Organic chemistry - Fuels (3)
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1.
State that hydrocarbons are compounds that contain hydrogen and carbon only. Explain briefly why this characteristic makes them important in the context of fuels.
Hydrocarbons are organic compounds composed exclusively of hydrogen (H) and carbon (C) atoms. This simple composition is crucial for their use as fuels. The carbon-hydrogen bonds are relatively weak, allowing for easy combustion. When hydrocarbons burn, they react with oxygen in a process called oxidation, releasing energy in the form of heat and light. This makes them readily combustible and suitable for powering engines and heating systems.
2.
Explain why petroleum is described as a mixture. In your answer, include the terms 'composition', 'variable', and 'separation'.
Petroleum is described as a mixture because its composition is variable. This means the exact types and proportions of hydrocarbons present in petroleum can differ depending on the source and geological conditions where it was formed. Because of this variable composition, petroleum can be separated into different fractions using fractional distillation. Each fraction contains a different range of hydrocarbons with similar boiling points.
3.
Describe the process of separating petroleum into useful fractions by fractional distillation. In your answer, include the following:
- The apparatus used.
- The principle on which the separation is based.
- The range of boiling points for different fractions.
- Two uses for one of the fractions obtained.
Fractional distillation is a process used to separate the different hydrocarbons present in petroleum (crude oil) based on their different boiling points. The process involves heating the crude oil in a furnace to a high temperature (around 400°C) to vaporize the hydrocarbons. This hot vapour is then passed into the bottom of a fractionating column, which is cooler at the bottom and gradually gets cooler towards the top.
The apparatus used in fractional distillation consists of a furnace, a fractionating column, and a condenser. The furnace heats the crude oil, converting it into vapour. This vapour enters the fractionating column. The column is designed with a temperature gradient – it’s hotter at the bottom and cooler at the top. As the vapour rises through the column, it gradually cools. Hydrocarbons with higher boiling points condense at the lower, hotter parts of the column and can be collected as liquids. Hydrocarbons with lower boiling points continue to rise further up the column before condensing at the cooler upper sections. Different fractions condense at different temperatures.
The separation is based on the difference in boiling points of the various hydrocarbon components of crude oil. Each fraction has a characteristic boiling point range. Typical fractions and their boiling point ranges are:
Fraction | Approximate Boiling Point Range (°C) | Typical Uses |
Gases (methane, ethane, propane, butane) | -161 to 0 | Fuel for cooking and heating, LPG |
| Petrol (gasoline) | 30 to 200 | Fuel for cars |
| Kerosene (paraffin oil) | 200 to 300 | Jet fuel, heating oil |
| Diesel oil | 300 to 350 | Fuel for diesel engines |
| Lubricating oil | 350 to 400 | Reducing friction in engines |
| Bitumen (asphalt) | 350 to 400 | Road surfacing |
Two uses for petrol are as a fuel for cars and as a component in petrol mixtures.