Calculate the energy released, in $\mathrm{kJ}$, from the complete combustion of $5.00 {\mathrm{dm}}^3$ of ethanol.

State a class of organic compounds found in gasoline.

Outline the advantages and disadvantages of using biodiesel instead of gasoline as fuel for a car. Exclude any discussion of cost.

A mixture of gasoline and ethanol is often used as a fuel. Suggest an advantage of such a mixture over the use of pure gasoline. Exclude any discussion of cost.

When combusted, all three fuels can release carbon dioxide, a greenhouse gas, as well as particulates. Contrast how carbon dioxide and particulates interact with sunlight.

Methane is another greenhouse gas. Contrast the reasons why methane and carbon dioxide are considered significant greenhouse gases.

Suggest a wavenumber absorbed by methane gas.

Determine the relative rate of effusion of methane ($M_{\mathrm{r}}=16.05$) to carbon dioxide ($M_{\mathrm{r}}=44.01$), under the same conditions of temperature and pressure. Use section 1 of the data booklet.

$«21 200 \mathrm{kJ} {\mathrm{dm}}^{-3}\times 5.00 {\mathrm{dm}}^3=»106000/1.06\times {10}^5«\mathrm{kJ}»$ ✔

alkane
OR
cycloalkane
OR
arene ✔

Accept “alkene”.
Do not accept just “hydrocarbon”, since given in stem.
Do not accept “benzene/aromatic” for “arene”.

Advantages: [2 max]

renewable ✔

uses up waste «such as used cooking oil» ✔

lower carbon footprint/carbon neutral ✔

higher flashpoint ✔

produces less ${\mathrm{SO}}_{\mathrm{x}}$/${\mathrm{SO}}_2$
OR
less polluting emissions ✔

has lubricating properties
OR
preserves/increases lifespan of engine ✔

increases the life of the catalytic converter ✔

eliminates dependence on foreign suppliers ✔

does not require pipelines/infrastructure «to produce» ✔

relatively less destruction of habitat compared to obtaining petrochemicals ✔

Accept “higher energy density” OR “biodegradable” for advantage.

Disadvantages: [2 max]

needs conversion/transesterification ✔

takes time to produce/grow plants ✔

takes up land
OR
deforestation ✔

fertilizers/pesticides/phosphates/nitrates «used in production of crops» have negative environmental effects ✔

biodiversity affected
OR
loss of habitats «due to energy crop plantations» ✔

cannot be used at low temperatures ✔

variable quality «in production» ✔

high viscosity/can clog/damage engines ✔

Accept “lower specific energy” as disadvantage.

Do not accept “lower octane number” as disadvantage”.

Any one:

uses up fossil fuels more slowly ✔

lower carbon footprint/CO2 emissions ✔

undergoes more complete combustion ✔

produces fewer particulates ✔

higher octane number/rating
OR
less knocking ✔

prevents fuel injection system build up
OR
helps keep engine clean ✔

Accept an example of a suitable advantage even if repeated from 11c.

carbon dioxide allows sunlight/short wavelength radiation to pass through AND particulates reflect/scatter/absorb sunlight ✔

Accept “particulates reflect/scatter/absorb sunlight AND carbon dioxide does not”.
Accept “$C{O}_{\mathit{2}}$ absorbs $IR$ «radiation» AND particulates reflect/scatter/absorb sunlight”.

Do not accept “traps” for “absorbs”.

carbon dioxide is highly/more abundant «in the atmosphere» ✔

methane is more effective/potent «as a greenhouse gas»
OR
methane/better/more effective at absorbing $\mathrm{IR}$ «radiation»
OR
methane has greater greenhouse factor
OR
methane has greater global warming potential/GWP✔

Accept “carbon dioxide contributes more to global warming” for M1.

any value or range within $2850–3090«{\mathrm{cm}}^{-1}»$ ✔

«rate of effusion of $\frac{{\mathrm{CH}}_4}{{\mathrm{CO}}_2}=\sqrt{\frac{44.01}{16.05}}=»1.656$ ✔

Almost all were able to calculate the energy released from the complete combustion of ethanol.

The majority cited correctly that alkanes are a class of organic compounds found in gasoline.

Most gained at least one mark for an advantage of using biodiesel instead of gasoline as fuel for a car and most scored one mark at least for a disadvantage of biodiesel. Many conveyed solid understanding, though the disadvantages were not as well articulated as the advantages. Some incorrectly based their responses on cost factors which were excluded as outlined in the stem of the question.

Most scored the one mark for this question, with "less knocking or higher octane number/rating" the most common correct answer seen.

The wording of this question was critical which involved contrasting how carbon dioxide and particulates interact with sunlight. Some missed the "Contrast" command term as the action verb. Loose, non-scientific syntax was often seen such as stating "traps" instead of "absorbs".

This was another "Contrast-type" question, which was better answered compared to (e)(i). Many scored both marks by stating that carbon dioxide is more abundant in the atmosphere whereas methane is more effective at absorbing IR radiation.

The main issue with this question was that a high percentage of candidates did not realise that wavenumber is the reciprocal of wavelength and hence wavenumber has typical units of cm^-1. Many incorrectly gave wavelength values, in nm, which did not answer the question posed.

The determination of the relative rate of effusion of methane to carbon dioxide was almost universally correctly computed as 1.656.

The Geobacter species of bacteria can be used in microbial fuel cells to oxidise aqueous ethanoate ions,
CH₃COO⁻(aq), to carbon dioxide gas.

State the half-equations for the reactions at both electrodes.

A concentration cell is an example of an electrochemical cell.

(i) State the difference between a concentration cell and a standard voltaic cell.

(ii) The overall redox equation and the standard cell potential for a voltaic cell are:

Zn (s) + Cu²⁺ (aq) → Zn²⁺ (aq) + Cu (s)     E^θ_cell = +1.10 V

Determine the cell potential E at 298 K to three significant figures given the following concentrations in mol dm⁻³:

[Zn²⁺] = 1.00 × 10⁻⁴       [Cu²⁺] = 1.00 × 10⁻¹

Use sections 1 and 2 of the data booklet.

(iii) Deduce, giving your reason, whether the reaction in (b) (ii) is more or less spontaneous than in the standard cell.

Dye-sensitized solar cells (DSSC) convert solar energy into electrical energy.

(i) Describe how a DSSC converts sunlight into electrical energy.

(ii) Explain the role of the electrolyte solution containing iodide ions, I⁻, and triiodide ions, I₃⁻, in the DSSC.

Negative electrode (anode): CH₃COO⁻ (aq) + 2H₂O (l) → 2CO₂ (g) + 7H⁺ (aq) + 8e⁻

Positive electrode (cathode): O₂ (g) + 4H⁺ (aq) + 4e⁻ → 2H₂O (l)

Accept equilibrium signs in equations.
Award [1 max] if correct equations are given at wrong electrodes.

i
concentration cell has different concentrations of electrolyte «solutions» «but same electrodes and electrolytes»
OR
standard voltaic cell has different electrodes/electrolytes «but same concentration of electrolytes»
Accept “both half-cells in concentration cell made from same materials”.

ii
«$E=1.10-\left(\frac{RT}{nF}\right)\ln \frac{\left[\text{Z}{\text{n}}^{2+}\right]}{\left[\text{C}{\text{u}}^{2+}\right]}=1.10-\left(\frac{8.31\times 298}{2\times 96500}\right)\ln \frac{{10}^{-4}}{{10}^{-1}}=1.10+0.0886=$»

(+) 1.19 «V»
3 significant figures needed for mark.

iii
more spontaneous because E > E^θ_cell

i

photon/«sun»light absorbed by the dye/photosensitizer/«transition» metal complex
OR
dye/photosensitizer/«transition» metal complex excited by photon/«sun»light 

electron«s» move«s» to conduction band
OR
electron«s» transferred to semiconductor/TiO₂     

ii

I₃⁻ + 2e⁻ → 3I⁻ «at cathode»
OR
triiodide ions/I₃⁻ reduced into/produce iodide ions/I⁻ «at cathode»

iodide ions/I⁻ reduce dye/act as reducing agent AND oxidized into/produce triiodide ions/I₃⁻ 
OR
dye⁺ + e⁻ → dye AND 3I^- → I₃⁻ + 2e⁻

Show that the mass of the ²³⁸U isotope in the rock is $0.639 \mathrm{kg}$.

The half-life of ²³⁸U is $4.46\times {10}^9$ years. Calculate the mass of ²³⁸U that remains after $0.639 \mathrm{kg}$ has decayed for $2.23\times {10}^{10}$ years.

Outline a health risk produced by exposure to radioactive decay.

Deduce the nuclear equation for the decay of uranium-238 to thorium-234.

Thorium-234 has a higher binding energy per nucleon than uranium-238. Outline what is meant by the binding energy of a nucleus.

Determine the nuclear binding energy, in $\mathrm{J}$, of ${}_{238}\mathrm{U}$ using sections 2 and 4 of the data booklet.

The mass of the ${}_{238}\mathrm{U}$ nucleus is $238.050786 \mathrm{amu}$.

$«\frac{\mathrm{mass} \%}{\mathrm{fraction} \mathrm{of} \mathrm{U} \mathrm{in} {\mathrm{UO}}_2}=»\frac{238.03}{238.03+2\times 16}$ /$0.881$/$88.1 \%$ ✔

$46.5«\mathrm{kg}»\times 0.0157\times 0.881\times 0.9928«=0.639 \mathrm{kg}»$ ✔

Award [1 max] for omitting mass composition (giving $\mathit{0}\mathit{.}\mathit{725}\mathit{ }kg$).

M2 is for numerical setup, not for final value of $\mathit{0}\mathit{.}\mathit{639}\mathit{ }kg$.

Alternative 1
$«\frac{2.23\times {10}^{10} \mathrm{year}}{4.46\times {10}^9 \mathrm{year}}=»5.00«\mathrm{half}-\mathrm{lives}»$ ✔

$«m=0.639 \mathrm{kg}\times (0.5{)}^5=»0.0200«\mathrm{kg}»$ ✔

Alternative 2
$«𝜆=\frac{\ln 2}{4.46\times {10}^9 \mathrm{year}}=»1.554\times {10}^{-10}«{\mathrm{year}}^{-1}»$ ✔

$«m=0.639 \mathrm{kg}\times {𝑒}^{–1.554\times {10}^{–10 {\mathrm{year}}^{–1}}\times 2.23\times {10}^{10 \mathrm{year}}}=»0.0200«\mathrm{kg}»$ ✔

Award [2] for correct final answer.

Any one:

«genetic» mutations ✔

«could cause» cancer ✔
Accept specific named types of cancer.

cells «in body» altered ✔

cells «in body» cannot function ✔

damaged DNA/proteins/enzymes/organs/tissue ✔

«radiation» burns ✔

hair loss ✔

damage in foetuses ✔

damages/weakens immune system ✔

${}_{92}{}^{238}\mathrm{U}\to {}_{90}{}^{234}\mathrm{Th}+{}_2{}^4\mathrm{He}$ ✔

Do not penalize missing atomic numbers in the equation.

Accept “$\alpha$” for "$He$”.

energy required to separate a nucleus into protons and neutrons/nucleons
OR
energy released when nucleus was formed from «individual/free/isolated» protons and neutrons/nucleons ✔

Do not accept “energy released when atom was formed”.

$238.050786«\mathrm{amu}»\times 1.66\times {10}^{-27}«\mathrm{kg} {\mathrm{amu}}^{-1}»$
OR

 ✔

$\left(92\times 1.672622\times {10}^{-27}\right)+\left(146\times 1.674927\times {10}^{-27}\right)-3.95\times {10}^{-25}$
OR
$3.42\times {10}^{-27}/3\times {10}^{-27}«\mathrm{kg}»$ ✔

$«E=m{c}^2=3.42\times {10}^{-27}\times (3.00\times {10}^8{)}^2=»3.08\times {10}^{-10}«\mathrm{J}»$ ✔

Accept answers in the range “$\mathit{2}\mathit{.}\mathit{7}\mathit{\times }{\mathit{10}}^{\mathit{-}\mathit{10}}\mathit{–}\mathit{3}\mathit{.}\mathit{1}\mathit{\times }{\mathit{10}}^{\mathit{-}\mathit{10}}\mathit{«}J\mathit{»}$”.

Award [3] for correct final answer.

This question was generally well answered. Many candidates approached this question using amount, in mol, and converting to mass at the end. Some candidates omitted the mass composition however, resulting in a mass of 0.725 kg, which yielded [1 max].

The question involving half-life was very well answered and most scored both marks giving a final answer of 0.0200 kg, via different methods of calculation.

This question on outlining a health risk produced by exposure to radioactive decay posed no difficulty and the most common, correct answers included "could cause cancer" and "can damage DNA".

Most scored the one mark for the nuclear equation for the decay of uranium-238 to thorium-234. The most common error was including a neutron on the product side

Although many scored the one mark for outlining what is meant by the binding energy of a nucleus, the question was often answered as energy involved in the formation of an atom. Some candidates did not understand the difference between nucleus and nucleons.

This demanding question on the determination of the nuclear binding energy was well executed and many scored all three marks. Even the weaker candidates still managed to gain an ECF mark for M3 by using the E = mc² equation. The general performance on this type of calculation was much better than in previous sessions.

Identify two ways in which the structure of the dye shown resembles the chlorophyll molecule. Use section 35 of the data booklet.

Both photosynthesis and the Grätzel cell use energy from sunlight to bring about reduction. Deduce an equation for the reduction reaction in the electrolyte of a Grätzel cell.

delocalized bonding/conjugated bonds

contain metal atom/ion coordinated to «organic» ligand(s)

involve bonds from nitrogen to the central metal ion

[2 marks]

I₃^– + 2e^– → 3I^–

Accept I₂ + 2e^– → 2I^–.

[1 mark]

Doping of silicon increases the conductivity in semiconductors.

Describe the doping in p-type and n-type semiconductors.

Doping of silicon increases the conductivity in semiconductors.

Explain how doping improves the conductivity of silicon.

p-type:
«small amount of» $\mathrm{B}/\mathrm{Al}/\mathrm{Ga}/\mathrm{In}/\mathrm{Tl}/$Group 13 element produces holes ✔

n-type:
«small amount of» $\mathrm{Sb}/\mathrm{P}/\mathrm{As}/\mathrm{Bi}/$Group 15 element adds extra electrons ✔

Award [1 max] for correct element type for p AND n OR p-type: “produces holes” AND n-type: adds extra electrons”.

electrons and holes flow in opposite directions
OR
electrons can flow into holes
OR
gap between valence and conduction bands becomes smaller ✔

The description of doping in p- and n-type semiconductors was well expressed and the majority scored both marks.

The better candidates were able to explain how doping improves the conductivity of silicon and typically stated that electrons can flow into holes. The weaker candidates had some idea of what was involved but often were not able to articulate precisely in a scientific manner the process and hence usually failed to score the mark.

The structures of 11-cis-retinal and β-carotene are given in section 35 of the data booklet. Suggest a possible wavelength of light absorbed by each molecule using section 3 of the data booklet.

both between 400–700 «nm»

β-carotene at higher wavelength than retinal

Accept any wavelength within the 400-700 nm visible region range for M1 and any higher wavelength for β-carotene
within the same region for M2.

[2 marks]

The natural absorption of light by chlorophyll has been copied by those developing dye-sensitized solar cells (DSSCs). Outline how a DSSC works.

Any three of:

dye has conjugated system

dye absorbs a photon «and injects an electron into TiO₂»

electrons transferred to semiconductor «and dye ionized»

dye oxidizes/takes electron from electrolyte

electron flows through external circuit «to reduce electrolyte»

M4 may also be scored from more detailed answers involving iodide species (eg “iodide/I^– oxidized to I₃^–/triiodide” OR “I^–/iodide reduces dye” OR “I^–/iodide releases electron to dye” OR “I₃^–/triiodide reduced to I^–/iodide”).

[Max 3 Marks]

Deduce the half-cell equations occurring at each electrode during discharge.

Outline the function of the proton-exchange membrane (PEM) in the fuel cell.

Explain how the flow of ions allows for the operation of the fuel cell.

Anode (negative electrode):
CH₃OH(aq) + H₂O(l) → 6H⁺(aq) + 6e⁻ + CO₂(g)

Cathode (positive electrode):
$\frac{3}{2}$O₂(g) + 6H⁺(aq) + 6e⁻ → 3H₂O(l)

Award [1 max] for correct equations at wrong electrode.

Accept “e” for “e^–”.

Accept “O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l)”.

[2 marks]

allows H⁺/ions pass through/diffuse/move «from anode to cathode but not electrons or small molecules»

Accept “acts as a salt bridge”.

[1 mark]

H⁺/ions pass through/diffuse/move from anode/negative electrode «through membrane» to cathode/positive electrode

H⁺/ions used to reduce oxygen at cathode/positive electrode

Oxygen must be mentioned for M2.

[2 marks]

The mass of X is 8.005305 amu and that of ${}_2^4\text{He}$ is 4.002603 amu. Determine the energy produced, in J, when one atom of ${}_6^{12}\text{C}$ is formed in this reaction. Use section 2 of the data booklet.

loss in mass = «8.005305 amu + 4.002603 amu – 12.000000 amu =» 0.007908 «amu»

= «0.007908 amu x 1.66 x 10^–27 kg amu^–1 =» 1.313 x 10^–29 «kg»

E = «mc² = 1.313 x 10^–29 kg x (3.00 x 10⁸ m$$s^–1)² =» 1.18 x 10^–12 «J»

Award [3] for correct final answer.

[3 marks]

Contrast how absorption of photons and charge separation occur in each device.

Suggest one advantage a DSSC has over a silicon based photovoltaic cell.

Accept “existence of holes AND electrons at p-n junction” for M2.

[4 marks]

Any of:

cheaper
OR
ease of fabrication
OR
use light of lower energy/lower frequency/longer wavelength
OR
plentiful and renewable resources «to construct DSSC cells»
OR
operate at lower «internal» temperatures/better at radiating heat away «since constructed with thin front layer of conductive plastic compared to glass box in photovoltaic cell»
OR
use of nanoparticles provides large surface area exposure to sunlight/sun/light
OR
can absorb better under cloudy conditions
OR
better conductivity
OR
more flexible

Accept “lower mass/lighter «so greater flexibility to integrate into windows etc.»” OR “greater power-conversion efficiency «with latest DSSC models»”.

[1 mark]

Complete the half-equations on the diagram and identify the species moving between the electrodes.

State the factor that limits the maximum current that can be drawn from this cell and how electrodes are designed to maximize the current.

Explain how the proportion of ²³⁵U in natural uranium is increased.

Accept any balanced equation which shows Li oxidized to Li⁺ for M3, such as

LiC₆ → Li⁺ + C₆ + e^–     or

Li_xC₆ → xLi⁺ + 6C + xe^–

[3 marks]

Limiting factor:

internal resistance «of the cell»

Electrodes design:

large surface area

Accept “time it takes ions to diffuse between electrodes”.

Accept specific ways of increasing surface area, such as “porous electrodes”.

Accept “close together/small separation”.

[2 marks]

uranium converted to uranium hexafluoride/UF₆ gas

ALTERNATIVE 1:

gas «allowed to» diffuse

lower mass isotope/²³⁵U passes through more rapidly

ALTERNATIVE 2:

use of centrifuge

higher mass isotope/²³⁸U moves/closer to outside of centrifuge

OR

lower mass isotope/²³⁵U stays in/removed from middle of centrifuge

[3 marks]

Deduce half-equations for the reactions at the two electrodes and hence the equation for the overall reaction.

Suggest a way in which they are similar.

Outline the difference between primary and rechargeable cells.

Identify one factor that affects the voltage of a cell and a different factor that affects the current it can deliver.

Anode: CH₃OH(aq) + H₂O(l) → CO₂(aq) + 6H⁺(aq) + 6e^–

Cathode: O₂(aq) + 4H⁺(aq) + 4e^– → 2H₂O(l)

Overall: 2CH₃OH(aq) + 3O₂(g) → 2CO₂(aq) + 4H₂O(l)

Accept correctly balanced equations with multiples of the coefficients given here.

Accept reversible or non-reversible arrows for all.

[3 marks]

«portable» sources of electrical energy/electricity
OR
convert chemical «potential» energy to electrical energy/electricity

[1 mark]

primary cells involve irreversible reactions AND rechargeable cells involve reversible reactions

Accept “primary cells have a limited life before going ‘flat’ AND rechargeable cells can be recharged when ‘flat’”.

[1 mark]

Voltage:
chemical nature of electrodes
OR
electrode reactions

Current:
diffusion rate
OR
internal resistance/resistance of the cell

Accept temperature for either but not both.

Accept concentration for either but not both.

Accept pH for either but not both.

Accept the current depends on the area/separation of the electrodes.

[2 marks]

Calculate the loss in mass, in kg, and the energy released, in J, when 0.00100 mol of ²²⁸Ac decays, each atom losing an electron. Use section 2 of the data booklet and E = mc².

²²⁸Ac → ${}_{-1}^0\text{e}$ + ²²⁸Th

Determine the energy released, in J, by 0.00100 mol of ²²⁸Ac over the course of 18 hours.

Outline how nuclear ionising radiation can damage DNA and enzymes in living cells.

Loss in mass:

«(3.78532 x 10^–25 kg – 9.109383 x 10^–31 kg – 3.78528 x 10^–25 kg) x 0.00100 x 6.02 x 10²³ =»1.86 x 10^–9 «kg»

Energy released:

«E = mc² = 1.86 x 10^–9 kg x (3.00 x 108 m s^–1)² =» 1.67 x 10⁸ «J»

«1.67 x 10⁸ J x $\frac{7}{8}$ =» 1.46 x 10⁸ «J»

production of radicals/•O₂^–/•OH

OR

direct effect such as breaking bonds/atom migration

Ignore missing dots on radical species.

Accept named radical eg “superoxide radical” OR “hydroxyl radical”.

An example must be given for second alternative.

Draw the Lewis (electron dot) structure for an appropriate doping element in the box in the centre identifying the type of semiconductor formed.

State the feature of the molecules responsible for the absorption of light.

Outline why complex B absorbs light of longer wavelength than complex A.

ALTERNATIVE 1

B/Ga in circle AND Type of semiconductor: p-type

showing 3 electron pairs AND one lone electron «and hole»

ALTERNATIVE 2

P/As in circle AND Type of semiconductor: n-type

showing 4 electron pairs AND one non-bonded electron

Accept any group 13 element labelled as p-type.

Accept showing 7 electrons.

Accept any group 15 element labelled as n-type.

Accept showing 9 electrons.

Accept dots or crosses for electrons.

[2 marks]

conjugated C=C/carbon–carbon double bonds

OR

«multiple» alternating C=C/carbon–carbon double bonds

OR

«extensive electron» conjugation/delocalization

OR

«many» fused/conjugated aromatic/benzene rings

[1 mark]

complex B has greater conjugation/delocalization

[1 mark]

Fuel cells have a higher thermodynamic efficiency than octane. The following table gives some information on a direct methanol fuel cell.

Determine the thermodynamic efficiency of a methanol fuel cell operating at 0.576 V.

Use sections 1 and 2 of the data booklet.

n = 6

«ΔG^Θ = –nFE^Θ = 6 mol × 96 500 C mol^–1 × 0.576 V =» –333 504 J/–334 kJ

«Efficiency = $\frac{\mathrm{\Delta }G}{\mathrm{\Delta }H}=\frac{-334}{-726}$ =» 0.459/45.9%

Award [3] for correct final answer.

[3 marks]

Describe the effect of infrared (IR) radiation on carbon dioxide molecules.

Outline one approach to controlling industrial emissions of carbon dioxide.

bond length/C=O distance changes
OR
«asymmetric» stretching «of bonds»
OR
bond angle/OCO changes [✔]

polarity/dipole «moment» changes
OR
dipole «moment» created «when molecule absorbs IR» [✔]

Note: Accept appropriate diagrams.

Any one of:
capture where produced «and store» [✔]

use scrubbers to remove [✔]

use as feedstock for synthesising other chemicals [✔]

carbon credit/tax/economic incentive/fines/country specific action [✔]

use alternative energy
OR
stop/reduce use of fossil fuels for producing energy [✔]

use carbon reduced fuels «such as methane» [✔]

increase efficiency and reduce energy use [✔]

Note: Do not accept “planting more trees”.

Accept specific correct examples.

This part was fairly well answered with most candidates receiving one of the two marks. There were many candidates who stated asymmetric stretching and bonds vibrate but missed writing polarity and dipole changes, which deprived them of the second mark.

This part was reasonably answered although there were many candidates who gave vague answers that did not receive marks.

Outline the major technical problem affecting the direct use of vegetable oils as fuels in internal combustion engines and the chemical conversion that has overcome this.

State the formula of a fuel that might be produced from the vegetable oil whose formula is shown.

viscosity «of vegetable oils is too high»

transesterification

OR

«conversion into» alkyl/methyl/ethyl esters

[2 marks]

R–CO–O–CH₃ / RCOOMe

OR

R–CO–O–C₂H₅ / RCOOEt

[1 mark]

Early photovoltaic cells were based on silicon containing traces of other elements. State the type of semiconductor produced by doping silicon with indium, In, giving a reason that refers to its electronic structure.

Dye-sensitized solar cells, DSSCs, use a dye to absorb the sunlight. State two advantages that DSSCs have over traditional silicon based photovoltaic cells.

The structure of two dyes used in DSSCs are shown.

Predict, giving a reason, which dye will absorb light of longer wavelength.

p-type AND has 3 «valence» electrons

OR

p-type AND fewer electrons «than silicon»

Do not accept “it is in group 3/13” as reason.

[1 mark]

Any two of:

cheaper

OR

ease of fabrication

use light of lower energy/lower frequency/longer wavelength

absorb wider range of wavelengths

dye converts most/all absorbed photons into electrons

plentiful /renewable resources «to construct DSSC cells»

operate at lower «internal» temperatures/better at radiating heat away «since constructed with thin front layer of conductive plastic compared to glass box in photovoltaic cell»

use of nanoparticles provides large surface area exposure to sunlight/sun/light

can absorb better under cloudy/low light conditions

better conductivity

more flexible

[2 marks]

B AND has greater/more «extensive» conjugation

Accept “more alternating single and double bonds”.

[1 mark]

Suggest another advantage and one disadvantage of solar energy.

State a physical property of vegetable oils that makes them very difficult to use as fuel in internal combustion engines.

Describe how vegetable oils can be converted to a more suitable fuel.

Contrast the importance of carbon dioxide and methane as greenhouse gases.

Explain, using an equation, the effect of increased carbon dioxide in the atmosphere on the pH of lake water.

Advantage:

renewable «energy source»

OR

does not produce greenhouse gases

OR

can be installed «almost» anywhere

OR

low maintenance costs ✔

Disadvantage:

widely dispersed/not concentrated «form of energy»

OR

geography/weather/seasonal dependent

OR

not available at night

OR

energy storage is difficult/expensive

OR

toxic/hazardous materials used in production

OR

concerns about space/aesthetics/local environment where installed

OR

need to be «constantly» cleaned ✔

Accept “can be used for passive/active heating”, “can be converted to electric energy”.

Accept any specific greenhouse gas name or formula for “greenhouse gases”.

Accept “solar cells require large areas”, “solar cell manufacture produces pollution/greenhouse gases”, “higher cost of solar cells «compared with traditional sources such as fossil fuels or hydroelectric»”.

high viscosity ✔

Accept “low volatility”, just “viscous/viscosity” OR “does not flow easily”.

convert to esters of monoatomic alcohols

OR

react with short-chain alcohols «in the presence of acid or base» ✔

Accept “convert to shorter «carbon chain» esters” OR “transesterification”.

Accept specific alcohols, such as methanol or ethanol.

carbon dioxide/CO₂ more/most abundant «GHG than methane/CH₄»

OR

carbon dioxide/CO₂ has «much» longer atmospheric life «than methane/CH₄» ✔

methane/CH₄ «much» better/more effective at absorbing IR radiation «than carbon dioxide/CO₂»

OR

methane/CH₄ has a greater greenhouse factor «than carbon dioxide/CO₂»

OR

methane/CH₄ has a greater global warming potential/GWP «than carbon dioxide/CO₂» ✔

Accept “carbon dioxide/CO₂ contributes more to global warming «than methane/CH₄»”.

CO₂ (g) + H₂O (l) $\rightleftharpoons$ H⁺ (aq) + HCO₃^– (aq)

OR

CO₂ (g) $\rightleftharpoons$ CO₂ (aq) AND CO₂ (aq) + H₂O (l) $\rightleftharpoons$ H⁺ (aq) + HCO₃^– (aq) ✔

«increasing [CO₂ (g)]» shifts equilibrium/reaction to right AND pH decreases ✔

Accept “H₂CO₃ (aq)” for “CO₂ (aq) + H₂O (l)”.

Equilibrium arrows required for M1.

State symbols required for CO₂ (g) $\rightleftharpoons$ CO₂ (aq) equation only for M1.

Accept “concentration of H⁺/[H⁺] increases AND pH decreases” for M2.

Formulate equation(s) for the conversion of coal and steam to methane.

Comment on the specific energies of hydrogen and methane.

Calculate the mass, in kg, of carbon dioxide produced by the complete combustion of 72.0 dm³ octane, C₈H₁₈.

Density of C₈H₁₈ = 703 g dm⁻³

C₈H₁₈ (l) + 12.5O₂ (g) → 8CO₂ (g) + 9H₂O (g)

ALTERNATIVE 1:

2C (s) + 2H₂O (g) → CH₄ (g) + CO₂ (g) ✔

ALTERNATIVE 2:

C (s) + 2H₂O (g) → CO (g) + H2 (g) AND 3H₂ (g) + CO (g) → CH₄ (g) + H₂O (g) ✔

Accept “3C (s) + 2H₂O (g) → CH₄ (g) + 2CO (g)”.

«$\frac{141.6}{55.5}$» hydrogen/H₂ produces 2.6 times/more than twice the energy of methane/CH₄ «per mass/g»

OR

less mass of hydrogen/H₂ required «to produce same amount of energy»

OR

hydrogen/H₂ more energy efficient ✔

Accept “hydrogen/H₂ produces «nearly» three times more energy than methane/CH₄ «per mass/g»”.

m_octane «= 72.0 dm³ × 703 g dm^–3» = 5.06 × 10⁴ «g»/50.6 «kg» ✔

m_{carbon dioxide} «= $\frac{8\times 44.01}{114.26}\times 50.6$» = 156 «kg» ✔

Award [2] for correct final answer.

Explain fusion reactions with reference to binding energy.

Outline why the term breeder is used for the reactors.

Deduce the fission reaction when ²³⁹Pu is bombarded with a neutron to produce ¹³³Xe and ¹⁰³Zr.

Nuclear disasters release radioactive caesium into the atmosphere, which presents serious health risks.

Cs-137 has a half-life of 30 years.

Calculate the percentage of Cs-137 remaining in the atmosphere after 240 years.

Deduce a Lewis (electron dot) structure of the superoxide, O₂^–, free radical.

Explain why free radicals are harmful to living cells.

small/lighter nuclei combine to form larger/heavier nuclei ✔

product has higher binding energy «per nucleon» ✔

Accept binding energy curve with explanation.

converts non-fissile «²³⁸U» material into fissile «²³⁹Pu» material

OR

produces more fissile material than it consumes ✔

²³⁹Pu + ¹n → ¹³³Xe + ¹⁰³Zr + 4¹n ✔

Accept equation with correct atomic numbers included.

Accept notation for neutrons of “n”.

Accept a correctly described equation in words.

ALTERNATIVE 1:

«$\frac{240}{30}=$» 8 $t_{\frac{1}{2}}$/8 half-lives «required» ✔

% remaining = «0.50⁸ × 100 =» 0.39 «%» ✔

ALTERNATIVE 2:

λ = «$\frac{0.693}{30}=$» 0.023 ✔

% remaining = «100 × e^{–0.023 × 240} =» 0.39 «%» ✔

Award [2] for correct final answer.

OR

Accept any combination of dots, crosses and lines to represent electrons.

Do not penalize missing brackets.

Penalize missing negative charge.

highly reactive

OR

start redox reactions ✔

damage/mutate DNA

OR

cause cancer

OR

damage enzymes ✔

Outline how a rechargeable battery differs from a primary cell.

Formulate half-equations for the reactions at the anode (negative electrode) and cathode (positive electrode) during discharge of a lithium-ion battery.

A voltaic cell consists of a nickel electrode in 1.0 mol dm⁻³ Ni²⁺ (aq) solution and a cadmium electrode in a Cd²⁺ (aq) solution of unknown concentration.

Cd (s) + Ni²⁺ (aq) → Cd²⁺ (aq) + Ni (s)               E^Θ_cell = 0.14 V

Determine the concentration of the Cd²⁺ (aq) solution if the cell voltage, E, is 0.19 V at 298 K. Use section 1 of the data booklet.

Identify the structural feature of the dye that allows the conversion of solar energy into electrical energy.

Outline the effect of sunlight on the dye in the solar cell.

State the purpose of TiO₂.

Deduce the reduction half-equation at the cathode.

«redox» reaction in rechargeable battery is reversible «but not in a primary cell»

OR

secondary cells need to be charged before use

OR

secondary cells have greater rate of self-discharge ✔

Accept “primary cells cannot be recharged/reused”, “primary cells can be used only once” OR “lithium batteries may explode”.

Anode (negative electrode):

Li (graphite) → Li⁺ (electrolyte) + e^–

OR

LiC6 (s) → 6C (s) + Li⁺ (electrolyte) + e^– ✔

Cathode (positive electrode):

Li⁺ (electrolyte) + e^– + MnO₂ (s) → LiMnO₂ (s)

OR

Li⁺ (electrolyte) + e^– + NiO₂ (s) → LiNiO₂ (s)

OR

Li⁺ (electrolyte) + e^– + CoO₂ (s) → LiCoO₂ (s)

OR

2Li⁺ (electrolyte) + 2e^– + 2CoO₂ (s) → Co2O₃ (s) + Li₂O (s) ✔

Accept “polymer” for “electrolyte”.

Award [1 max] if electrodes are reversed.

Do not accept “CO” for “Co”.

«E = E^Θ − $\left(\frac{RT}{nF}\right)$lnQ»

$0.19=0.14-\left(\frac{8.31\times 298}{2\times 96500}\right)\text{ln}\left(\frac{\left[\text{C}{\text{d}}^{2+}\right]}{\left[1\right]}\right)$

OR

0.05 = –0.01283 ln [Cd²⁺]

OR

ln[Cd²⁺] = – 3.897 ✔

[Cd²⁺] = 0.020 «mol dm^–3» ✔

Award [2] for correct final answer.

«extensive» conjugation

OR

«extensive» alternate single and double bonds ✔

Accept “delocalization”.

electrons excited/released «from dye» ✔

Accept “photooxidation/oxidizes dye”.

transfers e^– to external circuit ✔

Accept “provides large surface area”.

I₃^– (aq) + 2e^– → 3I^– (aq) ✔

Accept “3I₂ (aq) + 2e^– → 2I₃^– (aq)”.

Write the nuclear equation for this fission reaction.

Outline why the reaction releases energy.

The masses of the particles involved in this fission reaction are shown below.

Mass of neutron = 1.00867 amu
Mass of U-235 nucleus = 234.99346 amu
Mass of Ba-144 nucleus = 143.89223 amu
Mass of Kr-89 nucleus = 88.89788 amu

Determine the energy released, in J, when one uranium-235 nucleus undergoes fission. Use this data and information from sections 1 and 2 of the data booklet.

The critical mass for weapons-grade uranium can be as small as 15 kg. Outline what is meant by critical mass by referring to the equation in (a)(i).

The daughter product, ⁸⁹Kr, has a half-life of 3.15 min.

Calculate the time required, in minutes, for its radioactivity to fall to 10% of its initial value, using section 1 of the data booklet.

²³⁵U + ¹n → ¹⁴⁴Ba + ⁸⁹Kr + 3¹n   [✔]

greater binding energy per nucleon in products than reactants    [✔]

Note: Accept “mass of products less than mass of reactants” OR “mass converted to energy/E = mc²”.

«Δm  mass of reactants-mass of products»

Δm = «234.99346 – 143.89223 – 88.89788 – (2 × 1.00867) =» 0.18601 «amu»   [✔]

Δm = «0.18601 amu × 1.66 × 10⁻²⁷ kg amu^–1 =» 3.09 × 10^–28 «kg»    [✔]

E = «mc² = 3.09 × 10^–28 kg × (3.00 × 10⁸ m s^–1)² =» 2.78 × 10^–11 «J»    [✔]

Note: Award [3] for correct final answer.

mass/amount/quantity required so that «on average» each fission/reaction results in a further fission/reaction   [✔]

at least one of the «3» neutrons produced must cause another reaction    [✔]

Note: Accept “minimum mass of nuclear fuel needed for the reaction to be selfsustaining”.

$\lambda \left(=\frac{\ln 2}{t_{\frac{1}{2}}}=\frac{\ln 2}{3.15}\right)=0.220$ «min^–1»    [✔]

$t\left(=-\frac{1}{\lambda }\ln \frac{N}{N_0}=-\frac{\ln 0.1}{0.220}\right)=10.5$ «min»    [✔]

Note: Award [2] for correct final answer.

Required candidates to write a nuclear equation for a fission reaction with the question indicating that ²³⁵U was bombarded with a neutron to produce ¹⁴⁴Ba and ⁸⁹Kr. Despite this, some candidates did not include the initial neutron or omitted neutrons completely.

Outlining why the fission reaction releases energy was challenging. Some candidates simply said the reaction was exothermic. Some said that the products were smaller than the reactant. Very few candidates referred to binding energy per nucleon.

The calculation of the energy released in was done reasonably well. Some candidates answered this very well and scored full marks. However, many scored 1 or 2 marks out of 3 through ECF marks. Common errors were incorrect calculation of mass in amu, or omission of converting amu to kg. Here, it was apparent that good setting out of calculations was effective in scoring for partially correct responses.

The meaning of critical mass was answered reasonably well with most candidates scoring at least 1 out of 2.

The calculation of time taken for radioactivity to fall to 10% of its initial value was answered very well.

Calculate the energy released, in MeV, in this reaction, using section 36 of the data booklet.

ΔBE = BE(⁴He) − (BE(²H) + BE(³H))
OR
ΔBE = 4 x 7.1 − (2 x 1.1 + 3 x 2.8)
= 17.8 «MeV»

Accept answers in range 17.3 to 18.1 «MeV».

Award [1 max] for final answers in range of 3.0 to 3.4 «MeV».

Award [2] for correct final answer.

[2 marks]

Ethanol has a Research Octane Number (RON) of 108.6.

Outline how higher octane fuels affect engine performance.

Ethanol can be used in a direct-ethanol fuel cell (DEFC) as illustrated by the flow chart.

Deduce the half-equations occurring at electrodes A and B.

Electrode A: 

Electrode B:

State the name and function of X in the diagram in (b)(i).

Name:

Function:

Outline why aqueous ethanol, rather than pure ethanol, is used in a DEFC.

Biodiesel containing ethanol can be made from renewable resources.

Suggest one environmental disadvantage of producing biodiesel from renewable resources.

increased AND fuels can be compressed more «before ignition»   [✔]

Note: Accept “engines can be designed with higher compression ratio” OR “less chance of pre-ignition/auto-ignition/knocking occurring”.

Electrode A: C₂H₆O (aq) + 3H₂O (l) → 12H⁺ (aq) + 12e^– + 2CO₂ (g)   [✔]

Electrode B: 3O₂ (g) + 12H⁺ (aq) + 12e^– → 6H₂O (l)    [✔]

Note: Accept balanced equations with integer or fractional coefficients.

Penalize equilibrium arrows once only.

Name:
PEM/proton-exchange membrane/polymer exchange membrane/polymer electrolyte membrane   [✔]

Function:
allows the passage of protons/H⁺ ions «from anode to cathode but not electrons or molecules»   [✔]

Any one of:
water is a reactant/allows the cell to operate at a higher concentration of protons/H⁺ ions
OR
water is a stronger electrolyte and thus produces higher electric current    [✔]

less dangerous/flammable    [✔]

use of «farm» land «for production»
OR
deforestation «for crop production for fuel»
OR
can release more NO_x «than normal fuel on combustion»   [✔]

Note: Ignore any reference to cost.

One G2 respondent was concerned that “Research Octane Number” was used instead of “Octane Rating”, but most candidates correctly outlined how higher octane fuels affect engine performance.

Some candidates did very well and deduced the half-equations occurring in a DEFC as illustrated in a flow chart. A few used equilibrium arrows and lost a mark. Some candidates failed to balance the half-equations.

Was answered well.

Few candidates could outline why aqueous ethanol is used in a DEFC.

Most could suggest one environmental disadvantage of producing biodiesel from renewable resources.

Outline how a microbial fuel cell produces an electric current from glucose.

C₆H₁₂O₆ (aq) + 6O₂ (g) → 6CO₂ (g) + 6H₂O (l)

The cell potential for the spontaneous reaction when standard magnesium and silver half-cells are connected is +3.17 V.

Determine the cell potential at 298 K when:

     [Mg²⁺] = 0.0500 mol dm⁻³
     [Ag⁺] = 0.100 mol dm⁻³

Use sections 1 and 2 of the data booklet.

Outline one difference between a primary and a secondary cell.

Any three of:
C₆H₁₂O₆ (aq) + 6H₂O (l) → 6CO₂ (g) + 24H⁺ (aq) + 24e^–
OR
electrons released by oxidation of glucose [✔]

enzymes «in bacteria» oxidize glucose
OR
«bacteria» transfer «released» electrons directly to anode [✔]

24H⁺ (aq) + 6O₂ (g) + 24e^– → 12H₂O (l)
OR
electrons consumed by reduction of oxygen [✔]

PEM/membrane separates two half reactions
OR
PEM/membrane allows proton/H⁺ transfer from anode to cathode [✔]

electrons flow though external circuit [✔]

Note: Accept 4H⁺ (aq) + O₂ (g) + 4e^– → 2H₂O (l).

«E = Eᶱ $\frac{RT}{nF}$ × lnQ»

lnQ = « $\frac{[\text{M}{\text{g}}^{2+}]}{{[\text{A}{\text{g}}^{+}]}^2}=\frac{0.0500}{{0.100}^2}=\ln 5.00=$» 1.61  [✔]

E = «3.17 V $-\frac{8.31\text{ J }{\text{K}}^{-1}\text{ mo}{\text{l}}^{-1}\times 298\text{ K}}{2\times 96500\text{ J }{\text{V}}^{-1}\text{ mo}{\text{l}}^{-1}}\times \ln \frac{0.0500}{{0.100}^2}=3.17-0.021=+$»3.15 «V» [✔]

Note: Award [2] for correct final answer.

Any one of:

Note: Accept “primary cannot be recharged AND “secondary can be recharged”.

The question on microbial cell invited varied responses. Half equations for the oxidation of glucose or reduction of oxygen were rarely written. PEM/membrane separates two half- reactions and allows proton transfer from anode to cathode was missed by most of the candidates.

The cell potential was correctly calculated by several candidates with some candidates managed an ECF mark for an error in the calculation. Unfortunately, the ln Q part was frequently wrong due to candidates forgetting to square the denominator.

Most candidates were able to state one difference between a primary and a secondary cell.

Calculate the relative rate of effusion of ²³⁵UF₆(g) to ²³⁸UF₆(g) using sections 1 and 6 of the data booklet.

Explain, based on molecular structure and bonding, why diffusion or centrifuging can be used for enrichment of UF₆ but not UO₂.

M_r(²³⁵UF₆) = 235 + (19.00 × 6)/349

OR

M_r(²³⁸UF₆) = 238 + (19.00 × 6)/352

«$\frac{\text{rate of effusion of}{}^{235}\text{U}}{\text{rate of effusion of}{}^{238}\text{U}}=\sqrt{\frac{352}{349}}=$» 1.004

Award [2] for correct final answer.

Do not accept “1.00” OR “0.996”.

[2 marks]

UF₆: Structure: octahedral «solid»/square bipyramidal «solid»/«simple» molecular solid/simple molecule AND Bonding: covalent

UO₂: Structure: crystal/lattice/network «solid»/«resembles» fluorite AND Bonding: «partly» covalent

UF₆ sublimes/evaporates/boils at low temperature

Accept “UF₆: Structure: octahedral «solid»/square bipyramidal «solid»/«simple» molecular solid/simple molecule AND weak intermolecular/London/dispersion/van der Waals’/vdW forces”.

Accept “non-polar molecule” for “«simple» molecular solid”.

Accept “giant molecular” OR “macromolecular” for “network”.

Accept “ionic/electrostatic attractions «between ions»” for bonding in UO₂.

Award M2 for “UO₂: network covalent/covalent network/giant covalent” OR “UO₂: network ionic/giant ionic”.

For M1 and M2 award [1 max] for two correct structures OR two bonding types.

Accept any specified low temperature in the range 56–65 °C.

Deduce the half-equations for the reactions occurring at the electrodes.

Anode (negative electrode):

Cathode (positive electrode):

Calculate the cell potential, E^θ, in V, using section 24 of the data booklet.

Suggest how PEM fuel cells can be used to produce a larger voltage than that calculated in (b)(i).

Suggest an advantage of the PEM fuel cell over the lead-acid battery for use in cars.

Outline the functions of the dye, TiO₂ and the electrolyte in the operation of the DSSC.

Dye: 

TiO₂:

Electrolyte:

Suggest an advantage of the DSSC over silicon-based photovoltaic cells.

Anode (negative electrode):
H₂ (g) → 2H⁺ (aq) + 2e⁻ ✔

Cathode (positive electrode):
O₂ (g) + 4H⁺ (aq) + 4e⁻ → 2H₂O (l) ✔

NOTE: Accept any correct integer or fractional coefficients. 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.

(+)1.23 «V» ✔

NOTE: Do not accept “-1.23 «V»”.

connect several fuel cells in series
OR
increase pressure/concentration of reactant/hydrogen/oxygen ✔

NOTE: Do not accept changes in [H⁺]/pH as they do not affect cell potential in this case.
Do not accept reference to quantity for “concentration”.

liquid in cell is less/not corrosive
OR
does not contain lead/toxic chemicals
OR
larger energy density/charge capacity/current per unit mass
OR
does not have to be charged prior to use / is always ready for use «as long as fuel is available» ✔

Dye:
absorbs photons/light
OR
releases electrons ✔

TiO2:
conducts current/electricity
OR
semiconductor ✔

Electrolyte:
reduces/regenerates «the oxidized» dye ✔

Any one of:
cheaper/ease of manufacture
OR
plentiful and renewable resources «to construct DSSC cells» ✔

use light of lower energy/lower frequency/longer wavelength
OR
use of nanoparticles provides large surface area for exposure to sunlight/sun/light
OR
can absorb better under cloudy conditions ✔

operate at lower «internal» temperatures
OR
better at radiating heat away «since constructed with thin front layer of conductive plastic compared to glass box in photovoltaic cells» ✔

better conductivity ✔

more flexible/durable ✔

NOTE: Accept “lower mass/lighter «so greater flexibility to integrate into windows etc.»” OR “greater power-conversion efficiency «with latest DSSC models»”.

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.

Some solar cells use photovoltaic semi-conductors. Compare, giving reasons, the electrical conductivity of metals and semi-conductors as temperature increases.

Suggest one advantage of a dye-sensitized solar cell (DSSC) over a silicon based photovoltaic cell.

metal conductivity decreases AND semi-conductor conductivity increases    [✔]

metal: collisions between «already free moving» electrons/vibrating lattice ions and electrons increase    [✔]

semi-conductor:
provides sufficient energy for electrons to move to conduction band
OR
allows semiconductors to ionize forming freely moving electrons   [✔]

Any one of:

cheaper    [✔]

uses light of lower energy     [✔]

plentiful resources     [✔]

renewable resources     [✔]

use of nanoparticles provides large surface area exposure to sunlight     [✔]

can absorb better under cloudy conditions     [✔]

better conductivity     [✔]

more flexible     [✔]

Candidates struggled to express themselves adequately in comparing the change in electrical conductivity of metals and semi-conductors as temperature increases. Many scored 1 mark for stating the effect of increasing temperature, but very few scored any marks for the explanation.

Generally answered well with most candidates able to suggest one advantage of a DSSC over a silicon based photovoltaic cell.

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.

The regular rise and fall of sea levels, known as tides, can be used to generate energy.

State one advantage, other than limiting greenhouse gas emissions, and one disadvantage of tidal power.

Advantage:

Disadvantage:

Advantage
Any one of:
renewable  [✔]
predictable supply [✔]
tidal barrage may prevent flooding [✔]
effective at low speeds [✔]
long life-span [✔]
low cost to run [✔]

Disadvantage
Any one of:
cost of construction [✔]
changes/unknown effects on marine life [✔]
changes circulation of tides in the area [✔]
power output is variable [✔]
limited locations where feasible [✔]
equipment maintenance can be challenging [✔]
difficult to store energy [✔]

Note: Do not accept vague generalisations.

Do not accept economic issues for both advantage and disadvantage.

Do not accept sustainable.

Accept “energy” or “electricity” for “power”.

Many candidates performed well on this question, especially when identifying an advantage of tidal power. The candidates who struggled tended to either give vague or journalistic answers especially for the disadvantage of tidal power.

Deduce the half-equations and the overall equation for the reactions taking place in a direct methanol fuel cell (DMFC) under acidic conditions.

Outline one advantage and one disadvantage of the methanol cell (DMFC) compared with a hydrogen-oxygen fuel cell.

Negative electrode (anode):

CH₃OH (aq) + H₂O (l) → CO₂ (g) + 6H⁺ (aq) + 6e^–

Positive electrode (cathode):
O₂ (g) + 4H⁺ (aq) + 4e^– → 2H₂O (l)

Overall equation:

2CH₃OH (aq) + 3O₂ (g) → 2CO₂ (g) + 4H₂O (l)

Accept any whole or fractional coefficients in balanced equations.

Award [1 max] for correct half-equations at wrong electrodes for M1 and M2.

Advantage:

Any one of:

liquid methanol is easier to transport/store than gaseous hydrogen

OR

hydrogen is explosive

longer membrane life «as it operates in aqueous environment»

methanol has greater energy density than hydrogen

Disadvantage:

Any one of:

lower voltage

lower power per unit mass «of the cell»

lower efficiency

toxic/can be mistaken for ethanol

lower specific energy

Ignore any cost references throughout.

Accept “CO₂/greenhouse gas produced” OR “requires a more highly efficient catalyst”.

Do not award marks for converse statements for the advantage and disadvantage.

State the nuclear equation for the fusion reaction.

Explain why fusion is an exothermic process.

Calculate the heat energy released, in J, by the fusion reaction producing one atom of carbon-12. Use section 2 of the data booklet and E = mc².

Beryllium-8 is a radioactive isotope with a half-life of 6.70 × 10⁻¹⁷ s.

Calculate the mass of beryllium-8 remaining after 2.01 × 10⁻¹⁶ s from a sample initially containing 4.00 g of beryllium-8.

${}_2^4\text{He + }{}_4^8\text{Be}\to {}_6^{12}\text{C}$✔

NOTE: Do not penalize missing atomic numbers.

ALTERNATIVE 1
binding energy per nucleon is larger in carbon-12/product «than beryllium-8 and helium-4/reactants» ✔

difference in «total» binding energy is released «during fusion» ✔

ALTERNATIVE 2
mass of carbon-12/product «nucleus» is less than «the sum of» the masses of helium-4 and beryllium-8 «nuclei»/reactants
OR
two smaller nuclei form a lager nucleus ✔

mass lost/difference is converted to energy «and released»
OR
E = mc² ✔

Δm = «12.000000 amu − (4.002602 amu + 8.005305 amu) =» −0.007907 «amu» ✔
«0.007907 amu × 1.66 × 10⁻²⁷ kg amu⁻¹ =» 1.31 × 10⁻²⁹ «kg» ✔
«E = mc² = 1.31 × 10⁻²⁹ kg × (3.00 × 108 m s⁻¹)² =» 1.18 × 10⁻¹² «J» ✔

NOTE: Accept “0.007907 «amu»”.
Award [2 max] for “7.12 x 10¹⁴ «J»”.
Award [3] for correct final answer.

ALTERNATIVE 1
3 half-lives ✔
0.500 g «of beryllium-8 remain» ✔

ALTERNATIVE 2
$m=4.00{\left(\frac{1}{2}\right)}^{\frac{2.01\times {10}^{-16}}{6.70\times {10}^{-17}}}$ ✔
0.500 g «of beryllium-8 remain» ✔

ALTERNATIVE 3
λ = « $\frac{\ln 2}{6.70\times {10}^{-17}}$ »= 1.03 × 10¹⁶ «s⁻¹» ✔
m = « $4.00{\text{e}}^{-1.03\times {10}^{16}\times 2.01\times {10}^{-16}}\text{ = }$ » 0.500 «g» ✔

NOTE: Award [2] for correct final answer.

Crude oil can be converted into fuels by fractional distillation and cracking.

Contrast these two processes.

Determine the specific energy, in kJ g⁻¹, and energy density, in kJ cm⁻³, of hexane, C₆H₁₄. Give both answers to three significant figures.

Hexane: M_r = 86.2; ΔH_c = −4163 kJ mol⁻¹; density = 0.660 g cm⁻³

Specific energy:

Energy density:

Hydrocarbons need treatment to increase their octane number to prevent pre-ignition (knocking) before they can be used in internal combustion engines.

Describe how this is carried out and the molecular changes that take place.

Note: Award [1] for any two correct answers from one column OR one from each column.

Award [2] for any two correct from each column; eg: fractional distillation – any two correct award [1 max] AND
cracking – any two correct, award [1 max].

specific energy = « $\frac{4163\text{ kJ mo}{\text{l}}^{-1}}{86.2\text{ g mo}{\text{l}}^{-1}}$ =» 48.3 «kJ g^–1»  [✔]

energy density = «48.3 kJ g^–1 × 0.660 g cm^–3 =» 31.9 «kJ cm^–3»  [✔]

Note: Award [1 max] if either or both answers not expressed to three significant figures.

Any two of:
«hydrocarbons are heated with» catalyst  [✔]

long chains break and reform
OR
branching/aromatisation occurs
OR
isomerisation/reforming/platforming/cracking [✔]

zeolite separates branched from non-branched
OR
products are distilled
OR
«distillation» separates reformed and cracked products [✔]

Note: Accept a specific catalyst name or formula for M1 such as Pt/Re/Rh/Pd/Ir.

This part was not well answered. Many candidates didn’t answer the question as instructed. Candidates required two correct statements, either about fractional distillation or cracking as a process for one mark.

This part was very well answered by most candidates with the correct number of significant digits as specified in the question.

Candidates responded well to at least one mark of this question. There were several different ways to earn the two marks possible. The most common way candidates earned marks were by identifying the use of a catalyst and then the idea of the compound reforming into a smaller or branched compound. Very few candidates discussed the idea of purification or separation into individual compounds, which is another important part of this process.

Sketch graphs to show the general effect of increasing temperature on the electrical conductivity of semiconductors and metals on the axes below.

Explain the function of dyes in a dye-sensitized solar cell (DSSC).

Semiconductors:
increases  [✔]

Metals:
decreases  [✔]

Note: Accept any graph showing general increase for semiconductor.

Accept any graph showing general decrease for metal.

Accept a graph showing vertical section below transition temperature for a superconducting metal.

dye absorbs light  [✔]

electrons from «excited» dye pass to TiO₂/semiconductor/electrolyte/cell
OR
dye undergoes photo-oxidation [✔]

Several candidates managed one mark to show conductivity of semiconductors on increasing the temperature but were unable to show that generally conductivity decreases for metals when the temperature is increased.

The question on dye-sensitized solar cell invited mixed responses. While most candidates correctly stated that dyes absorb light but several failed to mention that electrons from the excited dye pass to TiO2/semiconductor.

This question is about biofuel.

Evaluate the use of biodiesel in place of diesel from crude oil.

Strength
Any one of:
less flammable «than diesel»  [✔]

recycles carbon «lower carbon footprint»
OR
lower greenhouse gas emissions [✔]

easily biodegradable «in case of spill» [✔]

renewable
OR
does not deplete fossil fuel reserves [✔]

economic security/availability in countries without crude oil [✔]

Limitation
Any one of:
more difficult to ignite inside the engine «than diesel» [✔]

more viscous «than diesel» [✔]

lower energy content/specific energy/energy density [✔]

uses food sources
OR
uses land that could be used for food [✔]

«production is» more expensive [✔]

less suitable in low temperatures [✔]

increased NO_x emissions for biodiesel [✔]

greenhouse gases still produced [✔]

Note: Accept “«close to» carbon neutral”, “produce less greenhouse gases/CO2”.

Accept “engines have to be modified if biodiesel used” as limitation.

Do not award marks for strength and limitation that are the same topic/concept.

This question was well answered, and many candidates received either one or both marks.

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.