Solved: Both HDPE (high density polyethene) and LDPE (low density polyethene) are... [IB Chemistry]

Question

SLPaper 2

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.

[1]

Verified

Solution

Thermoplastic polymers soften/melt when heated 1 mark

Thermoplastic polymers can be melted and moulded 1 mark

Accept "low melting point" OR "can be moulded when heated"

1 mark total

Suggest two of the major obstacles, other than collection and economic factors, which have to be overcome in plastic recycling.

[2]

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Solution

Many types of plastics exist 1 mark

Plastics require sorting by type 1 mark
Plastics need to be separated from non-plastic materials 1 mark

Often composites/moulded on/bound to non-plastic/other components 1 mark

Accept other valid factors such as thermal decomposition of some plastics, production of toxic fumes, etc.

2 marks total

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.

[1]

Verified

Solution

HDPE is more rigid/less flexible 1 mark

HDPE has a higher melting point 1 mark

HDPE has greater tensile strength 1 mark

Accept "HDPE has lower ductility".

1 mark total

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.

[1]

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Solution

Different classifications are appropriate for different properties/applications/purposes 1 mark

1 mark total

Compare and contrast the structures of HDPE and LDPE.

[2]

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Solution

Both HDPE and LDPE have long hydrocarbon chains 1 mark

OR
Both have chains comprising CH₂ units
HDPE has little/no branching AND LDPE has more branching 1 mark

Accept "CH₂–CH₂ units"

Accept "HDPE more crystalline"

2 marks total

The production of HDPE involves the use of homogeneous catalysts. Outline how homogeneous catalysts reduce the activation energy of reactions.

[1]

Verified

Solution

Form temporary activated complexes/reaction intermediates 1 mark

Accept "consumed in one reaction/step AND regenerated in a later reaction/step".

Accept "provides alternative mechanism".

1 mark total

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.

[1]

Verified

Solution

Inductively coupled plasma (ICP) spectroscopy using mass spectroscopy/mass spectrometry (MS/ICP-MS) 1 mark

Inductively coupled plasma (ICP) spectroscopy using optical emission spectroscopy (OES/ICP-OES) 1 mark

Accept "atomic absorption (AA) spectroscopy" or "MS/mass spectroscopy/mass spectrometry"

1 mark total

Both HDPE (high density polyethene) and LDPE (low density polyethene) are produced by the polymerization of ethene.

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