Chemistry SS1: Cracking and Reforming

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Questions and Answers

What is the primary purpose of reforming in the production of petrol?

To increase the octane number of petrol

To remove impurities from petrol

To break down large molecules into smaller ones

To change a compound into its isomers (correct)

What is the octane number of a straight-chain alkane like heptane?

100

0 (correct)

What is the result of petrol containing a higher percentage of straight-chain hydrocarbons in a car engine?

Reduced engine knock

Irregular motion of the pistons (correct)

Increased fuel efficiency

Smooth engine performance

What is the purpose of adding catalysts such as oxides of silicon and aluminium in the production of petrol?

To increase the octane number of petrol and produce high-grade petrol Signup and view all the answers

What is the composition of a petrol with an octane rating of 94?

94% iso-octane and 6% heptane Signup and view all the answers

What is the effect of knocking on a car engine?

It causes irregular motion of the pistons Signup and view all the answers

What is the primary purpose of cracking in the production of petrol?

To break down large molecules into smaller ones Signup and view all the answers

What is the octane number of a highly branched-chain alkane like 2,2,4 trimethylpentane?

100 Signup and view all the answers

What is the primary purpose of cracking in the petroleum industry?

To convert less volatile fractions into petrol Signup and view all the answers

What is the result of the cracking process on the petrol obtained from the petrol fractions?

It yields a higher grade of petrol Signup and view all the answers

What is the benefit of cracking in terms of the octane number of petrol?

It increases the octane number Signup and view all the answers

What is the by-product of the cracking process used for?

Manufacturing of plastics, synthetic rubber, and ethanol Signup and view all the answers

What is the purpose of reforming in the petroleum industry?

To improve the anti-knock properties of petrol Signup and view all the answers

Which fractions are the most difficult to cope with in terms of demand and supply?

Kerosene and diesel oil fractions Signup and view all the answers

What is the advantage of cracking over the distillation of crude oil?

It provides a more controllable process Signup and view all the answers

What is the range of carbon atoms in the fraction from which petrol is produced?

C6 – C12 Signup and view all the answers

Study Notes

Cracking

Cracking is the process of breaking down large hydrocarbon molecules into two or more smaller hydrocarbon molecules.
It is used to increase the quantity of petrol, as the fraction from which petrol is produced (C6 - C12) is small compared to other fractions with a greater number of carbon atoms.
There are two types of cracking in use in the petroleum industry:
- One type is more controllable, allowing for desirable products of certain chain lengths to be obtained.
- The other type yields a higher grade of petrol than the one obtained directly from the petrol fractions during the distillation of crude oil.

Benefits of Cracking

It increases the yield of petroleum.
It provides a petrol mixture rich in branched-chain hydrocarbons with an attendant increase in octane number.
It yields as a by-product, a large quantity of ethane, propane, butane, etc. used for making plastics, synthetic rubber, detergent, and many important chemicals like ethanol and phenol.

Reforming

Reforming is the process of converting long-chain hydrocarbons to shorter and branched-chain molecules to improve its anti-knock properties.
It usually takes place in the presence of catalysts such as oxides of silicon and aluminium at about 6000C and pressure between 8 and 15 atm.
Reforming is an isomerisation process, changing a compound into its isomers.

Octane Number

The octane number or octane rating of petrol is a measure of the proportion of branched-chain hydrocarbons to the straight-chain hydrocarbons in a given blend of gasoline (petrol).
Straight-chain hydrocarbons (e.g. n-heptane) burn too rapidly in the car engine, causing irregular motion of the pistons and resulting in rattling noise (knocking).
Branched-chain hydrocarbons (e.g. 2,2,4 trimethylpentane) burn very smoothly in engines.
Octane number is a measure of the performance of the fuel in engines and is rated according to the percentage of branched-chain hydrocarbons to straight-chain hydrocarbons present.

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Description

Learn about the process of cracking and reforming in chemistry, including breaking down large hydrocarbon molecules and increasing petrol quantity.