DP Chemistry (last assessment 2024)

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Question 19M.3.hl.TZ1.5

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Date May 2019 Marks available [Maximum mark: 3] Reference code 19M.3.hl.TZ1.5
Level hl Paper 3 Time zone TZ1
Command term Explain, Label Question number 5 Adapted from N/A
5.
[Maximum mark: 3]
19M.3.hl.TZ1.5

Liquid-crystal displays (LCDs) have many uses.

A molecule which acts as a chiral nematic thermotropic liquid-crystal is given.

(a)

Label with an asterisk, *, the chiral carbon atom.

[1]

Markscheme

  [✔]

Examiners report

Identifying a chiral carbon atom was answered reasonably well.

(b)

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

 

Low temperature:

High temperature:

[2]

Markscheme

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”.

Examiners report

Explaining effects of very low and very high temperatures on liquid-crystal behaviour proved difficult for most candidates. Responses lacked the required detail about intermolecular forces between molecules.

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