Describe the structure and bonding of a carbon nanotube.

Structure:

Bonding:

Suggest one application for carbon nanotubes.

Structure:
giant covalent/network covalent  [✔]

Note: Accept “cylindrical/tube shaped”.

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

Note: 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 answerssuch as “medicine” or “defence”.

Most students were aware that nanotubes have a tubular structure, but answers to the bonding were rarely detailed enough to gain the second mark.

Only a few students gained this mark and they usually gave the use of nanotubes for a reinforcing.

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.

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  [✔]

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» tertiary/quaternary structures altered [✔]

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

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

The similarity in bonding between the 2 types of secondary structures was answered well but the difference was not. Most students were not descriptive enough to receive the second mark or simply repeated the idea of proteins containing an alpha-helix and beta-pleated sheets rather than describing something different about them.

This was another question 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 of the active site shape changing or substrate molecules not longer fitting into the active site.

While many candidates did receive two marks for this question 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.

Students tend to struggle with these questions and end up giving journalistic or vague answers that cannot be awarded marks. It is important for teachers to instruct students to give more specific answers directly related to the topics presented.

Outline the impact of antibiotic waste on the environment.

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

«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 ✔

accumulate in groundwater
OR
have limited biodegradability ✔

cost of disposal ✔

NOTE: Do not accept “harmful to the environment”.
Do not accept just “pollutes water”.
Do not accept “hazard of disposal”.
Accept “ozone depletion” only if there is some reference to chlorinated solvents.

Discuss the data.

In a natural gas power station, 1.00 tonne of natural gas produces 2.41 × 10⁴ MJ of electricity.

Calculate the percentage efficiency of the power station.

1 tonne = 1000 kg
Specific energy of natural gas used = 55.4 MJ kg⁻¹

«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 input =» 5.54 ×10⁴ «MJ» ✔
«efficiency = $\frac{2.41\times {10}^4 \mathrm{MJ}}{5.54\times {10}^4 \mathrm{MJ}}\times 100=$» 43.5 «%» ✔

NOTE: Award [2] for correct final answer.

The graph shows the relationship between the temperature and the rate of an enzyme-catalysed reaction.

State one reason for the decrease in rate above the optimum temperature.

Explain why a change in pH affects the tertiary structure of an enzyme in solution.

State one use of enzymes in reducing environmental problems.

enzyme denatures
OR
change of conformation/shape of active site
OR
substrate cannot bind to active site/binds less efficiently ✔

NOTE: Accept “change in structure” or “substrate doesn't fit/fits poorly into active site”

Any two of:
acidic/basic/ionizable/COOH/carboxyl/NH₂/amino groups in the R groups/side chains «react» ✔
exchange/lose/gain protons/H⁺ ✔
change in H-bonds/ionic interactions/intermolecular forces/London dispersion forces ✔

NOTE: Do not accept “enzyme denatures” OR “change of conformation/tertiary structure” OR “substrate cannot bind to active site/binds less efficiently” as this was the answer to 8(a).

breakdown of oil spills/industrial/sewage waste/plastics
OR
production of alternate sources of energy «such as bio diesel»
OR
involve less toxic chemical pathway «in industry» ✔

NOTE: Accept “«enzymes in» biological detergents can improve energy efficiency”.

Draw a circle around the functional group that can be converted to the carboxylate by hydrolysis.

Suggest a reason for using a phosphate salt of oseltamivir in oral tablets.

Anti-HIV drugs, such as zidovudine, often become less effective over time.

Explain the development of resistant virus strains in the presence of antiviral drugs.

 [✔]

Note: Accept circles that include the alkyl side chain.

more soluble «in water»  [✔]

viruses undergo «rapid» mutation  [✔]

mutation causes a change in viral protein
OR
drug to no longer binds to virus  [✔]

Note: Accept “rapid reproduction «allows resistant viruses to multiply»”.

Many students erroneously identified the amide as the required group, failing to realise that its hydrolysis would give the carboxylate ion of the side chain lost to the drug.

About a third of the candidates realised that producing a salt would increase the drug’s aqueous solubility, though many just stated “increased bioavailability” without explaining how this came about.

Another question where well argued responses were rare, though many students gained credit for mentioning the ease of mutation and the speed of reproduction of viruses.

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

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.

⁹⁰Sr, a common product of fission, has a half-life of 28.8 years.

Determine the number of years for the activity of a sample of ⁹⁰Sr to fall to one eighth ($\frac{1}{8}$) of its initial value.

${}_{40}^{103}\text{Zr}$  [✔]

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 [✔]

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 disasters.

86.4 «years»  [✔]

This question was well answered.

This question was also fairly well answered although some students missed the concept of maintaining a chain reaction.

This question was reasonable answered by many students, but some gave very vague or general answers.

This was a well answered question with most student referring to fusion having less or no nuclear waste. There were many different possible correct answers.

This was a well answered question with most students solving for the number of years correctly.

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.

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.

« $\frac{891\text{kJmo}{\text{l}}^{-1}}{\text{16}\text{.05gmo}{\text{l}}^{-1}}$ = 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»  [✔]

About half the candidates were able to locate the appropriate data and use it to calculate the specific energy of methane.

Many students were aware that methane is the major component of natural gas and could use the efficiency data to calculate the electrical energy available from methane.

This seemed to cause quite a lot of difficulties, especially as some students appeared totally unaware of what hydroelectric power was, with a number discussing it as if it were some kind of fuel. The most usual mark gained was from discussing environmental concerns as a reason for its decreased use.

Having been given it is a gas, it is difficult to know why probably only about a third of the candidates could identify where methane would appear on a fractionating column.

Again surprisingly poorly done. Firstly there appeared to be some confusion about the term “volatility” with listing in the reverse order being quite common. Secondly many seemed unaware of the nature of “asphalt” as it was the one most frequently misplaced.

Comprehensive answers were rare. Many students gained a mark for correct statements about methane’s molecular geometry or polarity, though quite a few totally disregarded the instruction to refer to these. Some seemed aware of the link to vibrational motion and the better ones also identified the need for a change in dipole moment.

Quite a few candidates were aware of the relative atmospheric abundances of carbon dioxide and methane as well as their relative potency for enhancing the greenhouse effect.

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 differ, using section 26 of the data booklet.

Identify a hazardous product of the incineration of polychloroethene.

Explain how plasticizers affect the properties of plastics.

Suggest why the addition of plasticizers is controversial.

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 one of:
HCl  [✔]

Cl₂ [✔]

dioxins [✔]

C [✔]

CO [✔]

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 [✔]

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

Note: Accept “plasticizers cannot be recycled”.

Quite a few candidates scored at least one mark although most either scored both or none for this polymer structure.

Almost all students who attempted this question received the mark for identifying the correct absorption band.

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

Many students received a mark for this question although some did not because their answers were too vague.

Discuss why different methods of reduction are needed to extract metals.

Determine the percentage of ionic bonding in alumina using sections 8 and 29 of the data booklet.

Write half-equations for the electrolysis of molten alumina using graphite electrodes, deducing the state symbols of the products.

Anode (positive electrode):

Cathode (negative electrode):

ions of more reactive metals are harder to reduce
OR
more reactive metals have more negative electrode potentials ✔

electrolysis is needed/used for most reactive metals
OR
carbon is used to reduce metal oxides of intermediate reactivity/less reactive than carbon
OR
heating ore is sufficient for less reactive metals ✔

NOTE: Award [1 max] for “«ease of reduction/extraction» depends on reactivity”.

electronegativity difference = 1.8 «and average electronegativity = 2.5» ✔
57 «%» ✔

NOTE: Accept any value in the range 52−65 %.
Award [2] for correct final answer.

Anode (positive electrode):
2O²⁻ → 4e⁻ + O₂(g)
OR
2O²⁻ + C → 4e⁻ + CO₂ (g) ✔

NOTE: Award [1 max] for M1 and M2 if correct half-equations are given at the wrong electrodes OR if incorrect reversed half-equations are given at the correct electrodes.

Cathode (negative electrode):
Al³⁺ + 3e⁻ → Al (l) ✔
O₂ gas AND Al liquid ✔

NOTE: Only state symbols of products required, which might be written as (g) and (l) in half-equations. Ignore any incorrect or missing state symbols for reactants.

The melting points of cocoa butter and coconut oil are 34 °C and 25 °C respectively.

Explain this in terms of their saturated fatty acid composition.

Fats contain triglycerides that are esters of glycerol and fatty acids. Deduce an equation for the acid hydrolysis of the following triglyceride.

The addition of partially hydrogenated cocoa butter to chocolate increases its melting point and the content of trans-fatty acids (trans-fats).

Outline two effects of trans-fatty acids on health.

coconut oil has higher content of lauric/short-chain «saturated» fatty acids
OR
cocoa butter has higher content of stearic/palmitic/longer chain «saturated» fatty acids [✔]

longer chain fatty acids have greater surface area/larger electron cloud  [✔]

stronger London/dispersion/instantaneous dipole-induced dipole forces «between triglycerides of longer chain saturated fatty acids»  [✔]

Note: Do not accept arguments that relate to the melting points of saturated and unsaturated fats.

correct products  [✔]

correctly balanced  [✔]

Any two of:
«increased risk of» coronary/heart disease  [✔]

«increased risk of» stroke [✔]

«increased risk of» atherosclerosis [✔]

«increased risk of type-2» diabetes [✔]

increase in LDL cholesterol [✔]

decrease in HDL cholesterol [✔]

«increased risk of» obesity [✔]

A classic instance of candidates answering the question they thought (or hoped?) they had been asked rather than the one that was asked. Almost all answers referred to the differing amounts of saturated and unsaturated fatty acids present, totally ignoring the fact that the question clearly stated “their saturated fatty acid composition”, where the relative lengths of the chains was the key point. Nevertheless some who went on to discuss the nature of the intermolecular forces between the chains gained some credit.

A disappointingly small number of candidates gained any marks for deducing the equation for the hydrolysis of the given lipid.

Almost all students were aware of negative health effects of trans-fats, though quite a few lost marks by just stating “cholesterol” without specifying HDL or LDL.

Estimate the time at which the powdered zinc was placed in the beaker.

State what point Y on the graph represents.

The maximum temperature used to calculate the enthalpy of reaction was chosen at a point on the extrapolated (dotted) line.

State the maximum temperature which should be used and outline one assumption made in choosing this temperature on the extrapolated line.

Maximum temperature:

Assumption:

To determine the enthalpy of reaction the experiment was carried out five times. The same volume and concentration of copper(II) sulfate was used but the mass of zinc was different each time. Suggest, with a reason, if zinc or copper(II) sulfate should be in excess for each trial.

The formula q = mcΔT was used to calculate the energy released. The values used in the calculation were m = 25.00 g, c = 4.18 J g⁻¹ K⁻¹.

State an assumption made when using these values for m and c.

Predict, giving a reason, how the final enthalpy of reaction calculated from this experiment would compare with the theoretical value.

100 «s»  [✔]

Note: Accept 90 to 100 s.

highest recorded temperature
OR
when rate of heat production equals rate of heat loss  [✔]

Note: Accept “maximum temperature”.

Accept “completion/end point of reaction”.

Maximum temperature:
73 «°C»  [✔]

Assumption:
«temperature reached if» reaction instantaneous
OR
«temperature reached if reaction occurred» without heat loss  [✔]

Note: Accept “rate of heat loss is constant” OR “rate of temperature decrease is constant”.

Any one of:
copper(II) sulfate AND mass/amount of zinc is independent variable/being changed.
OR
copper(II) sulfate AND with zinc in excess there is no independent variable «as amount of copper(II) sulfate is fixed»   [✔]

copper(II) sulfate AND having excess zinc will not yield different results in each trial  [✔]

zinc AND results can be used to see if amount of zinc affects temperature rise «so this can be allowed for» [✔]

zinc AND reduces variables/keeps the amount reacting constant  [✔]

Note: Accept “copper(II) sulfate/zinc sulfate” for “solution”.

lower/less exothermic/less negative AND heat loss/some heat not accounted for
OR
lower/less exothermic/less negative AND mass of reaction mixture greater than 25.00 g
OR
greater/more exothermic /more negative AND specific heat of solution less than water  [✔]

Note: Accept “temperature is lower” instead of “heat loss”.

Accept “similar to theoretical value AND heat losses have been compensated for”.

Accept “greater/more exothermic/more negative AND linear extrapolation overestimates heat loss”.

Almost all candidates identified 100 s as the time at which the reaction was initiated.

Many students gained this mark through stating this was the highest temperature recorded, though even more took advantage of the acceptance of the completion of the reaction, expressed in many different ways. Very few answered that it was when heat loss equalled heat production.

Even though almost all students recognised 100 seconds as the start time of the reaction less than 50% chose the extrapolated temperature at this time. Predictably the most common answer was the maximum of the graph, followed closely by the intercept with the y-axis. With regard to reasons, again relatively few gained the mark, though most who did wrote “no loss of heat”, even though it was rare to find this preceded by “the temperature that would have been attained if …”.

The correct answer depended on whether students considered the object of the additional trials was to investigate the effect of a new independent variable (excess copper(II) sulphate) or to obtain additional values of the same enthalpy change so they could be averaged (excess zinc). Answers that gave adequate reasons were rare.

Again relatively few gained these marks for stating that it was assumed the density and specific heat of the solution were the same as water.

Only about a third of the students correctly deduced that loss of heat to the environment means that the experimental value is lower than the theoretical one, though other answers, such as “higher because linear extrapolation over-compensates for the heat losses” were also accepted.

Draw a best-fit line on the graph.

Determine the initial rate of reaction of limestone with nitric acid from the graph.

Show your working on the graph and include the units of the initial rate.

Explain why the rate of reaction of limestone with nitric acid decreases and reaches zero over the period of five days.

Suggest a source of error in the procedure, assuming no human errors occurred and the balance was accurate.

Justify this hypothesis.

The student obtained the following total mass losses.

She concluded that nitric acid caused more mass loss than sulfuric acid, which did not support her hypothesis.

Suggest an explanation for the data, assuming that no errors were made by the student.

best-fit smooth curve ✔

NOTE: Do not accept a series of connected lines that pass through all points OR any straight line representation. 

tangent drawn at time zero ✔
g day⁻¹ ✔
0.16 ✔

NOTE: Accept other reasonable units for initial rate eg, mol dm⁻³ s⁻¹, mol dm⁻³ min⁻¹, g s⁻¹ OR g min⁻¹.

M3 can only be awarded if the value corresponds to the correct unit given in M2.
Accept values for the initial rate for M3 in the range: 0.13 − 0.20 g day⁻¹ OR 1.5 × 10⁻⁶ g s⁻¹ − 2.3 × 10⁻⁶ g s⁻¹ OR 7.5 × 10⁻⁸ − 1.2 × 10⁻⁷ mol dm⁻³ s⁻¹ OR 4.5 × 10⁻⁶ − 6.9 × 10⁻⁶ mol dm⁻³ min⁻¹ OR 9.0 × 10⁻⁵ − 1.4 × 10⁻⁴ g min⁻¹ OR a range based on any other reasonable unit for rate.

Ignore any negative rate value.
Award [2 max] for answers such as 0.12/0.11 g day⁻¹, incorrectly obtained by using the first two points on the graph (the average rate between t = 0 and 1 day).
Award [1 max] for correctly calculating any other average rate.

acid used up
OR
acid is the limiting reactant ✔

concentration of acid decreases
OR
less frequent collisions ✔

NOTE: Award [1 max] for "surface area decreases" if the idea that CaCO₃ is used up/acts as the limiting reactant” is conveyed for M1.

Do not accept “reaction reaches equilibrium” for M2.

surface area not uniform
NOTE: Accept “acids impure.

OR
limestone pieces do not have same composition/source
NOTE: Accept “«limestone» contains impurities”.

OR
limestone absorbed water «which increased mass»

OR
acid removed from solution when limestone removed
NOTE: Accept “loss of limestone when dried" OR "loss of limestone due to crumbling when removed from beaker”.

OR
«some» calcium sulfate deposited on limestone lost

OR
pieces of paper towel may have stuck to limestone

OR
beakers not covered/evaporation

OR
temperature was not controlled ✔

sulfuric acid is diprotic/contains two H⁺ «while nitric acid contains one H⁺»/releases more H⁺ «so reacts with more limestone»
OR
higher concentration of protons/H⁺ ✔

NOTE: Ignore any reference to the relative strengths of sulfuric acid and nitric acid.
Accept “sulfuric acid has two hydrogens «whereas nitric has one»”.
Accept "dibasic" for "diprotic".

calcium sulfate remained/deposited on limestone «in sulfuric acid»
OR
reaction prevented/stopped by slightly soluble/deposited/layer of calcium sulfate ✔

NOTE: Answer must refer to calcium sulfate.