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Date May 2017 Marks available 2 Reference code 17M.3.sl.TZ1.10
Level SL Paper 3 Time zone TZ1
Command term Explain Question number 10 Adapted from N/A

Question

Infrared (IR) spectroscopy is often used for the identification of polymers, such as PETE, for recycling.

LDPE and high density polyethene (HDPE) have very similar IR spectra even though they have rather different structures and physical properties.

Below are the IR spectra of two plastics (A and B); one is PETE, the other is low density polyethene (LDPE).

Deduce, giving your reasons, the identity and resin identification code (RIC) of A and using sections 26 and 30 of the data booklet.

[3]
a.

Describe the difference in their structures.

[1]
b.i.

Explain why the difference in their structures affects their melting points.

[2]
b.ii.

Markscheme

A RIC: 1 AND B RIC: 4

ALTERNATIVE 1:
«only» PETE contains carbonyl/C=O/ester/COO groups
carbonyl groups absorb at 1700–1750 «cm–1»

ALTERNATIVE 2:
LDPE contains more C–H bonds «than PETE»
C–H bonds absorb at 2850–3090 «cm–1»

 

For either, accept specific frequencies in these ranges (eg 1735 «cm–1» or 2900 «cm–1»).

[3 marks]

a.

HDPE less branched
OR
LDPE more branched

 

Accept “no branching in HDPE AND branching in LDPE”.

[1 mark]

b.i.

HDPE «polymer» chains/molecules can pack together more closely «than LDPE chains»
OR
HDPE «polymer» chains/molecules have a higher contact surface area «than LDPE chains»

stronger intermolecular/dispersion/London/van der Waals’ forces in HDPE AND higher melting point

 

Accept converse arguments.

[2 marks]

b.ii.

Examiners report

[N/A]
a.
[N/A]
b.i.
[N/A]
b.ii.

Syllabus sections

Core » Topic 10: Organic chemistry » 10.1 Fundamentals of organic chemistry
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