(i) Uranium hexafluoride, UF₆, is used in the uranium enrichment process that produces fuel for nuclear reactors.

State the molecular shape of uranium hexafluoride.

(ii) Explain why uranium dioxide, UO₂, has a very high melting point whereas uranium hexafluoride vapourises easily into gas.

i

octahedral

Accept “square bipyramidal”

ii

UO₂ strong bonding throughout crystal structure

UF₆ molecular «covalent bonds between atoms» AND London/dispersion/instantaneous induced dipole-induced dipole forces between molecules 

Accept “UO₂ has ionic lattice”

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".

Describe how the monomers of addition polymers and of condensation polymers differ.

Identify the type of intermolecular bonding that is responsible for Kevlar^®’s strength.

addition: C=C

AND

condensation: two functional groups needed on each monomer

Accept "alkene/alkenyl" OR "double bond" OR "multiple bond".

hydrogen bonds

Accept “$\pi -\pi$ stacking/interactions”.

Proteins are polymers of amino acids.

A paper chromatogram of two amino acids, A1 and A2, is obtained using a non-polar solvent.

© International Baccalaureate Organization 2020.

Determine the ${\mathrm{R}}_{\mathrm{f}}$ value of A1.

Proteins are polymers of amino acids.

The mixture is composed of glycine, $\mathrm{Gly}$, and isoleucine, $\mathrm{Ile}$. Their structures can be found in section 33 of the data booklet.

Deduce, referring to relative affinities and ${\mathrm{R}}_{\mathrm{f}}$, the identity of A1.

Proteins are polymers of amino acids.

Glycine is one of the amino acids in the primary structure of hemoglobin.

State the type of bonding responsible for the α-helix in the secondary structure.

Proteins are polymers of amino acids.

Sketch and label two oxygen dissociation curves, one for adult hemoglobin and one for foetal hemoglobin.

Proteins are polymers of amino acids.

Explain why the affinity for oxygen of foetal hemoglobin differs from that of adult hemoglobin.

$0.70$ ✔

Accept any value within the range “$\mathit{0}\mathit{.}\mathit{67}\mathit{-}\mathit{0}\mathit{.}\mathit{73}$”.

Ile AND larger R_f ✔

more soluble in non-polar solvent «mobile phase»
OR
not as attracted to polar «stationary» phase ✔

Only award M2 if Ile is identified in M1.

hydrogen/$\mathrm{H}$ bonding «between amido hydrogen and carboxyl oxygen atoms» ✔

both curves sigmoidal shape AND starting at zero ✔

foetal hemoglobin showing greater affinity/steeper/higher gradient ✔

Do not penalize if convergence is not approached for M1.

Both curves must be labelled to score M2.

Any two of:

contains two gamma/$\gamma$ units «instead of two beta/$\beta$ units found in adults»
OR
differs in amino acid sequence «from the two beta//$\beta$ units found in adults» ✔

less sensitive to inhibitors/2,3-BPG/DPG ✔

receives ${\mathrm{O}}_2$ from «partly deoxygenated» blood so can work at low $p{\mathrm{O}}_2$ ✔

low $p{\mathrm{CO}}_2$ in foetal blood increases affinity for ${\mathrm{O}}_2$ ✔

hemoglobin concentration in foetal blood greater than in the mother ✔

It was surprising that more did not manage to determine the Rf value of A1 within the acceptable range of "0.67 to 0.73". An out of range value of 0.75 was frequently seen.

Few candidates recognised that A1 was Ile so no marks were scored. Even of those candidates that did identify A1 as IIe, many did not mention larger Rf and therefore M1 was lost. There appeared to be in general, poor understanding of the basic principles of paper chromatography.

A majority stated that hydrogen bonding is the bonding responsible for the alpha-helix in the secondary structure.

The better candidates scored both marks in this question on oxygen dissociation curves. Many scored M2 for showing foetal hemoglobin having a higher gradient. Frequently sigmoidal curves were not drawn which lost M1 and often both curves were not explicitly labelled which was necessary to score M2.

This question required an explanation as to why the affinity for oxygen of foetal hemoglobin differs from that of adult hemoglobin. This question was poorly answered and only the stronger candidates managed to score both marks, usually by stating that foetal hemoglobin contains two gamma units instead of the two beta units found in adult hemoglobin, required for M1 and often by stating that there is less sensitivity to inhibitors for M2.

List two components of nucleotides.

Explain how the double-helical structure of DNA is stabilized once formed.

Any two correct for [1]:
pentose «sugar»
OR
deoxyribose ✔

phosphate/phosphato «group»/residue of phosphoric acid ✔

NOTE: Accept “−OPO₃²⁻/−OPO₃H⁻/−OPO₃H₂” but not “PO₄³⁻”.

«organic» nitrogenous base
OR
nucleobase
OR
nucleic base
OR
purine
OR
pyrimidine ✔

NOTE: Accept the four bases together: “adenine/A, guanine/G, cytosine/C, thymine/T”.
Accept names or formulas.

Any two of:
H-bonding between bases in each pair ✔
hydrophobic interactions/π-stacking between bases ✔
polar/charged/hydrophilic groups in sugar-phosphate backbone interactions with aqueous solution/water
OR
H-bonding AND ion-dipole interactions between phosphato «groups» andwater/histones ✔

Accept "phosphate groups are hydrophilic and form H-bonds with water".
Accept “H-bonding with histones”.

Deduce the structural formula of the by-product of this reaction.

Aspirin crystals are rinsed with water after recrystallization to remove impurities.
Suggest why cold water is used.

The solubility of aspirin is increased by converting it to an ionic form. Draw the structure of the ionic form of aspirin.

Comment on the risk of overdose when taking aspirin as an analgesic, referring to the following values, for a person weighing $70 \mathrm{kg}$:

Minimum therapeutic dose $=0.5 \mathrm{g}$

Estimated minimum lethal dose $=15 \mathrm{g}$

Explain how IR spectroscopy can be used to distinguish aspirin from salicylic acid.

OR

${\mathrm{CH}}_3\mathrm{COOH}$ ✔

Accept full OR condensed structural formula.

to avoid dissolving the crystals/aspirin ✔

Accept “to avoid loss of product” OR “aspirin is less soluble in cold water”.

✔

Accept a positive metal ion next to the $CO{O}^{\mathit{-}}$ such as “$N{a}^{-}\mathit{/}{K}^{\mathit{+}}$”.

Accept “$-ONa/-OK$” without showing the charges.

Accept notations such as “$RCO{O}^{-}$” OR “$RCOONa$” OR “$RCOOK$” but not “$R{O}^{-}$” OR “$RONa$” OR “$ROK$”.

low/medium risk «of overdosing» AND «estimated» lethal dose is $30$ times/much larger than therapeutic dose OR
$30$ times the dose results in chance of dying ✔

Accept “$\mathit{30}$ and low/medium risk due to large therapeutic index”.
Do not accept “low/medium risk AND large therapeutic window”.
Do not accept “$\mathit{30}$ times the dose” alone for the mark.

salicylic acid contains absorption in the range $3200-3600 «{\mathrm{cm}}^{-1}»$ ✔

due to phenol/hydroxyl/$\mathrm{OH}$ group not present in aspirin ✔

Award [2] for “additional $OH$ «stretch» in IR for salicylic acid at higher wavenumber than corresponding $OH$  «stretch» in aspirin” OR “aspirin has two absorption bands/one stronger absorption band in $\mathit{1700}\mathit{–}\mathit{1750}\mathit{ }\mathit{«}c{m}^{\mathit{–}\mathit{1}}\mathit{»}$while salicylic acid has one/weaker absorption band in that region”.

Award [1 max] for “fingerprint regions will be different for both”.

Most candidates were able to deduce a correct structural formula (either full or condensed) for ethanoic acid. A minority did not read the question fully and gave the structure of aspirin instead of the by-product of the reaction. Another incorrect answer cited as the by-product was water.

Many were unable to explain why aspirin should be washed with cold water, namely, to avoid dissolving crystals. Surprisingly, the incorrect term "melt" was frequently used instead of "dissolve".

A drawing of the structure of the ionic form of aspirin was required for this question. This question was poorly answered by a significant number of candidates, and lots of basic chemical errors were seen, such as incorrect valencies, writing RCO- instead of RCOO-, showing a cationic structure instead of an anionic structure etc. A couple of candidates also lost the mark by drawing square brackets with a negative charge both inside and outside the bracket.

Few scored this mark. Most knew the overdose risk was low but referred to a large therapeutic window instead of a large therapeutic index. Many also did not quantify the therapeutic index by working out that the estimated lethal dose is actually 30 times the therapeutic dose.

This question which asked for an explanation of how IR spectroscopy can be used to distinguish aspirin from salicyclic acid was generally very well answered. The majority stated that salicyclic acid contains an absorption in the IR spectrum in the 3200-3600 cm^-1 range due to the phenolic OH group, which is not present in aspirin. A few stated that aspirin has a methyl group and hence the CH stretch will appear in the 2850-3090 cm_-1 region of the IR spectrum in aspirin (using Section 26 of the Data Booklet) which will not appear in the corresponding IR spectrum for salicyclic acid. This is somewhat incorrect as in salicyclic acid the benzene ring will also have CH bonds and the CH stretch for the benzene ring will occur in a similar region of the IR spectrum (as indicated in Section 26 of the Data Booklet) and hence cannot be used to distinguish fully between the two structures per se if using the Data Booklet range. Of course, in practice the alkyl CH stretch would be at a slightly lower wavenumber (e.g. 2850-2950 cm^-1) in the IR spectrum compared to the aromatic CH stretch (3030 cm^-1), but virtually no candidate gave this type of precise detail.

Suggest why aspirin is slightly soluble in water. Refer to section 37 of the data booklet.

A student prepares aspirin from salicylic acid in the laboratory, extracts it from the reaction mixture, ensures the sample is dry and determines its melting point.

Suggest why the melting point of the student’s sample is lower and not sharp compared to that of pure aspirin.

Organic molecules can be characterized using infrared (IR) spectroscopy.

Compare and contrast the infrared peaks above 1500 cm⁻¹ in pure samples of aspirin and salicylic acid using section 26 of the data booklet.

Some mild analgesics contain a solid mixture of acidic aspirin and a non-acidic organic chemical of similar polarity to asprin.

Discuss how acid-base properties and the process of solvent extraction can be used to separate aspirin from the mixture.

The pharmaceutical industry is one of the largest producers of waste solvents.

State a green solution to the problem of organic solvent waste.

presence of «large» benzene/arene ring AND non-polar/hydrophobic
OR
presence of «large» benzene/arene ring AND cannot form H-bond with water

contain COOH/carboxyl/–OH/hydroxyl «and ester group» AND polar/hydrophilic
OR
contain COOH/carboxyl/–OH/hydroxyl «and ester group» AND can form H-bonds with water

Accept “phenyl” for “benzene ring”.

Accept "carboxylic acid" for "carboxyl".

Do not accept "alcohol" for "hydroxyl".

[2 marks]

«student’s» sample impure

lattice disrupted/not uniform «due to presence of impurities»
OR
fewer interparticle/intermolecular forces «due to presence of impurities»

Accept converse arguments.

[2 marks]

One similarity:
peak at 2500–3000 «cm^–1»/peak due to O–H/hydroxyl in carboxylic acids
OR
peak at 1700–1750 «cm^–1»/peak due to C=O/carbonyl
OR
peak at 2850–3090 «cm^–1»/peak due to C–H of arene

One difference:
peak at 3200–3600 «cm^–1» in salicylic acid/ peak due to O–H in phenol in salicylic acid
OR
«two» peaks at 1700–1750 «cm^–1» in aspirin AND one peak «in the same area» in salicylic acid

Accept “peak at 1600 cm^–1 for arene/benzene ring” – not in the data booklet.

Accept “2500–3600 cm^–1 «overlapping absorptions of two O–H» in salicylic acid”.

Accept “stronger/broader/split peak at 1700–1750 cm^–1 in aspirin”.

[2 marks]

dissolve compounds in an organic solvent

add NaOH(aq)/OH^–(aq) «to the mixture» to convert aspirin to its water soluble salt

separate the two «immiscible» layers

convert salt «in aqueous layer» back to aspirin by reacting with acid/H⁺

«evaporate solvents and dry»

Accept organic solvents immiscible with water such as hexane, ethyl ethanoate, butyl acetate.

Accept any other base.

Need explanation for mark.

[3 marks]

«use of» alternative solvents such as supercritical/liquid CO₂
OR
use of water «as solvent»
OR
solvent-free reactions «for example, polymerization of propene»
OR
solid-state chemistry
OR
recycle «waste» solvents
OR
catalysis that leads to better/higher yield
OR
reducing number of steps

Do not accept political/regulatory solutions.

“catalysis” not sufficient for mark.

[1 mark]

Describe the function of chlorophyll in photosynthesis.

Compare and contrast the structures of starch and cellulose.

One similarity:

One difference:

Explain why maltose, C₁₂H₂₂O₁₁, is soluble in water.

absorbs/traps light «energy» ✔

initiates redox reactions
OR
transfers electrons ✔

One similarity:
1−4/glycosidic linkage
OR
glucose monomers/residues ✔

NOTE: Accept “both are polysaccharides”.

One difference:
starch has α-glucose AND cellulose has β-glucose «monomers»
OR
starch can form coiled/spiral/helical chains «and straight chains» AND cellulose cannot/can only form straight chains/can only form a linear structure
OR
starch «in amylopectin» also has 1−6 glycosidic links AND cellulose does not ✔

NOTE: Accept "cellulose has alternate glucose monomers upside down with respect to each other AND starch does not".

«solubility depends on forming many» H-bonds with water ✔
maltose has many hydroxyl/OH/oxygen atom/O «and forms many H-bonds» ✔

NOTE: Reference to “with water” required.
Accept “hydroxy” for “hydroxyl” but not “hydroxide/OH^–”.
Reference to many/several OH groups/O atoms required for M2.