Explain these properties of carbon nanotubes.

CNT can act as Type 2 superconductors. Outline why Type 2 superconductors are generally more useful than Type 1.

Explain the role of electrons in superconducting materials in terms of the Bardeen–Cooper–Schrieffer (BCS) theory.

Alloying metals changes their properties. Suggest one property 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 $10.0$ hours. Use charge :(Q) = current (I) × time (t)  and section 2 of the data booklet.

Suggest a gas which should be continuously passed over the molten magnesium in the electrolytic cell.

Zeolites can be used as catalysts in the manufacture of CNT. Explain, with reference to their structure, the high selectivity of zeolites.

Experiments have been done to explore the nematic liquid crystal behaviour of CNT. Justify how CNT molecules could be classified as nematic.

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.

Any two of:
have higher critical temperatures/T_c «than Type 1»
OR
can act at higher temperatures ✔

have higher critical magnetic fields/B_c «than Type 1» ✔

less time needed to cool to operating temperature ✔

less energy required to cool down/maintain low temperature ✔

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 ✔

Any of:
ductility ✔
strength/resistance to deformation ✔
malleability ✔
hardness ✔
resistance to corrosion/chemical resistance ✔
range of working temperatures ✔
density ✔

Do not accept “conductivity”.

$«Q=I\times t=3.00\times 10.0\times 3600=»108 000 \mathrm{C}$ ✔

$«\frac{Q}{F}=\frac{108 000 C}{96 500 C {\mathrm{mol}}^{-1}}=»1.12«\mathrm{mol} {\mathrm{e}}^{-}»$ ✔

$«\frac{1.12 \mathrm{mol}}{2}=0.560 \mathrm{mol} \mathrm{Mg}»$
$«m=0.560 \mathrm{mol}\times 24.31 \mathrm{g} {\mathrm{mol}}^{-1}=»13.6«\mathrm{g}»$ ✔

Award [3] for correct final answer.

argon/$\mathrm{Ar}$/helium/$\mathrm{He}$ ✔

Accept any identified noble/inert gas.
Accept name OR formula.

Do not accept “nitrogen/$N_2$“.

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.

rod-shaped molecules
OR
«randomly distributed but» generally align
OR
no positional order AND have «some» directional order/pattern ✔

Accept “linear” for “rod-shaped”.

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.

Most gained at least one mark here for stating that Type 2 superconductors have higher critical temperatures than Type 1.

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. 

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.

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.

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).

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.

Most gained the one mark here, justifying how CNT molecules can be classified as nematic, by stating that they are "rod-shaped molecules".

Deduce, giving a reason, the group of elements in the periodic table most likely to undergo sublimation.

Describe the density trend across periods 4 and 5 of the periodic table.

Suggest, with a reason, whether the lanthanoids or actinoids of the f-block would have the higher density.

Compare the ease of oxidation of s-block and d-block metals to their melting points and densities. Use section 25 of the data booklet.

Sketch how the first ionization energies of elements vary with their atomic radius.

group 18/noble gases     [✔]

smallest difference between melting and boiling points
OR
weakest intermolecular forces «in that period»     [✔]

Note: Accept “group 17/halogens”.

density increases «to a maximum in the transition elements» AND then decreases     [✔]

actinoids AND density increases down all groups «due to large increase in atomic mass for small increase in atomic volume»
OR
actinoids AND «much» greater atomic mass with similar type of bonding
OR
actinoids AND density «of actinoids» atomic number 90 to 95 is greater than corresponding lanthanoids    [✔]

Note: Accept “actinoids AND on graph actinoids have «much» greater density than lanthanoids”.

Alternative 1:
«metals with» low densities oxidize easier    [✔]

«metals with» low melting points oxidize easier     [✔]

Alternative 2:
in s-block «metals with» high densities oxidize easier
OR
in s-block «metals with» low melting points oxidize easier     [✔]

in d-block «metals with» low densities oxidize easier
OR
in d-block «metals with» low melting points oxidize easier     [✔]

Note: Award [1 max] for “s-block metals more easily oxidized” OR “s-block metals have lower melting points” OR “s-block metals have lower densities”.

Accept “have greater activity” for “oxidize easier”.

    [✔]

Note: Accept any negative sloping line.

Do not award mark if line touches either axis.

Most candidates correctly identified the group of elements most likely to undergo sublimation but did not score for the reason as they referred to low melting and boiling points, rather than the smallest difference between these temperatures. There were several G2 comments that “the group of elements” was a confusing requirement as elements could be grouped in many ways, including for instance, from B to Ne. The Chemistry Guide clearly states that a group on the periodic table refers to a vertical column of elements. A few complaints were received about the inclusion of a question on sublimation, but the question was designed to make candidates think, and did not require knowledge of phase diagrams.

Required candidates to consider density trends. Most candidates correctly described trends across periods 4 and 5 but had difficulty predicting and explaining whether lanthanoids or actinoids would have the higher density. Many said that actinoids would have higher density because they have more protons and neutrons / greater atomic number / greater mass with no further detail about having similar bonding and hence similar volume. Some G2 comments complained about the inclusion of lanthanoids and actinoids in this question. However, the Chemistry Guide clearly states that these terms should be known.

Most candidates scored at least 1 mark for comparing the s-block and d-block metals and most drew a line with a negative slope.

Although many here failed to score because the line crossed or touched one of the axes. A few sketched a graph reminiscent of first ionization energy against atomic number.