The many compounds in crude oil are separated by fractional distillation. The oil is heated until it vaporises, and the vapour enters a tall column that is hot at the bottom and progressively cooler towards the top.
As the vapour rises it cools. Each fraction condenses at the level where the temperature falls below its boiling point, so the fractions with the highest boiling points — the largest molecules — condense low down, while those with the lowest boiling points condense near the top or leave as gases. The fractions are drawn off at different heights.
Each fraction contains molecules with a similar number of carbon atoms, and so a similar boiling point. Separation is possible precisely because boiling point depends so strongly on molecular size.
The fractions are processed to produce fuels and other useful materials. Fuels include petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases. Crude oil is also the main source of feedstock for the petrochemical industry, which uses it to make solvents, lubricants, polymers and detergents.
Trends in properties
The properties of hydrocarbons depend on the size of the molecules, and three trends follow the same underlying cause.
As molecules get larger, boiling point increases. This is because the intermolecular forces between larger molecules are stronger, so more energy is needed to separate them. Note carefully that it is the forces between molecules, not the covalent bonds within them, that are overcome on boiling.
As molecules get larger, viscosity increases, meaning the liquid flows less easily. Heavy fuel oil is thick and sticky; petrol is thin and runny.
As molecules get larger, flammability decreases, meaning they are harder to ignite. This is why petrol, with small molecules, is used in car engines while heavy fuel oil is used where a less easily ignited fuel is acceptable.
These three trends explain why particular fractions suit particular uses, and questions very often ask you to justify a use in terms of one of them.
Combustion
When hydrocarbons burn in plenty of air, complete combustion occurs. Both the carbon and the hydrogen in the fuel are oxidised, producing carbon dioxide and water, and energy is released. This makes hydrocarbons useful as fuels.
When the supply of air is limited, incomplete combustion occurs instead. The products then include carbon monoxide, a toxic gas, and solid particles of carbon known as soot or particulates. Carbon monoxide is dangerous because it is colourless and odourless, so it cannot be detected by the senses.
During combustion, the carbon and hydrogen in the fuel are oxidised, which is why combustion is described as an oxidation reaction. The overall process is strongly exothermic.
Cracking
The fractions containing the largest molecules are produced in greater quantity than they are needed, while shorter-chain fractions such as petrol are in high demand. Cracking solves this mismatch by breaking large hydrocarbon molecules into smaller, more useful ones.
There are two methods. Catalytic cracking passes hydrocarbon vapours over a hot catalyst. Steam cracking mixes the vapours with steam and heats them to a very high temperature. Both break the long chains.
The products of cracking are alkanes and alkenes. The shorter alkanes are valuable as fuels, so cracking increases the supply of the fractions that are most in demand. The alkenes are useful as starting materials for making other compounds, particularly polymers.
The economic argument is examinable in its own right: cracking converts a low-demand product into high-demand ones, so it increases the value obtained from a barrel of crude oil and matches supply to demand.
Alkenes and the bromine water test
Alkenes contain a carbon–carbon double bond, which makes them unsaturated: further atoms can be added across the double bond without removing anything.
This difference is the basis of the test that distinguishes them. Bromine water is orange. When it is added to a saturated alkane and shaken, no reaction occurs and the orange colour remains. When it is added to an unsaturated alkene, the bromine adds across the double bond and the solution is decolourised, turning from orange to colourless.
The correct wording matters. The bromine water is decolourised, meaning it loses its colour; it does not turn clear, since it was already clear in the sense of transparent. Examiners reject the word clear for this reason, and it is one of the most reliable ways to lose an otherwise easy mark.
Worked examples
Example 1: Explaining a trend (3 marks)
Explain why the boiling point of butane is higher than the boiling point of methane.
A butane molecule contains four carbon atoms while a methane molecule contains only one, so butane molecules are larger. The intermolecular forces between the larger butane molecules are stronger than those between methane molecules. More energy is therefore needed to overcome these forces and separate the molecules, so butane boils at a higher temperature.
Example 2: Identifying combustion products (4 marks)
A gas heater burns methane. Describe the products formed when the air supply is plentiful and when it is restricted, and explain why the restricted supply is dangerous.
With a plentiful supply of air, complete combustion occurs and the products are carbon dioxide and water, with energy released. With a restricted air supply, incomplete combustion occurs and the products include carbon monoxide and solid particles of carbon. This is dangerous because carbon monoxide is toxic and is also colourless and odourless, so a person cannot detect it before being affected by it.
Example 3: Explaining why cracking is carried out (3 marks)
Explain why an oil company cracks long-chain hydrocarbons.
Fractional distillation produces more of the long-chain fractions than there is demand for, while there is high demand for shorter-chain fuels such as petrol. Cracking breaks the long molecules into shorter alkanes, which are more useful as fuels, and into alkenes, which are used to make polymers and other compounds. This matches supply to demand and increases the value obtained from the crude oil.
Common mistakes and how to avoid them
The most commonly penalised error in this unit is writing that bromine water turns clear. It becomes colourless. Since the solution is transparent both before and after, clear describes no change at all.
Students frequently explain boiling point trends by saying that the covalent bonds are stronger in larger molecules. The covalent bonds are not broken on boiling; the intermolecular forces are overcome, and those are stronger between larger molecules.
Another regular slip is stating that crude oil is a compound. It is a mixture, which is exactly why it can be separated by a physical process.
In combustion questions, many answers give carbon dioxide as a product of incomplete combustion without mentioning carbon monoxide or particulates. Incomplete combustion is defined by those products.
Finally, students often say cracking produces only alkenes. It produces both smaller alkanes and alkenes, and the alkanes are the part that answers the fuel demand.
Exam technique for "Chemistry: Organic Chemistry"
When asked to justify a use for a fraction, name the trend you are relying on. Saying petrol is used in cars because it is a small molecule scores nothing; saying it has a low boiling point and is easily ignited because its molecules are small scores the marks.
For formulae, count carefully and use the general formula of the alkanes as a check: hydrogens should equal twice the carbons plus two. If they do not, either the molecule is not an alkane or you have miscounted.
In fractional distillation questions, describe the temperature gradient explicitly — hot at the bottom, cooler towards the top — and link condensation to boiling point. Many answers describe the column without ever saying why separation happens.
When a question asks for a test, give the reagent and both the positive and negative results. Bromine water with alkene gives orange to colourless; with alkane it stays orange. Giving both halves makes the answer complete.
Quick revision summary
Crude oil is a finite mixture formed from ancient biomass, consisting mainly of hydrocarbons, most of them alkanes. Alkanes are saturated with the general formula carbon n hydrogen two n plus two, the first four being methane, ethane, propane and butane. Fractional distillation separates crude oil using a column hot at the bottom and cooler at the top, so fractions condense according to their boiling points. Larger molecules have higher boiling points and higher viscosity but lower flammability, because intermolecular forces increase with size. Complete combustion gives carbon dioxide and water; incomplete combustion gives carbon monoxide and particulates, and carbon monoxide is toxic, colourless and odourless. Cracking, by catalytic or steam methods, breaks long chains into shorter alkanes for fuel and alkenes for making polymers, matching supply to demand. Alkenes contain a carbon–carbon double bond and decolourise orange bromine water; alkanes do not.