Crude oil is a complex mixture of hydrocarbons, with a varying composition depending on its source. The hydrocarbons in crude oil have different boiling points, according to the number of carbon atoms their molecules contain and how they are arranged. Fractional distillation uses the difference in boiling point to separate the hydrocarbons in crude oil. The fractionating column is cooler at the top than the bottom, so the vapours cool as they rise. Vapours condense onto a tray when they reach the part of the column which is cooler than their boiling point, they are therefore now separated.
There is a greater demand for ‘lighter’ short chain hydrocarbons such as petrol and naphtha and the demand for ‘heavy’ long chain hydrocarbons is much less. E.g. bitumen
Cracking is a form of thermal decomposition and is the process in which shorter more useful carbon chain hydrocarbons are produced from longer less useful chains by heating. This is done by breaking the carbon-carbon bonds which are very strongly attracted so the process of cracking has high activation energy. It is a free radical process and so it makes a mixture of products.
Catalytic cracking is a Carbo Cation mechanism. It uses catalyst: ‘zeolite’ at slight pressure- 4/5 atmospheres to make more fuels. It also makes ‘aromatic’ hydrocarbons.
Shape selectivity by a zeolite catalyst – separation of isomers by a molecular sieve
The advantages of using fossils is that very large amounts of electricity can be generated in one place using coal, fairly cheaply. Transporting oil and gas to the power stations is also very easy. Gas-fired power stations are very efficient and a fossil-fuelled power station can be built almost anywhere, so long as you can get large quantities of fuel to it.
However there are many disadvantages the most common and serious being pollution. Spillage of fuels can cause great damage in streams and ponds resulting in immense loss of animal and plant life and an enormous cost of cleaning it up. Oxidation of carbon-based compounds produces vast amounts of carbon dioxide which contributes to the green house effect causing an increase in atmospheric temperatures which is then likely to cause changes in climate and weather patterns. A large variety of compounds, including carcinogens, appear in the smoke from burning coal and wood.
Inefficient burning of carbon-based fuels in defective furnace and domestic gas fires produces poisonous gas carbon monoxide. This can cause problems in health as if it is breathed in it can take up the oxygen space in hemoglobin and react with the iron, this would mean that the body would not function properly and death will follow.
Burning fuels also produces two other poisonous gases such as various nitrogen oxides and sulphur oxides. The sulphur oxide comes from the sulphur impurities in the fossil fuels however the nitrogen oxides are produced by the reaction of nitrogen and oxygen in the air caused by the heat of the burning. When these gases mix with clouds they form dilute sulphuric acid and dilute nitric acid. This then falls as acid rain which kills fish, trees and limestone buildings. Mining coal can be difficult and dangerous. Strip mining destroys large areas of the landscape.
This image shows dead Effects of acid on stone
Fish as a result of pollution sculpture
There are some alternatives to fossil fuels that may provide a better environment these include the following:
- BIOMASS: plants can be used directly as fuels e.g wood or they can be grown for conversion into fuels e.g sugar from sugar cane
Advantage: renewable, helps to reduce waste and used with simple technology
Disadvantage: not large enough to replace fossil fuels at present rates of use.
- METHANOL: this alcohol can be used in racing cars and it is made quite cheaply from methane
Advantage: methanol does not produce a lot of carbon monoxide when burnt
Disadvantage: mixture of methanol and petrol absorb water and may cause corrosion of car engines
- NUCLEAR FUELS: when the nuclei of atoms of isotopes of uranium undergo fission (splitting) in a chain reaction very large amounts of energy is released
Advantage: there are no carbon, nitrogen and sulphur produced.
Disadvantage: Radioactive waste products are difficult to store and treat.
- MOVING AIR: WIND The energy of moving air is transferred into the motion of windmills and wind turbines
Advantage: no pollution is produced and it is renewable
Disadvantage: can be expensive to generate electricity for a large scale. Is not very reliable as it needs wind which is not always available.
- MOVING: WATER Stored water behind dams or from waterfalls can be released through turbines and generate electricity
Advantage: can be used on large scale and is quite predictable
Disadvantage: quite costly
- SUNLIGHT: Solar panels are used to heat water and photovoltaic cells are used to convert light into electricity
Advantage: pollution free with no waste products
Disadvantage: the sun is not always available in countries like the UK and also there is no sun at night time.
- GEOTHERMAL: Water is pumped into wells in the hot rock zone is heated and used to heat buildings
Advantage: available in large quantities and no pollution is produced
Disadvantage: expensive and has some technological problems
- HYDROGEN: Hydrogen is extracted from water by electrolysis and used on transport systems
Advantage: available in large quantities and causes no air pollution
Disadvantage: regarded as too dangerously explosive and it is too difficult to store
Bibliography
- OCR text book: Chemistry 1
- AS and A level chemistry revision guide
- GCSE double science chemistry revision guide
- Google research
