Question 20N.3.hl.TZ0.4
| Date | November 2020 | Marks available | [Maximum mark: 15] | Reference code | 20N.3.hl.TZ0.4 |
| Level | hl | Paper | 3 | Time zone | TZ0 |
| Command term | Calculate, Explain, Justify, Outline, Suggest | Question number | 4 | Adapted from | N/A |
There has been significant growth in the use of carbon nanotubes, CNT.
Explain these properties of carbon nanotubes.
[2]
Excellent strength: defect-free AND rigid/regular 2D/3D ✔
Excellent conductivity: delocalized electrons ✔
Accept “carbons/atoms are all covalently bonded to each other” for M1.
The stronger candidates knew that the excellent conductivity associated with CNTs is associated with delocalised electrons but few scored the mark for citing the property associated with excellent strength, which can be attributed to being defect-free and having a rigid/regular 2D/3D structure.
CNT can act as Type 2 superconductors. Outline why Type 2 superconductors are generally more useful than Type 1.
[2]
Any two of:
have higher critical temperatures/Tc «than Type 1»
OR
can act at higher temperatures ✔
have higher critical magnetic fields/Bc «than Type 1» ✔
less time needed to cool to operating temperature ✔
less energy required to cool down/maintain low temperature ✔
Most gained at least one mark here for stating that Type 2 superconductors have higher critical temperatures than Type 1.
Explain the role of electrons in superconducting materials in terms of the Bardeen–Cooper–Schrieffer (BCS) theory.
[3]
Any three of:
passing electrons «slightly» deform lattice/displace positive ions/cations ✔
electrons couple/form Cooper pairs/condense with other electrons ✔
energy propagates along the lattice in wave-like manner/as phonons ✔
Cooper pair/electron condensate/pair of electrons moves through lattice freely
OR
phonons are «perfectly» elastic/cause no energy loss ✔
The role of electrons in superconducting materials in terms of the Bardeen-Cooper-Schrieffer (BCS) theory was very well understood and many scored all three marks.
Alloying metals changes their properties. Suggest one property of magnesium that could be improved by making a magnesium–CNT alloy.
[1]
Any of:
ductility ✔
strength/resistance to deformation ✔
malleability ✔
hardness ✔
resistance to corrosion/chemical resistance ✔
range of working temperatures ✔
density ✔
Do not accept “conductivity”.
This question proved to be difficult and few could suggest a suitable property (such as ductility) of magnesium that could be improved by making a magnesium-CNT alloy.
Pure magnesium needed for making alloys can be obtained by electrolysis of molten magnesium chloride.
© International Baccalaureate Organization 2020
Calculate the theoretical mass of magnesium obtained if a current of 3.00 A is used for hours. Use charge :(Q) = current (I) × time (t) and section 2 of the data booklet.
[3]
✔
✔
✔
Award [3] for correct final answer.
The better candidates scored all three marks for the electrolysis calculation. Even the weaker candidates managed to score at least one mark for calculating Q = 108,000 C.
Suggest a gas which should be continuously passed over the molten magnesium in the electrolytic cell.
[1]
argon//helium/ ✔
Accept any identified noble/inert gas.
Accept name OR formula.
Do not accept “nitrogen/“.
The most common error here was "nitrogen" as the gas that should be continuously passed over the molten magnesium in the electrolytic cell. Magnesium can react with nitrogen forming magnesium nitride, which makes this choice of gas unsuitable (unlike argon for example).
Zeolites can be used as catalysts in the manufacture of CNT. Explain, with reference to their structure, the high selectivity of zeolites.
[2]
pores/cavities/channels/holes/cage-like structures ✔
«only» reactants with appropriate/specific size/geometry/structure fit inside/go through/are activated/can react ✔
Accept “molecules/ions” for “reactants” in M2.
The explanation of the high selectivity of zeolites, in terms of their structure, was very well answered and many scored both marks. A thorough understanding of zeolites was much better conveyed in N20 compared to previous sessions.
Experiments have been done to explore the nematic liquid crystal behaviour of CNT. Justify how CNT molecules could be classified as nematic.
[1]
rod-shaped molecules
OR
«randomly distributed but» generally align
OR
no positional order AND have «some» directional order/pattern ✔
Accept “linear” for “rod-shaped”.
Most gained the one mark here, justifying how CNT molecules can be classified as nematic, by stating that they are "rod-shaped molecules".