Hydrocarbons and polymers (Extended response)

Question 1

Tetrafluoroethene, C[2]F[4], is used to make poly(tetrafluoroethene).

This is an addition polymer often called PTFE. It is used as the non-stick coating in pans and for lining pipes carrying corrosive liquids.

Explain how PTFE molecules are formed from tetrafluoroethene molecules, and how the properties of PTFE are related to its uses.

State one problem associated with the disposal of PTFE, and give a reason for your answer. [6]

Answer 1

• tetrafluoroethene can act as a monomer because its molecules contain a C=C bond
• this bond can open up, allowing many molecules to react together to form a long PTFE molecule

(relevant diagrams here)

• PTFE is very slipperly, so food will not stick to it in a pan
• its molecules consist of long chains of carbon atoms, with strong intermolecular forces between molecules, so its melting point is high

-PTFE does not react with acids and alkalis, making it suitable for lining chemical pipes.

• PTFE is not biodegradable
• this means that it will persist in the environment if disposed of in a landfill site
• it may produce harmful gases if disposed of by combustion in an incinerator

Question 2

Ethane and ethene are compounds produced from crude oil. Both undergo complete combustion but only one of them can take part in an addition reaction with bromine.

Describe these reactions, and explain how to distinguish between the two compounds using a simple laboratory test. In your answer, draw their structures and include balanced equations. [6]

Answer 2

ethane:

• C[2]H[6]
• structure: C-C with six C-H bonds
• saturated
• complete combustion produces CO2 and water

2C[2]H[6] + 7O[2] -> 4CO[2] + 6H[2]O

ethene:

• C[2]H[4]
• structure: C=C with four C-H bonds
• unsaturated
• complete combution produces CO2 and water

(combustion)
C[2]H[4] + 3O[2] -> 2CO[2] + 2H[2]O

differences:
-less oxygen is needed when ethene burns
-less water is produced when ethene burns
-ethene reacts with bromine water in an addition reaction/ethane does not
C[2]H[4] + Br[2] -> C[2]H[4]Br[2]

laboratory test:

• bromine water is decolourised/changes from orange-brown to colourless in ethene
• no change in colour in ethane