| Date | May 2018 | Marks available | 1 | Reference code | 18M.3.sl.TZ1.4 |
| Level | SL | Paper | 3 | Time zone | TZ1 |
| Command term | State | Question number | 4 | Adapted from | N/A |
Question
Both HDPE (high density polyethene) and LDPE (low density polyethene) are produced by the polymerization of ethene.
Both of these are thermoplastic polymers. Outline what this term means.
Compare and contrast the structures of HDPE and LDPE.
State one way in which a physical property of HDPE, other than density, differs from that of LDPE as a result of this structural difference.
The production of HDPE involves the use of homogeneous catalysts. Outline how homogeneous catalysts reduce the activation energy of reactions.
Trace amounts of metal from the catalysts used in the production of HDPE sometimes remain in the product. State a technique that could be used to measure the concentration of the metal.
Suggest two of the major obstacles, other than collection and economic factors, which have to be overcome in plastic recycling.
Suggest why there are so many different ways in which plastics can be classified. HDPE can, for example, be categorized thermoplastic, an addition polymer, having Resin Identification Code (RIC) 2, etc.
Markscheme
soften/melt when heated
OR
can be melted and moulded
Accept “low melting point” OR “can be moulded when heated”.
[1 mark]
both have «long» hydrocarbon chains
OR
both have chains comprising CH2 units
HDPE has little/no branching AND LDPE has «more» branching
Accept “CH2–CH2 units”.
Accept “HDPE more crystalline”.
[2 marks]
HDPE is more rigid/less flexible
OR
HDPE has a higher melting point
OR
HDPE has greater «tensile» strength
Accept “HDPE has lower ductility”.
[1 mark]
form «temporary» activated complexes/reaction intermediates
Accept “consumed in one reaction/step AND regenerated in a later reaction/step”.
Accept “provides alternative mechanism”.
[1 mark]
inductively coupled plasma/ICP spectroscopy using mass spectroscopy/mass spectrometry/MS/ICP-MS
OR
inductively coupled plasma/ICP spectroscopy using optical emission spectroscopy/OES/ICP-OES
Accept “atomic absorption/aa spectroscopy” or “MS/massspectroscopy/mass spectrometry”.
[1 mark]
Any two of:
many types «of plastics» exist
OR
«plastics» require sorting «by type»
«plastics» need to be separated from non-plastic materials
OR
«often» composites/moulded on/bound to non-plastic/other components
Accept other valid factors such as thermal decomposition of some plastics, production of toxic fumes, etc.
[2 marks]
«different classifications are appropriate for» different properties/applications/purposes
[1 mark]
