Crude oil is a mixture of a very large number of compounds.
It is formed from the remains of plants and animals which died millions of years ago.
This is why it is called a fossil fuel.
Most of the compounds in crude oil consist of molecules made up of hydrogen and carbon atomsonly, we call these type of compounds hydrocarbons.
We represent hydrocarbons in the following ways:
The many hydrocarbons in crude oil may be separated into fractions, each of which contains molecules of a similar size, by evaporating the oil and allowing it to condense at a number of different temperatures.
This process is called fractional distillation.
cracking and products
There is a much greater demand for shorter hydrocarbon than there is for the longer hydrocarbons.
Long chain hydrocarbons can be broken into smaller hydrocarbons, by heating with a catalyst.
This is a thermal decomposition reaction known as cracking.
Cracking produces two types of hydrocarbon
1. Alkanes with only single covalent bonds
2. Alkenes with one or more double covalent bonds
Test for alkenes: alkenes turn bromine water from brown to colourless.
Most fuels contain carbon and/or hydrogen and may also contain some sulphur.
The gases released into the atmosphere when a fuel burns may include:
- carbon dioxide
- water (vapour), which is an oxide of hydrogen
- sulphur dioxide (This gas dissolves in rain and forms acid rain)
:Alkenes are reactive and so are useful for making many other substances including polymers.
Polymers have very large molecules. They are formed when many small molecules join together. This process is called polymerisation.
When alkenes join together to form a polymer with no other substance being produced in the reaction, the process is called addition polymerisation.
Plastics are polymers and are made by polymerisation.
Example: poly(ethene) (often called polythene) is made by polymerising the simplest alkene, ethene.
metal ores and reactivity
Rocks from the Earth contain many useful metals.
Most metals are combined with other elements in materials called ores and have to be extracted using various methods.
How each metal is extracted depends on how reactive it is.
Gold is a very unreactive metal and is found as a pure metal, because of this it has been in use for many thousands of years despite of being a very rare metal.
Iron and copper are more reactive than gold but less reactive than carbon, these can be extracted from their ores by simply heating with coke these have been known for several thousand years
Aluminium is the most common metal in the Earth’s crust, however, was only discover 200 years ago because it is a relatively reactive metal which is hard to extract from its ore.
A metal such as iron, which is less reactive than carbon, can be extracted from its ore using carbon
Reactions in the blast furnace:
- C + O2 > CO2
- The coke burns, to form carbon dioxide and to produce heat
- CO2 + C > 2CO
- The carbon dioxide reacts with more hot coke to produce carbon monoxide gas.
- 3CO + Fe2O3 > 2Fe + 3CO2
- The carbon monoxide removes the oxygen from the iron ore this is called reduction.
- The main impurity in the iron ore is silica this reacts with the limestone to produce **** (calcium silicate)
Copper can be extracted from its ore by reduction with carbon, however, this is only 98% pure.
Copper can be purified by electrolysis using a positive electrode made of the impure copper and a negative electrode of pure copper in a solution containing copper ions.
When the current is switched on copper ions in solution are attracted to the negative and electrode and deposited there.
Copper atoms in the impure block lose electrons and become positive ions and go into the solution, replacing those which were deposited at the negative electrode.
Eventually the impure block disappears leaving behind the impurities and the pure block becomes larger.
- Nitrogen and Hydrogen are needed to make Ammonia.
- Nitrogen is obtained from the air.
- Hydrogen is obtained from water and natural gas.
- The Haber process is a reversible reaction
- This means that the reaction occurs in both directions
- High pressures favour the production of ammonia, however it expensive to make industrial equipment to cope with high pressures.
- Low temperatures favour the production of ammonia, however at low temperatures the reaction would be too slow to be commercially viable.
- The Haber process makes a compromise with these two and recycles the unreacted hydrogen and nitrogen
- N2(g) + 3H2(g) <> 2NH3(g) . <> menas the reaction is reversible