8.1–8.2, 8.6Crude 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.
8.3–8.4Fractional 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.
8.5Properties 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.
Before you practise
- A hydrocarbon contains carbon and hydrogen only — nothing else.
- Crude oil is a mixture of hydrocarbons, mostly alkanes, formed from the remains of ancient plankton.
- Fractional distillation works because the fractions have different boiling points: the column is hottest at the bottom, and each fraction condenses where it cools to its own boiling point.
- Learn the fractions in order with a use each: gases, petrol, kerosene, diesel, fuel oil, bitumen.
- Longer chains mean higher boiling points, higher viscosity and lower flammability.
- A homologous series shares a general formula, differs by CH₂ each step, and has similar chemical properties.
- The alkanes are CₙH₂ₙ₊₂ — methane, ethane, propane, butane.