Mixture of Hydrocarbons
- Crude oil is a complex mixture of lots of different hydrocarbon compounds of different sizes.
- It is a thick, sticky, black liquid that is found under porous rock (under the ground and under the sea).
Chains & Rings
- The hydrocarbon molecules in crude oil consist of a carbon backbone which can be in a ring or chain, with hydrogen atoms attached to the carbon atoms.
- The mixture contains molecules with many different ring sizes and chain lengths.
Crude oil is a mixture containing different sized hydrocarbon molecules
- Crude oil is the main source of hydrocarbons which are used for producing fuels such as petrol and diesel.
- It is also a main source of raw materials (called feedstock) for the petrochemical industry.
- Crude oil formed over millions of years from the effects of high pressures and temperatures on the remains of plants and animals.
- Since it is being used up much faster than it is being formed crude oil is a finite resource.
- Crude oil as a mixture isn’t a very useful substance but the different hydrocarbons that make up the mixture, called fractions, are useful, with every fraction having different applications.
- Each fraction consists of groups of hydrocarbons of similar chain lengths.
- The fractions in petroleum are separated from each other in a process called fractional distillation.
- The molecules in each fraction have similar properties and boiling points, which depend on the number of carbon atoms in the chain.
- The boiling point and viscosity of each fraction increases as the carbon chain gets longer.
Fractional distillation of crude oil
Diagram showing the process of fractional distillation to separate crude oil in a fractionating column
- Fractional distillation is carried out in a fractionating column which is very hot at the bottom and cool at the top.
- Crude oil enters the fractionating column and is heated so vapours rise.
- Vapours of hydrocarbons with very high boiling points will immediately condense into liquid at the higher temperatures lower down and are tapped off at the bottom of the column.
- Vapours of hydrocarbons with low boiling points will rise up the column and condense at the top to be tapped off.
- The different fractions condense at different heights according to their boiling points and are tapped off as liquids.
- The fractions containing smaller hydrocarbons are collected at the top of the fractionating column as gases.
- The fractions containing bigger hydrocarbons are collected at the lower sections of the fractionating column.
- Number of Carbon & Hydrogen Atoms
- The size and length of each hydrocarbon molecule determines in which fraction it will be separated into.
- The size of each molecule is directly related to how many carbon and hydrogen atoms the molecule contains.
- Most fractions contain mainly alkanes, which are compounds of carbon and hydrogen with only single bonds between them.
Decane is an alkane, C10H22 and is a component in some fuels
- Boiling point
- As the molecules get larger, the intermolecular forces of attraction between the molecules becomes greater as there is more surface area contact between them.
- This means that more heat is needed to separate the molecules, hence with increasing molecular size there is an increase in boiling point.
- Viscosity refers to the ease of flow of a liquid.
- High viscosity liquids are thick and flow less easily.
- Viscosity also increases with increasing chain length.
Trend in boiling point of the main fractions
- Ease of Ignition
- Molecular size again influences the ease of ignition.
- Smaller hydrocarbon molecules are more flammable and are easier to ignite than larger molecules.
Summary of the effects of chain length on the properties of the fractions of crude oil