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Why is there a gradual change in the physical properties of petroleum fractions?

Why is there a gradual change in the physical properties of petroleum fractions?

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isaaq
1 week ago
Crude oil is a finite resource. Petrol and other fuels are produced from it using fractional distillation and cracking. One family of hydrocarbons produced from crude oil is called the alkanes.
Each crude oil fraction contains a mixture of hydrocarbons. The hydrocarbons in a fraction are mostly hydrocarbons called alkanes. The alkanes in each fraction have similar (but not identical):

numbers of hydrogen and carbon atoms in their molecules
boiling points
ease of ignition (how easily they are set on fire)
viscosity (a measure of how they flow)
For example, the gases fraction contains hydrocarbons with one to four carbon atoms. These have boiling points below room temperature. They are very flammable (easily set on fire) and have a low viscosity (because they are gases).

The hydrocarbons in different fractions differ in these properties. For example, the bitumen fraction contains hydrocarbons with more than 35 carbon atoms. These have boiling points well above room temperature. They are very difficult to ignite and have a high viscosity (they flow with difficulty).

Alkanes as a homologous series
A homologous series is series of compounds which:

have the same general formula
differ by CH2 in the molecular formula of neighbouring compounds
show a gradual variation in physical properties, such as their boiling points
have similar chemical properties
Each crude oil fraction contains a mixture of hydrocarbons. The hydrocarbons in a fraction are mostly hydrocarbons called alkanes. The alkanes in each fraction have similar (but not identical):

numbers of hydrogen and carbon atoms in their molecules
boiling points
ease of ignition (how easily they are set on fire)
viscosity (a measure of how they flow)
For example, the gases fraction contains hydrocarbons with one to four carbon atoms. These have boiling points below room temperature. They are very flammable (easily set on fire) and have a low viscosity (because they are gases).

The hydrocarbons in different fractions differ in these properties. For example, the bitumen fraction contains hydrocarbons with more than 35 carbon atoms. These have boiling points well above room temperature. They are very difficult to ignite and have a high viscosity (they flow with difficulty).

Alkanes as a homologous series
A homologous series is series of compounds which:

have the same general formula
differ by CH2 in the molecular formula of neighbouring compounds
show a gradual variation in physical properties, such as their boiling points
have similar chemical properties
The table shows four alkanes, their molecular formulae and their structures.
The general formula for the alkanes is CnH2n+2 (where n stands for the number of carbon atoms). As the number of carbon atoms increases:

the intermolecular forces between alkane molecules become stronger
more energy is needed to overcome these forces
the boiling point increases
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