17.3 Cracking hydrocarbons

Question 1

What happens after fractional distillation?

Answer 1

Petroleum is separated into fractions by fractional distillation. But that is not the end of the story. The fractions all need further treatment before they can be used.

Question 2

Treatment 1

Answer 2

They contain impurities – mainly sulfur compounds. If left in the fuels, these will burn to form harmful sulfur dioxide gas.

Question 3

Treatment 2

Answer 3

Some fractions are separated further into single compounds, or smaller groups of compounds. For example the gas fraction is separated into methane, ethane, propane, and butane. (We buy butane in canisters.)

Question 4

Treatment 3

Answer 4

Part of a fraction may be cracked. Cracking breaks molecules down into smaller ones.

Question 5

Cracking a hydrocarbon in the lab

Answer 5

This experiment is carried out using a hydrocarbon oil from petroleum. The product is a gas, collected over water in an inverted test tube.

Question 6

Smell of the reactant vs the product

Answer 6

no smell to pungent smell

Question 7

Appearance of the reactant vs the product

Answer 7

thick colourless liquid to colourless gas

Question 8

Flammability of the reactant vs the product

Answer 8

difficult to burn to burns readily

Question 9

Reactions of the reactant vs the product

Answer 9

few chemical reactions to many chemical reactions

Question 10

Which process has taken place in cracking hydrocarbons?

Answer 10

The product is quite different from the reactant. Heating has caused the hydrocarbon to break down. A thermal decomposition has taken place.

Question 11

What did the flammability of the reactant indicate?

Answer 11

The reactant had a high boiling point and was not flammable - which means it had large molecules, with long chains of carbon atoms.

Question 12

What did the boiling point of the product indicate?

Answer 12

The product had a low boiling point and is very volatile - so it must have small molecules, with short carbon atoms.

Question 13

What can we say about the atoms in the product?

Answer 13

The product must also be a hydrocarbon since nothing new was added.

Question 14

Why could the product be a more useful chemical.

Answer 14

The product is more reactive.

Question 15

Cracking in the refinery

Answer 15

The long- chain hydrocarbon is heated to vaporise it.
The vapour is usually passed over a hot catalyst
Thermal decomposition takes place

Question 16

How does cracking help you make the best use of petroleum?

Answer 16

Cracking helps you make the best use of petroleum. Suppose you have too much of the naphtha fraction, and too little of the gasoline fraction. You can crack some naphtha to get molecules the right size for petrol.

Question 17

What type of chains does cracking always produce?

Answer 17

Cracking always produces short-chain compounds with a carbon-carbon double bond- This bond makes the compounds reactie. So they can be used to make plastics and other substances.