Polymers and Alkenes

Question 1

What is cracking?

Answer 1

A chemical reaction that produces smaller hydrocarbons including alkanes and alkenes.

Question 2

What does cracking do?

Answer 2

Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules.

Question 3

How does cracking work?

Answer 3

Fractions containing large hydrocarbon molecules are heated to vaporise them. They are then either:
passed over a hot catalyst, or
mixed with steam and heated to a very high temperature.

These processes break chemical bonds in the molecules, causing thermal decomposition reactions.

Question 4

What are the products of cracking?

Answer 4

The products of cracking include alkenes (for example ethene and propene). The alkenes are a family of hydrocarbons that share the same general formula:

CnH2n

Question 5

What does the general formula for cracking mean?

Answer 5

The general formula means that the number of hydrogen atoms in an alkene is double the number of carbon atoms.

Question 6

What are alkenes?

Answer 6

Alkenes are unsaturated hydrocarbons. They contain a double covalent bond, which is shown as two lines between two of the carbon atoms.

Question 7

What can alkenes be used to make?

Answer 7

Alkenes can be used to make polymers. Polymers are very large molecules made when many smaller molecules join together, end-to-end.

Question 8

What is the name of the smaller molecules that make up the larger molecules in polymers?

Answer 8

Monomers

Lots of monomer molecules → a polymer molecule.

Question 9

What can alkenes act as?

Answer 9

Alkenes can act as monomers because they are unsaturated, meaning they have a double bond.

Question 10

Give some examples of polymers and their uses.

Answer 10

Polythene is used for plastic bags and plastic bottles.
Polypropene is used for ropes and crates.
Polychloroethene is used for waters pipes and insulation for electricity cables.

Question 11

Give some examples of modern day polymers.

Answer 11

Modern polymers have many uses, including:
new packaging materials
waterproof coatings for fabrics (such as for outdoor clothing)
fillings for teeth
dressings for cuts
hydrogels (soft contact lenses and disposable nappy liners)
smart materials (shape memory polymers for shrink-wrap packaging).

Question 12

Name one advantage and one disadvantage of polymers.

Answer 12

Advantage
They are unreactive, so they are suitable for storing food and chemicals safely.
Disadvantage
Unfortunately, this property makes it difficult to dispose of polymers. They can cause litter and are usually sent to landfill sites.

Question 13

Name a disadvantage of recycling polymers.

Answer 13

Polymers must be separated from each other before they are recycled and this can be difficult and expensive.

Question 14

What is ethanol?

Answer 14

Ethanol is the type of alcohol found in alcoholic drinks such as wine and beer. It is also used as a fuel. Ethanol molecules contain carbon, hydrogen and oxygen.

Question 15

How is ethanol made?

Answer 15

Ethanol can be made by reacting ethene (from cracking crude oil fractions) with steam. A catalyst of phosphoric acid is used to ensure a fast reaction.

ethene + steam → ethanol

C2H4 + H2O → C2H5OH

Question 16

What is ethanol made from?

Answer 16

Ethene and Steam

Question 17

Name one problem with ethene.

Answer 17

Ethene is made from crude oil, which is a non-renewable resource. It cannot be replaced once it is used up and it will run out one day.

Question 18

How does fermentation work?

Answer 18

Sugar from plant material is converted into ethanol and carbon dioxide by fermentation. The enzymes found in single-celled fungi (yeast) are the natural catalysts that can make this process happen.
Unlike ethene, sugar from plant material is a renewable resource.

Question 19

Name another way of making ethanol.

Answer 19

Fermentation.