4.7.1.1Crude oil, hydrocarbons and alkanes
Crude oil is a finite resource found in rocks. It is the remains of an ancient biomass consisting mainly of plankton that was buried in mud.
Crude oil is a mixture of a very large number of compounds. Most of the compounds in crude oil are hydrocarbons, which are molecules made up of hydrogen and carbon atoms only.
Most of the hydrocarbons in crude oil are hydrocarbons called alkanes. The general formula for the homologous series of alkanes is CnH₂ₙ₊₂.
- Hydrocarbon
- A molecule made up of hydrogen and carbon atoms only.
- Homologous series
- A family of compounds with the same general formula and similar chemical properties.
- Finite resource
- A resource that is being used up faster than it is formed, so supplies will eventually run out.
Alkane molecules can be represented as a molecular formula such as C₂H₆, or as a displayed structural formula in which every bond is drawn as a line. You should be able to recognise substances as alkanes given their formulae in these forms.
The test with a molecular formula is whether it fits CnH₂ₙ₊₂. With a displayed formula, look for single bonds only, and check that every carbon has four bonds. You do not need to know the names of specific alkanes other than methane, ethane, propane and butane.
4.7.1.2Fractional distillation and petrochemicals
The many hydrocarbons in crude oil may be separated into fractions, each of which contains molecules with a similar number of carbon atoms, by fractional distillation.
The fractions can be processed to produce fuels and feedstock for the petrochemical industry. Many of the fuels on which we depend for our modern lifestyle — petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases — are produced from crude oil. Many useful materials on which modern life depends are produced by the petrochemical industry, such as solvents, lubricants, polymers and detergents.
You should be able to explain how fractional distillation works in terms of evaporation and condensation.
The explanation in order: the crude oil is heated until most of it evaporates, and the vapour enters a tall column that is hot at the bottom and cooler towards the top. As the vapour rises it cools. Each hydrocarbon condenses back to a liquid when it reaches the level where the temperature is below its boiling point, so the fractions with the largest molecules and the highest boiling points condense low down, and those with the smallest molecules rise furthest before condensing. Knowledge of the names of other specific fractions or fuels is not required.
The vast array of natural and synthetic carbon compounds occurs because of the ability of carbon atoms to form families of similar compounds — chains and rings of carbon atoms bonded to one another.
Common mistake
Fractional distillation separates the mixture by boiling point, and each fraction is still a mixture — of hydrocarbons with a similar number of carbon atoms — not a pure compound.
4.7.1.3Properties of hydrocarbons
Some properties of hydrocarbons depend on the size of their molecules, including boiling point, viscosity and flammability. These properties influence how hydrocarbons are used as fuels.
You should be able to recall how boiling point, viscosity and flammability change with increasing molecular size. Knowledge of trends in the properties of hydrocarbons is limited to those three.
The combustion of hydrocarbon fuels releases energy. During combustion, the carbon and hydrogen in the fuels are oxidised. The complete combustion of a hydrocarbon produces carbon dioxide and water.
hydrocarbon + oxygen → carbon dioxide + water
You should be able to write balanced equations for the complete combustion of hydrocarbons with a given formula.
Worked example
Write the balanced equation for the complete combustion of propane, C₃H₈.
- Write the products first: every carbon becomes CO₂ and every two hydrogens become one H₂O.
- 3 carbons give 3CO₂; 8 hydrogens give 4H₂O.
- C₃H₈ + O₂ → 3CO₂ + 4H₂O. Count the oxygen on the right: (3 × 2) + 4 = 10 atoms.
- 10 oxygen atoms means 5 O₂ molecules.
Answer: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Common mistake
Balance the carbon and the hydrogen first and leave the oxygen until last. Because oxygen appears in both products, it is the only element you can fix without disturbing anything else.
4.7.1.4Cracking and alkenes
Hydrocarbons can be broken down — cracked — to produce smaller, more useful molecules.
Cracking can be done by various methods including catalytic cracking and steam cracking. You should be able to describe in general terms the conditions used for each:
• catalytic cracking: the vapour is passed over a hot catalyst • steam cracking: the vapour is mixed with steam and heated to a very high temperature.
The products of cracking include alkanes and another type of hydrocarbon called alkenes.
There is a high demand for fuels with small molecules, and so some of the products of cracking are useful as fuels. Alkenes are used to produce polymers and as starting materials for the production of many other chemicals.
You should be able to give examples to illustrate the usefulness of cracking, and to explain how modern life depends on the uses of hydrocarbons. The economic argument is the one to make: fractional distillation produces far more of the long-chain fractions than anyone wants, and far less petrol than is needed. Cracking converts the surplus into what is in demand — and produces the alkenes that polymers, and so most modern plastics, are made from.
You should be able to balance chemical equations as examples of cracking, given the formulae of the reactants and products. Cracking equations balance like any other: the atoms on the left must all appear on the right.
Worked example
Decane, C₁₀H₂₂, is cracked to produce octane, C₈H₁₈, and one other product. Identify the product.
- Count the carbon atoms on the left: 10. Octane accounts for 8, so 2 are left.
- Count the hydrogen atoms on the left: 22. Octane accounts for 18, so 4 are left.
- The missing product is therefore C₂H₄.
- Check it against CnH₂ₙ: for n = 2 that is C₂H₄, so it is an alkene — ethene.
Answer: C₁₀H₂₂ → C₈H₁₈ + C₂H₄. The second product is ethene, an alkene.
Alkenes are more reactive than alkanes and react with bromine water, which is used as a test for alkenes. You should be able to recall the colour change when bromine water reacts with an alkene: the orange bromine water is decolourised, turning colourless. With an alkane there is no change, and the bromine water stays orange.
Common mistake
Decolourised means the colour disappears — the mixture goes colourless, not white and not clear. "Clear" describes transparency, and orange bromine water is already clear.
Before you practise
- Crude oil is a finite resource, the remains of an ancient biomass consisting mainly of plankton buried in mud.
- Hydrocarbons contain hydrogen and carbon only; alkanes have the general formula CnH₂ₙ₊₂ — methane, ethane, propane, butane.
- Fractional distillation separates crude oil into fractions of similar chain length by evaporation and condensation at different heights of the column.
- As molecules get larger, boiling point and viscosity increase but flammability decreases.
- Complete combustion of a hydrocarbon gives carbon dioxide and water; balance carbon and hydrogen first, oxygen last.
- Cracking breaks long molecules into smaller, more useful ones — by catalytic cracking over a hot catalyst, or steam cracking with steam at a very high temperature.
- Cracking produces alkanes and alkenes; alkenes are used to make polymers and as starting materials for other chemicals.
- Bromine water is the test for an alkene: orange to colourless.