5 b) crude oil Flashcards
5.6 understand that crude oil is a mixture of hydrocarbons
Crude oil is a mixture of hydrocarbons - compounds containing carbon and hydrogen only.
5.7 describe and explain how the industrial process of fractional distillation separates crude oil into fractions
The process of refining involves separating the hydrocarbons into fractions or batches using a technique called fractional distillation. Each fraction separates as they have different boiling points. The crude oil is heated in a furnace to around 400oC. This allows all of the hydrocarbons in the crude oil to move into the bottom of the fractionating tower. The tower is hottest at the bottom and coolest at the top
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The smallest molecules contained in the crude oil have lower boiling points and so move to the top of the tower. This is because the forces between these molecules are weak, so little energy is required to vaporise them. Larger molecules will remain lower down the tower as they have higher boiling points. This is because forces between the molecules are stronger.
5.8 recall the names and uses of the main fractions obtained from crude oil: refinery gases, gasoline, kerosene, diesel, fuel oil and bitumen
Refinery gases: refinery gases are a mixture of methane, ethane, propane and butane, which can be separated into individual gases if required. These gases are commonly used as LPG (liquefied petroleum gas) for domestic heating and cooking.
Gasoline (petrol): As with all other fractions, petrol is a mixture of hydrocarbons with similar boiling points. It is used in cars mainly.
Kerosine: Kerosine is used as fuel for jet aircraft, as domestic heating oil and as as ‘paraffin’ for small heaters and lamps.
Diesel oil (gas oil): This is used for buses, Iorries, some cars, and railway engines where the line hasn’t been electrified. Some is also cracked to make other organic chemicals and produced more petrol
Fuel oil: This is used for ships’ boilers and for industrial heating.
Bitumen: Bitumen is a thick black material, which is melted and mixed with rock chippings to make the top surfaces of roads.
5.9 describe the trend in boiling point and viscosity of the main fractions
As the molecules get bigger, the following changes occur:
- boiling point increases
- The liquids become less volatile.
- The liquid flow less easily (they become more vicous)
- Bigger hydrocarbons do not burn as easily as smaller ones.
5.10 understand that incomplete combustion of fuels may produce carbon monoxide and explain that carbon monoxide is poisonous because it reduces the capacity of the blood to carry oxygen
If there isn’t enough air (or isn’t enough oxygen), you get incomplete combustion. This leads to the formation of carbon or carbon monoxide instead of carbon dioxide.
2CH4(g) + 3O2(g) ==> 2CO(g) + 4H2O(l)
Carbon monoxide is colourless, odourless and is very poisonous. Carbon monoxide is poisonous because it combines with hemoglobin, preventing it from carrying oxygen. People can be made ill or even die, because of lack of oxygen in your body.
5.11 understand that, in car engines, the temperature reached is high enough to allow nitrogen and oxygen from air to react, forming nitrogen oxides
In car engines there is a high enough temperature to cause a reaction between oxygen and nitrogen in the air.
N2(g) + O2(g) ==> NO(g)
5.12 understand that nitrogen oxides and sulfur dioxide are pollutant gases which contribute to acid rain, and describe the problems caused by acid rain
Acid rain is formed when acidic air pollutants such as sulphur dioxide and nitrogen dissolve in rainwater. Sulphur dioxide dissolves in water to form sulphurous acid (H2SO3).
SO2(g) + H2O(l) ==> H2SO3(aq)
In the presence of oxygen in the air, the acid is slowly oxidized to sulphuric acid (H2SO4).
Oxides of nitrogen also contribute to acid rain. In the presence of oxygen and water, nitrogen dioxide is converted to nitric acid.
4NO2(g) + 2H2O(l) + O2(g) ==> 4HNO3(aq)
Carbon dioxide in the air dissolves in rainwater to form carbonic acid, which is a weak acid.
CO2(g) + H2O(l) ==> H2CO3(aq)
The pH value of normal rain is slightly below 7. The pH value of acid rain is approximately 3.4.
Problems of acid rain:
Acid rain reacts with metals and with carbonates in marble and limestone. When this happens, metal bridges and stone buildings are damaged.
It can reduce the pH value of natural water bodies from 6.5 and 8.5 to below 4. This will kill fish and other aquatic life.
It also leaches important nutrients from the soil and destroys plants. Without these nutrients, plant growth is stunted. In some cases, acid rain dissolves aluminium hydroxide in the soil to produce aluminium ions, which are toxic to plants.
5.13 understand that fractional distillation of crude oil produces more long-chain hydrocarbons than can be used directly and fewer short-chain hydrocarbons than required and explain why this makes cracking necessary
The amounts of each fraction you get will depend on the proportions of the various hydrocarbons in the original crude oil, not in the amount in which they are needed.
Long chain hydrocarbons which can’t be used directly are become less flammable, more viscous and therefore less useful.
Short chain hydrocarbons burn well and flow well. Therefore, they are useful but these are produced less in the fractional distillation of crude oil. Thus chemists convert these large, less useful, heavy fractions into smaller, more useful ones by means of cracking.
5.14 describe how long-chain alkanes are converted to alkenes and shorter-chain alkanes by catalytic cracking, using silica or alumina as the catalyst and a temperature in the range of 600-700
o C.
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The gas oil fraction is heated to give a gas and then passed over a catalyst of mixed silicon dioxide and aluminium oxide at about 600-700o C. Cracking can also be carried out at higher temperature without a catalyst.
Cracking is simply splitting of larger molecules to simpler ones. The molecules are broken up in an random way which produce a mixture of alkanes and alkenes.
