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CRUDE OIL. Definition and Formation of Crude Oil Fractional Distillation of Crude Oil Cracking and Reforming of Crude Oil Fractions Impact of the Oil Industry on the Environment . Definition and Formation of Crude Oil. Crude oil is a complex mixture of naturally

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Definition and formation of crude oil
Definition and Formation of Crude Oil

  • Crude oil is a complex mixture of naturally

    occurring hydrocarbons found within the earth.

    It consists mainly of a variety of alkanes,

    cycloalkanes and aromatic hydrocarbons.

  • Crude oil was formed from the remains of

    microscopic plants and animals that were buried

    and preserved in rocks millions of years ago.

These remains slowly decayed as they were buried

deeper within layers of rocks and with time and

temperature they were converted to crude oil.

  • The composition and appearance of crude oil

    depends on its source. Most crude oil looks

    like thin, brown treacle (molasses), while some

    are almost colourless volatile(easily evaporated at

    normal temp.) liquids and others are thick black


  • Crude oil must be separated into various components

    and selectively modified before it can be useful.

    This process is called refining and occurs in an

    oil refinery. The primary process for separating the

    components of crude oil is fractional distillation.

The typical fractions of crude oil and their boiling

point ranges are shown in the table below.

Cracking of crude oil fractions
Cracking Of Crude Oil Fractions

Cracking is a process in which large hydrocarbon

molecules are broken down into smaller molecules.

For example, dodecane could break into decane and ethene:

C₁₂H₂₆ C₁₀H₂₂ + C₂H₄

or into nonane and propene:

C₁₂H₂₆ C₉H₂₀ + C₃H₆

Cracking is carried out either using heat (thermal cracking)

or a catalyst (catalytic cracking).

Thermal cracking
Thermal Cracking

Thermal cracking involves rapidly heating the

hydrocarbon to temperatures of about 800⁰C and

then cooling it. This process occurs within a second.

The high temperatures can cause the C - C bond

to undergo homolytic fission, leaving each carbon

with a single unpaired electron. For example,

octane could break down into a hexyl radical and

an ethyl radical:

C₁₂H₂₆ C₁₀H₂₁˙ + C₂H₅ ̇

The raised dot ( ̇) indicates an unpaired electron.

These free radicals can then undergo further reactions.

For example, they can lose a hydrogen atom to form

a stable molecule. The ethyl radical becomes ethene

in such a reaction:

C₂H₅ ̇ C₂H₄ + H ̇

The hydrogen atom, which is a free radical, can combine

with the decyl radical to form decane.

H˙ + C₁₀H₂₁˙ C₁₀H₂₂

Or two hydrogen atoms can combine together to form

hydrogen gas:

H ̇ + H ̇ H₂

Thermal cracking is generally used for cracking the

residue fractionof crude oil.

Catalytic cracking
Catalytic Cracking

Catalytic cracking (cat-cracking) involves the use of

a catalyst at lower temperatures to break the bonds

of the hydrocarbon molecules. The reaction is

usually catalyzed by a powdered mixture of

alumina and silica (Al₂O₃/SiO₂) at about 500⁰C.

The C-C bond undergoes heterolytic fission which

results in a mechanism involving carbocations.

Catalytic cracking is used to crack the distilled

fractions such as diesel oil and kerosene into

smaller molecules. The cracked compounds are

important to produce more valuable fuel as well

a chemical feedstock for the petrochemical industry.

Past paper question
Past Paper Question

1.) a). Briefly describe the principles involved in

i). fractional distillation (3 marks)

ii). cracking. (2 marks)

b). Give a balanced equation to illustrate cracking.

(1 mark)

c). Explain the importance of EITHER fractional

distillation OR cracking. (1 mark)

d). Suggest ONE adverse effect associated with the extraction

of crude oil and comment on the environmental problems

that result. (3 marks)