| Date | May 2019 | Marks available | 2 | Reference code | 19M.3.hl.TZ1.5 |
| Level | HL | Paper | 3 | Time zone | TZ1 |
| Command term | Explain | Question number | 5 | Adapted from | N/A |
Question
Liquid-crystal displays (LCDs) have many uses.
A molecule which acts as a chiral nematic thermotropic liquid-crystal is given.
Label with an asterisk, *, the chiral carbon atom.
Explain the effects of very low and high temperatures on the liquid-crystal behaviour of this molecule.
Low temperature:
High temperature:
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
Identifying a chiral carbon atom was answered reasonably well.
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.
Syllabus sections
-
18M.3.sl.TZ1.3b.iv:
State the property of carbon nanotubes that enables them to form a nematic liquid crystal phase.
- 16N.3.sl.TZ0.7b: Explain the effect of increasing the temperature of a nematic liquid crystal on its...
- 18N.3.sl.TZ0.4b: The structure of biphenyl nitrile is shown. Describe, giving a reason, a feature of the...
- 18N.3.sl.TZ0.4a: Outline two observations that he could have made.
-
18M.3.sl.TZ1.4a:
Both of these are thermoplastic polymers. Outline what this term means.
-
19M.3.sl.TZ1.5a:
State the name of the functional group which allows the molecule to be responsive to applied electric fields.
-
19M.3.hl.TZ1.5a:
Label with an asterisk, *, the chiral carbon atom.
- 16N.3.sl.TZ0.7a: Outline how a lyotropic liquid crystal differs from a thermotropic liquid crystal.
- 19N.3.sl.TZ0.6b: State how liquid crystals are affected by an electric field.
- 19N.3.sl.TZ0.6a: Describe the arrangement of soap molecules in the nematic liquid crystal phase.
-
19M.3.sl.TZ1.5b:
Explain the effects of very low and high temperatures on the liquid-crystal behaviour of this molecule.
Low temperature:
High temperature:
-
18M.3.hl.TZ2.6b:
MWCNT are very small in size and can greatly increase switching speeds in a liquid crystal allowing the liquid crystal to change orientation quickly.
Discuss two other properties a substance should have to be suitable for use in liquid crystal displays.
-
17M.3.sl.TZ2.6a:
Two important properties of a liquid crystal molecule are being a polar molecule and having a long alkyl chain. Explain why these are essential components of a liquid crystal molecule.
-
18M.3.sl.TZ2.5b:
Discuss three properties a substance should have to be suitable for use in liquid crystal displays.
-
19M.3.hl.TZ2.3:
Describe the characteristics of the nematic liquid crystal phase.
Shape of molecules:
Distribution:
- 20N.3.hl.TZ0.4e: Experiments have been done to explore the nematic liquid crystal behaviour of CNT. Justify...
-
19M.3.sl.TZ2.3:
Describe the characteristics of the nematic liquid crystal phase and the effect that an electric field has on it.
Shape of molecules:Distribution:
Effect of electric field:
- 20N.3.sl.TZ0.4d: Experiments have been done to explore the nematic liquid crystal behaviour of CNT. Justify...
-
17N.3.sl.TZ0.7a:
Outline two properties a substance should have to be used as liquid-crystal in a liquid-crystal display.
