Describe the structure and bonding of a carbon nanotube.

Structure:

Bonding:

Suggest one application for carbon nanotubes.

Structure:
giant covalent/network covalent    [✔]

Bonding:
each carbon covalently bonded to 3 other carbons
OR
each bond has order of 1.5    [✔]

Note: Accept “cylindrical/tube shaped”.

Accept “has delocalized electrons” OR “has sp2 hybridization”.

Any one of:
3D electrodes   [✔]

catalysts    [✔]

biosensors    [✔]

molecular stents    [✔]

body armour    [✔]

synthetic muscles    [✔]

micro transistors/circuitry/capacitors/electrodes    [✔]

reinforcing phase in a matrix/composite material «such as concrete»    [✔]

micro antenna    [✔]

stealth technology    [✔]

water/air filtration    [✔]

solar cells    [✔]

tennis racquets    [✔]

microelectronic circuits    [✔]

Note: Do not accept just general answers such as “medicine” or “defence”.

Describing the structure and bonding of a carbon nanotube was generally answered satisfactorily, although some candidates simply said the bonding was covalent with no further detail.

There were some vague responses for applications of carbon nanotubes when specific details were needed to score the mark in (b).

Determine the percentage of technetium-99m remaining after 24.0 hours.

Technetium-99 decays further, emitting beta radiation. Formulate the equation for the decay of technetium-99.

Outline what is meant by low-level waste.

Outline the disposal of LLW.

Magnetic resonance imaging (MRI) is an application of NMR technology using radiowaves.

Suggest why MRI is much less dangerous than imaging techniques such as X-rays and radiotracers. Use section 3 of the data booklet.

Alternative 1
half-lives = « $\frac{24}{6.0}$ =» 4.0  [✔]

«N(t) (%) = 100(0.5)⁴ =» 6.3 «%»   [✔]

Note: Accept “6.25 «%»”.

Alternative 2
λ =«$\frac{\ln 2}{t_{\frac{1}{2}}}=\frac{\ln 2}{6.0}$» 0.116 hour^–1 OR  $\frac{N_t}{N_0}$=e^{–0.116 × 24}  [✔]

6.3 «%»  [✔]

Note: Award [2] for correct final answer.

⁹⁹Tc → ⁹⁹Ru + β^–

Ru  [✔]

mass number of Ru AND beta product  [✔]

Note: Accept “e/e–/$\frac{0}{-1}$ e” for “β^–”.

small/low amounts of radiation AND for a short time  [✔]

Note: Accept “weakly ionizing radiation” instead of “small amounts of radiation”.

Accept “short half-lives” instead of “for a short time”.

stored in shielded containers until radiation level drops «to a safe level»  [✔]

lower frequency/longer wavelength/lower energy
OR
does not use ionizing radiation/radionuclides  [✔]

Note: Do not accept “does not cause cancer”.

Most candidates correctly determined the % of technetium-99m remaining after 24.0 hours. Some candidates did not read the question properly and forgot to convert to a percentage.

A common incorrect answer was to give Tc as a product or to give an incorrect symbol for beta radiation. Most candidates scored a mark for the correct mass number of the product and beta radiation.

Low level nuclear waste was poorly outlined with many superficial responses. Many candidates only gave half the required answer.

Outlining the disposal of LLW was also challenging with many candidates saying that it should be put in a container without saying the container should be shielded.

Suggesting why MRI is less dangerous than using X-rays and radiotracers was mostly answered well, but some candidates were confused and linked longer wavelengths with higher energy.

Explain how the beta-lactam ring is responsible for the antibiotic properties of penicillin. Refer to section 37 of the data booklet.

Outline the impact of antibiotic waste on the environment.

Suggest a concern about the disposal of solvents from drug manufacturing.

Discuss two difficulties, apart from socio-economic factors, associated with finding a cure for AIDS.

ring is «sterically» strained
OR
angles of 90° instead of 109.5/109/120° angles
OR
angles smaller than 109.5/109/120°/tetrahedral/trigonal planar/triangular planar angle ✔

ring breaks up/opens/reacts «easily»
OR
amido/amide group «in ring» is «highly» reactive ✔

«irreversibly» binds/bonds to enzyme/transpeptidase
OR
inhibits enzyme/transpeptidase «in bacteria» that produces cell walls
OR
prevents cross-linking of «bacterial» cell walls ✔

NOTE: Accept arguments using correct descriptions of hybridization for M1.
Do not accept "breaks/binds to cell walls" – a reference to the enzyme is needed for alternatives 1 and 2 for M3.
Do not accept "cell membrane" for "cell wall".

«leads to bacterial» resistance «to antibiotics»
OR
destroys useful/beneficial bacteria
OR
useful/beneficial/less harmful bacteria replaced with «more» harmful bacteria ✔

NOTE: Accept “affects/disturbs micro-ecosystems”.

Any one of:
«most are» toxic «to living organisms»
OR
incomplete combustion/incineration can produce toxic products/dioxins/phosgene
OR
carcinogenic/can cause cancer ✔
NOTE: Do not accept “harmful to the environment”.

accumulate in groundwater
OR
have limited biodegradability ✔
NOTE: Do not accept just “pollutes water”.

cost of disposal ✔
NOTE: Do not accept “hazard of disposal”.

NOTE: Accept “ozone depletion” only if there is some reference to chlorinated solvents.

Any two of:

HIV difficult to detect/remains dormant ✔

HIV mutates rapidly/quickly ✔

HIV replicates rapidly/quickly ✔

HIV destroys «T-»helper cells/white blood cells/lymphocytes
OR
HIV attacks immune system ✔

HIV has several «significantly different» strains/subtypes ✔

NOTE: Accept “virus” for “HIV”.
Do not accept “AIDS mutates” without mention of the HIV/virus.
Penalize the use of “AIDS” for “HIV” once only.
Accept “HIV metabolism linked to that of host cell” OR “drugs harm host cell as well as HIV”.

Some proteins form an α-helix. State the name of another secondary protein structure.

Compare and contrast the bonding responsible for the two secondary structures.

One similarity:

One difference:

Explain why an increase in temperature reduces the rate of an enzyme-catalyzed reaction.

State and explain how a competitive inhibitor affects the maximum rate, V_max, of an enzyme-catalyzed reaction.

Suggest two reasons why oil decomposes faster at the surface of the ocean than at greater depth.

Oil spills can be treated with an enzyme mixture to speed up decomposition.

Outline one factor to be considered when assessing the greenness of an enzyme mixture.

β/beta pleated/sheet  [✔]

One similarity:
hydrogen bonding
OR
attractions between C=O and N–H «on main chain»  [✔]

One difference:
α-helix has hydrogen bonds between amino acid residues that are closer than β-pleated sheet
OR
H-bonds in α-helix parallel to helix axis AND perpendicular to sheet in β-pleated sheet
OR
α-helix has one strand AND β-pleated sheet has two «or more» strands
OR
α-helix is more elastic «since H-bonds can be broken easily» AND β-pleated sheet is less elastic «since H-bonds are difficult to break»  [✔]

Note: Accept a diagram which shows hydrogen bonding between O of C=O and H of NH groups for M1.

Accept “between carbonyl/amido/amide/carboxamide” but not “between amino/amine” for M1.

enzyme denatured/ loss of 3-D structure/conformational change
OR
«interactions responsible for» for tertiary/quaternary structures altered  [✔]

shape of active site changes
OR
fewer substrate molecules fit into active sites  [✔]

V_max unchanged  [✔]

at high substrate concentration substrate outcompetes inhibitor/need a higher
substrate concentration to reach V_max  [✔]

Note: Accept suitable labelled diagram.

Any two of:
surface water is warmer «so faster reaction rate»/more light/energy from the sun  [✔]

more oxygen «for aerobic bacteria/oxidation of oil» [✔]

greater surface area [✔]

Any one of:
non-hazardous/toxic to the environment/living organisms  [✔]

energy requirements «during production» [✔]

quantity/type of waste produced «during production»
OR
atom economy [✔]

safety of process [✔]

Note: Accept “use of solvents/toxic materials «during production»”.

Do not accept “more steps involved”.

This question was well answered with many scoring the mark although there were quite a few incorrect responses that answered “beta-helix” rather than “beta-pleated sheet”.

Almost all the candidate’s stated hydrogen bonding as the similarity between the 2 types of secondary structures but lost marks on the difference between them.

This question was well answered where most candidates received one mark for identifying that the enzyme will denature with an increase in temperature. However, many candidates did not continue with the explanation that the shape of the active site changes.

Many candidates stated correctly that Vmax remains unchanged but only some mentioned that a higher substrate concentration was required to reach Vmax for the second mark.

Many candidates received two marks for this part while some candidates only suggested one reason or repeated the same reason (for example - heat and energy from the sun) even though the question clearly asked for two reasons.

The candidates struggled with this part and gave journalistic or vague answers that cannot be awarded marks. Atom economy was mentioned correctly by a few candidates.

Determine the other product of the fission reaction of plutonium-239.

$${}_{94}^{239}\text{Pu + }{}_0^1\text{n}\to {}_{54}^{134}\text{Xe + }.................{\text{ + 3}}_0^1\text{n}$$

Outline the concept of critical mass with respect to fission reactions.

Outline one advantage of allowing all countries access to the technology to generate electricity by nuclear fission.

State one advantage of using fusion reactions rather than fission to generate electrical power.

Outline how the energy of a fission reaction can be calculated.

Calculate the half-life of an isotope whose mass falls from 5.0 × 10⁻⁵ g to 4.0 × 10⁻⁵ g in 31.4 s, using section 1 of the data booklet.

${}_{40}^{103}\text{Zr}$   [✔]

minimum mass to «self-»sustain chain reaction
OR
if mass of fissile material is too small, too many neutrons produced pass out of the nuclear fuel
OR
at least one neutron produced causes further reaction  [✔]

Any one of:
reduction in emission of greenhouse gases «from burning fossil fuels»  [✔]

economic independence/self-sufficiency «from crude oil/producing states» [✔]

uranium is more abundant on Earth «in terms of total energy that can be produced from this fuel» than fossil fuels [✔]

Note: Accept specific greenhouse gases (such as carbon dioxide/CO2) but not pollutants or other general statements.

Any one of:
fuel is inexpensive/readily available [✔]
no/less radioactive waste is formed [✔]
lower risk of accidents/large-scale disasters [✔]
impossible/harder to use for making materials for nuclear weapons [✔]
larger amounts of energy released per unit mass [✔]
does not require a critical mass [✔]
can be used continuously [✔]

Note: Accept “higher specific energy for fusion”.

Do not accept “no/less waste produced for fusion”.

Accept specific example for a disaster.

mass difference between reactants and products AND E = mc²  [✔]

«N = N₀e^–λt»

λ«= $\frac{-\ln \left(\frac{N}{N_0}\right)}{t}=\frac{-\ln \left(\frac{4.0\times {10}^{-5}}{5.0\times {10}^{-5}}\right)}{31.4\text{ s}}$ »

= 7.106 × 10^–3 s^–1  [✔]

« $t_{\frac{1}{2}}=\frac{\ln 2}{\lambda }$ =» 98/97.5 «s» [✔]

Note: Award [2] for correct final answer.

This part was well answered.

This part was also fairly well answered although some candidates missed the concept of minimum mass to sustain a chain reaction.

This part saw some reasonable answers, but some other candidates wrote very vague or general answers.

This was a well-answered question with most candidates referring to fusion having less or no radioactive waste.

Most of the candidates were able to state correctly the mass difference between reactants and products and E = mc².

Many candidates were able to calculate the half-life of an isotope correctly.

Discuss the data.

Outline what is meant by the degradation of energy.

«similar specific energy and» pentane has «much» larger energy density ✔

Any two for [2 max]:
similar number of bonds/«C and H» atoms in 1 kg «leading to similar specific energy» 
OR
only one carbon difference in structure «leading to similar specific energy» ✔
NOTE: Accept “both are alkanes” for M2.

pentane is a liquid AND butane is a gas «at STP» ✔
NOTE: Accept “pentane would be easier to transport”.

1 m³ of pentane contains greater amount/mass than 1 m³ of butane ✔
NOTE: Accept “same volume” for “1 m³” and “more moles” for “greater amount” for M4.

energy converted to heat
OR
energy converted to less useful/dispersed forms
OR
energy converted to forms that have lower potential to do work
OR
heat transferred to the surroundings ✔

NOTE: Reference to energy conversion/transfer required. Do not accept reference to loss of energy.

Outline what is meant by genetically modified organisms.

Outline one benefit of the use of these products.

organism whose genetic material/DNA has been altered by genetic engineering techniques «involving transferring DNA between species»   [✔]

Note: Accept “any living organism that possesses a novel combination of genetic material obtained through the use of modern biotechnology”.

Any one of:
increased resistance to pests/micro-organisms    [✔]

increased shelf-life of food    [✔]

increased nutritional value    [✔]

greater crop yield     [✔]

greater tolerance of crops to adverse climatic/soil/growing condition    [✔]

Candidates had difficulty outline the meaning of genetically modified organisms with many simply repeating the question by stating modified DNA without including technology or engineering techniques.

Outlining one benefit of using GMOs was well answered.

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”.

Calculate the specific energy of methane, in MJ kg⁻¹, using sections 1, 6 and 13 of the data booklet.

Calculate the maximum electric energy output, in MJ, which can be obtained from burning 1.00 kg of methane by using your answer from (a).

Hydroelectric power plants produced 16% of the world’s energy in 2015, down from 21% in 1971.

Suggest why hydroelectric power production has a higher efficiency than the other sources given in (b) and why its relative use has decreased despite the high efficiency.

Reason for higher efficiency:

Reason for decreased use:

Methane can also be obtained by fractional distillation of crude oil.

[Source: Image used with kind permission of science-resources.co.uk]

Draw a circle on the diagram to show where the methane fraction is withdrawn.

List the following products, which are also obtained by fractional distillation, according to decreasing volatility: asphalt, diesel, gasoline, lubricating motor oil.

Explain how methane absorbs infrared (IR) radiation by referring to its molecular geometry and dipole moment.

Compare methane’s atmospheric abundance and greenhouse effect to that of carbon dioxide.

« $\frac{891\text{kJ mo}{\text{l}}^{-1}}{16.05\text{g mo}{\text{l}}^{-1}}$ =» 55.5 kJ g^–1 =» 55.5 «MJ kg^–1» ✔

«55.5 MJ × 58 % =» 32.2 «MJ»   [✔]

Reason for higher efficiency:
no heat/energy loss in producing steam
OR
no need to convert chemical energy of the fuel into heat and then heat into mechanical energy
OR
direct conversion of «gravitational» potential energy to mechanical energy    [✔]

 Note: Accept “less energy lost as heat” but do not accept “no energy lost”.

Reason for decreased use:
limited supply of available hydroelectric sites
OR
rapid growth of electrical supply in countries with little hydroelectric potential
OR
not building «new hydroelectric» dams because of environmental concerns    [✔]

Note: Accept “new/alternative/solar/wind power sources «have taken over some of the demand»”.

Accept “lower output from existing stations due to limited water supplies”.

  [✔]

gasoline > diesel > lubricating motor oil > asphalt   [✔]

Note: Accept products written in this order whether separated by >, comma, or nothing.

methane is tetrahedral
OR
methane has zero dipole moment/is non-polar/bond polarities cancel    [✔]

Any two of:
IR absorption can result in increased vibrations/bending/stretching    [✔]

only modes that cause change in dipole absorb IR    [✔]

for methane this is asymmetric bending/stretching    [✔]

methane is less abundant AND has a greater effect «per mol»   [✔]

Calculations of specific energy of methane and the maximum electric energy output in parts (a) and (b)(i) were done well.

Calculations of specific energy of methane and the maximum electric energy output in parts (a) and (b)(i) were done well.

Suggesting reasons for hydroelectric power having higher efficiency but lower relative use than other energy sources in was not answered well by most candidates. Often the reasons for higher efficiency were given in vague terms that did not meet the detail required.

Required candidates to circle a fractionating tower to show where the methane fraction could be withdrawn. Despite the expectation that candidates know methane is a gas at room temperature, there were many varied answers to this question.

Required products of fractional distillation of crude oil to be ranked according to decreasing volatility. This should have been able to be worked out from first principles and did not have to be memorized as one G2 respondent suggested.

Many candidates scored the first mark for stating that methane is tetrahedral. Further details to explain how methane absorbs IR radiation were generally insufficient. Many candidates referred to “dipole movements” despite dipole moment being in the stem of the question.

Most candidates correctly answered comparing methane’s atmospheric abundance and greenhouse effect to that of carbon dioxide.

Draw a section of isotactic polychloroethene (polyvinylchloride, PVC) showing all the atoms and all the bonds of four monomer units.

The infrared (IR) spectrum of polyethene is given.

Suggest how the IR spectrum of polychloroethene would diff er, using section 26 of the data booklet.

Explain how plasticizers affect the properties of plastics.

Suggest why the addition of plasticizers is controversial.

Outline, giving a reason, how addition and condensation polymerization compare with regard to green chemistry.

Draw the full structural formula of the organic functional group formed during the polymerization of the two reactants below.

correct bonding  [✔]

Cl atoms all on same side and alternate  [✔]

Note: Continuation bonds must be shown.

Award [1 max] if less than or more than four units shown.

Accept a stereo formula with all atoms and bonds shown.

«strong additional» absorption at 600–800 «cm^–1»  [✔]

Any two of:
embedded/fit between chains of polymers  [✔]

prevent chains from forming crystalline regions  [✔]

keep polymer strands/chains/molecules separated/apart [✔]

increase space/volume between chains  [✔]

weaken intermolecular/dipole-dipole/London/dispersion/instantaneous dipoleinduced dipole/van der Waals/vdW forces «between chains» [✔]

increase flexibility/durability/softness [✔]

make polymers less brittle [✔]

Note: Accept “lowers density/melting point”.

leach into foodstuffs/environment
OR
«unknown» health/environmental consequences  [✔]

Note: Accept “plasticizers cannot be recycled”.

addition produces only the polymer «AND more green»
OR
addition has no by/side-product/condensation produces by-product/small molecules/HCl/NH₃ «AND less green»
OR
addition has high atom economy/condensation has lower atom economy «AND less green»
OR
condensation polymers «often» more biodegradable than addition polymers «AND more green»  [✔]

Note: Accept “if water produced by condensation «AND condensation and addition equally green»”.

Accept for addition “all of reactants change into products”.

 [✔]

Note: Continuation bonds must be shown.

Do not accept condensed formula.

Few candidates scored at least one mark although most either scored both or none for this polymer structure. Some did not read that only four monomer units are required.

Almost all candidates received the mark for identifying the correct absorption band for polychloroethene.

This was a well-answered question; with most candidates identifying at least one method plasticizers affect the properties of plastic.

Several candidates wrote vague answers as to why the addition of plasticizers is controversial.

Candidates seemed to have difficulty in comparing addition and condensation polymerisation with regard to green chemistry.

Several candidates struggled to draw the full structural formula of the peptide linkage formed during the polymerisation of the two reactants.