Question 19M.3.sl.TZ1.5
| Date | May 2019 | Marks available | [Maximum mark: 3] | Reference code | 19M.3.sl.TZ1.5 |
| Level | sl | Paper | 3 | Time zone | TZ1 |
| Command term | Explain, State | Question number | 5 | Adapted from | N/A |
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]
nitrile [✔]
Note: Accept “cyano".
A few students correctly named the nitrile group as being the one responsible for the polarity of the liquid crystal molecule.
Explain the effects of very low and high temperatures on the liquid-crystal behaviour of this molecule.
Low temperature:
High temperature:
[2]
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”.
Very poorly answered with candidates failing to link the change in liquid crystal properties to changes in molecular motion.
