Hydrocarbon
Hydrocarbon is a compound that contains carbon, oxygen and hydrogen only.
Crude Oil
- Crude oil is a finite resource found in the Earth's crust.
- It is the remains of organisms that died millions of years ago.
- It is a complex mixture of hydrocarbons.
Separation of Crude Oil with Fractional Distillation
Crude oil can be separated into different fractions using fractional distillation since different fractions of crude oil have different boiling points.
- Vaporized crude oil enters the fractionating column at the bottom.
- The column is hot at the bottom and cool at the top.
- Vaporized crude oil rises through the column.
- When the vapor reaches a part of the column that has a temperature lower than its boiling point, it will condense into liquid.
- Different fractions of the crude oil leave as liquid at different heights.
For IGCSE chemistry, you should be able to identify the names and uses of each fraction.
Trends in color, boiling point and viscosity of the fractions
- Fractions collected at the bottom have high boiling points.
- Fractions at the bottom have high boiling points since large molecules have stronger intermolecular forces and need more energy to separate the molecules.
- Lower fractions also have a higher viscosity because the attraction between the hydrocarbon molecules increases as the number of carbons increases, making it less easy to flow.
- The color gets darker as you go to the bottom of the column.
Catalytic Cracking
Catalytic cracking is breaking down larger hydrocarbon molecules into smaller, more useful hydrocarbon molecules.
- Fractions that contain larger hydrocarbons are heated at 600 - 700 °C to vaporize them.
- Vapors are then passed through a hot catalyst of aluminum or silica.
- This process causes thermal decomposition of longer-chain alkanes into alkenes and shorter-chain alkanes.
- Demands for shorter chain hydrocarbons are bigger than what is produced through fractional distillation, while demands for larger chain hydrocarbons are smaller than what is produced. Therefore, it is necessary to turn longer chain hydrocarbons into shorter chain hydrocarbons.
- Catalytic cracking produces alkenes, which are raw material in the plastic industry.
- Cracking also produces hydrogen, which can be used in the Haber process to produce ammonia.
Formation of Harmful Gases
I. Carbon monoxide
When there is a limited supply of oxygen, hydrocarbons undergo incomplete combustion. During incomplete combustion of hydrocarbons, carbon monoxide is produced.
hydrocarbon + oxygen → carbon monoxide + water
Carbon monoxide reduces the capacity of blood to transport oxygen, and is poisonous to us.
II. Oxides of nitrogen
When fuels are burned at high temperatures in car engines, nitrogen and oxygen in the air can form nitrogen oxides. (e.g. nitrogen monoxide, nitrogen dioxide)
When nitrogen oxides react with water droplets and oxygen in the cloud, nitric acid and nitrous acid will be formed, which causes acid rain.
III. Sulfur dioxide
Fuels may contain impurities, such as sulfur. During combustion, sulfur will be oxidized to sulfur dioxide.
When sulfur dioxides react with water droplets and oxygen in the cloud, sulfuric acid will be formed, which causes acid rain.