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Date May 2019 Marks available 1 Reference code 19M.3.sl.TZ1.5
Level SL Paper 3 Time zone TZ1
Command term State Question number 5 Adapted from N/A

Question

Liquid-crystal displays (LCDs) have many uses.

A molecule which acts as a thermotropic liquid crystal is shown.

State the name of the functional group which allows the molecule to be responsive to applied electric fields.

[1]
a.

Explain the effects of very low and high temperatures on the liquid-crystal behaviour of this molecule.

 

Low temperature: 

High temperature:

[2]
b.

Markscheme

nitrile  [✔]

Note: Accept “cyano".

a.

Low temperature:
intermolecular forces prevent molecules moving AND solid/«normal» crystal formation  [✔]

High temperature:
«above a critical temperature» disrupts alignment of molecules AND behaves as fluid/liquid  [✔]

 

Note: Accept “weak intermolecular forces break AND behaves as fluid/liquid”.

b.

Examiners report

A few students correctly named the nitrile group as being the one responsible for the polarity of the liquid crystal molecule.

a.

Very poorly answered with candidates failing to link the change in liquid crystal properties to changes in molecular motion.

b.

Syllabus sections

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