Chemistry 11 Notes for Chemistry Notes

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Introduction to petroleum
Petroleum is a complex mixture of hydrocarbons and a number of different compounds whose composition varies according to its place of occurrence.

The word petroleum is composed of two Latin words Petra means rock and oleum means oil. Petroleum is a dark brown, thick and viscous liquid which occurs below the earth’s surface. Petroleum is a complex mixture of hydrocarbons and different compounds of sulphur, oxygen and nitrogen in small amounts. Usually it contains alkanes, alkenes, cyclo alkanes, aromatic hydrocarbons etc.

Refining of petroleum
The process of dividing petroleum into fractions with different boiling range volatilities and free from impurities is called refining.
Petroleum is refined by fractional distillation.

Fractional distillation
The process of separating a mixture into a series of fractions of different volatilities by means of distillation is known as fractional distillation.

In the process of fractional distillation, a mixture of different liquids is evaporated followed by condensation. Different liquids are evaporated according to their boiling point, and they are collected in different chambers of distillation tower.

Fractional distillation of crude oil
Generally crude oil contains compounds having boiling points upto 400 OC. In this process, crude oil is heated to above 400 OC in a pipe. Its vapors are allowed to enter into a fractionating tower having different compartment to collect different volatilities. These are now condensed to liquid and are separated.


More than 500 different compounds are obtained in the process of the distillation of petroleum as shown in the table.


Gasoline which is commonly known as petrol is a mixture of n-hexane and n-heptane. During the process of fractional distillation, it is obtained in the upper most portion of the tower.

Octane number
Octane number is a standard which determines the knocking ability and quality of gasoline. Higher is the octane number of a gasoline, lower is the knocking it produces.
Octane number can be increased by the following two methods:

  • Reforming
  • By adding TEL (tetraethyl lead)

Knocking is a sharp metallic sound produced in the internal combustion engine. Knocking is caused by the low octane number of gasoline.

Reforming of petroleum
The conversion of straight chain hydrocarbon into branched chain hydrocarbon is called reforming of petroleum.
By the process of reforming, the octane number of a gasoline is improved. The process of reforming is carried out in the presence of catalyst n-alkanes burn in internal combustion engine with explosion and produce knocking but branched chain hydrocarbons burn smoothly.
Reforming is a process similar to cracking, which converts n-alkanes into branched alkanes.
The octane number of gasoline may also be improved by adding tetraethyl lead (TEL) Pb(C2H5)4 which is a knock inhibitor. But the use of compounds containing lead cause environment pollution.

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