DP Chemistry (last assessment 2024)

Question 20N.3.sl.TZ0.4

Date	November 2020	Marks available	[Maximum mark: 12]	Reference code	20N.3.sl.TZ0.4
Level	sl	Paper	3	Time zone	TZ0
Command term	Calculate, Explain, Justify, Suggest, Write	Question number	4	Adapted from	N/A

[Maximum mark: 12]

20N.3.sl.TZ0.4

There has been significant growth in the use of carbon nanotubes, CNT.

(a)

Explain these properties of carbon nanotubes.

[2]

Markscheme

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.

Examiners report

This was a very popular option with approximately 34% of candidates attempting Option B. Many students appeared well prepared for this option. Some candidates continue to provide answers with a heavy Biology bias that often make them lose valuable points.

(b(i))

Alloying metals changes their properties. Suggest one property of magnesium that could be improved by making a magnesium–CNT alloy.

[1]

Markscheme

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

Do not accept “conductivity”.

Examiners report

(b(ii))

Pure magnesium needed for making alloys can be obtained by electrolysis of molten magnesium chloride.

Write the half-equations for the reactions occurring in this electrolysis.

[2]

Markscheme

Anode: $2 {Cl}^{-} \to {Cl}_{2} (g) + 2 e^{-}$ ✔

Cathode: ${Mg}^{2 +} + 2 e^{-} \to Mg (l)$ ✔

Accept $C l^{-} \to C l_{2} (g) + e^{-}$ .

Award [1 max] for correct equations at incorrect electrodes.

Examiners report

(b(iii))

Calculate the theoretical mass of magnesium obtained if a current of $3.00 A$ is used for $10.0$ hours. Use charge $(Q) = c u r r e n t (I) \times t i m e (t)$ and section 2 of the data booklet

[3]

Markscheme

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

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

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

Award [3] for correct final answer.

Examiners report

(b(iv))

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

[1]

Markscheme

argon/ $Ar$ /helium/ $He$ ✔

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

Do not accept “nitrogen/ $N_{2}$ “.

Examiners report

(c)

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

[1]

Markscheme

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.

Examiners report

(d)

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

[1]

Markscheme

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

Accept “linear” for “rod-shaped”.

Examiners report

(e)

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

[1]

Markscheme

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

Accept “linear” for “rod-shaped”.

Examiners report

Question 20N.3.sl.TZ0.4

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