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Question
Alloys containing at least $60 \%$ copper reduce the presence of bacteria on their surface. The percentage of copper in brass, an alloy of copper and zinc, can be determined by UV-vis spectrometry.
A sample of brass is dissolved in concentrated nitric acid and then made up to $250.0 \mathrm{~cm}^3$ with water before analysis.
$$
\begin{gathered}
\mathrm{Cu}(\mathrm{s})+4 \mathrm{HNO}_3(\mathrm{aq}) \rightarrow \mathrm{Cu}\left(\mathrm{NO}_3\right)_2(\mathrm{aq})+2 \mathrm{NO}_2(\mathrm{~g})+2 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \\
3 \mathrm{Zn}(\mathrm{s})+8 \mathrm{HNO}_3(\mathrm{aq}) \rightarrow 3 \mathrm{Zn}\left(\mathrm{NO}_3\right)_2(\mathrm{aq})+2 \mathrm{NO}(\mathrm{g})+4 \mathrm{H}_2 \mathrm{O}(\mathrm{l})
\end{gathered}
$$
The concentration of copper(II) ions in the resulting solution is then determined from a calibration curve, which is plotted by measuring the light absorbance of standard solutions.
Titration is another method for analysing the solution obtained from adding brass to nitric acid.
a. Outline why the initial reaction should be carried out under a fume hood.
b. Deduce the equation for the relationship between absorbance and concentration.
c. Outline how a solution of $0.0100 \mathrm{~mol} \mathrm{dm}^{-3}$ is obtained from a standard $1.000 \mathrm{~mol} \mathrm{dm}^{-3}$ copper(II) sulfate solution, including two essential pieces of glassware you would need.
d.i. The original piece of brass weighed $0.200 \mathrm{~g}$. The absorbance was 0.32 .
Calculate, showing your working, the percentage of copper by mass in the brass.
d.ii.Deduce the appropriate number of significant figures for your answer in (d)(i).
e.i. Comment on the suitability of using brass of this composition for door handles in hospitals.
If you did not obtain an answer to (d)(i), use $70 \%$ but this is not the correct answer.
e.ii.Suggest another property of brass that makes it suitable for door handles.
f.i. Copper(II) ions are reduced to copper(I) iodide by the addition of potassium iodide solution, releasing iodine that can be titrated with sodium thiosulfate solution, $\mathrm{Na}_2 \mathrm{~S}_2 \mathrm{O}_3(\mathrm{aq})$. Copper(I) iodide is a white solid.
$$
\begin{gathered}
4 \mathrm{I}^{-}(\mathrm{aq})+2 \mathrm{Cu}^{2+}(\mathrm{aq}) \rightarrow 2 \mathrm{Cul}(\mathrm{s})+\mathrm{I}_2(\mathrm{aq}) \\
\mathrm{I}_2(\mathrm{aq})+2 \mathrm{~S}_2 \mathrm{O}_3{ }^{2-}(\mathrm{aq}) \rightarrow 2 \mathrm{I}^{-}(\mathrm{aq})+\mathrm{S}_4 \mathrm{O}_6{ }^{2-}(\mathrm{aq})
\end{gathered}
$$
Deduce the overall equation for the two reactions by combining the two equations.
f.ii. Suggest why the end point of the titration is difficult to determine, even with the addition of starch to turn the remaining free iodine black.
▶️Answer/Explanation
Markscheme
a. $\mathrm{NO}_2 / \mathrm{NO} / \mathrm{NO}_{\mathrm{x}} / \mathrm{HNO}_3 /$ gas is poisonous/toxic/irritant
Accept formula or name.
Accept ” $\mathrm{HNO}_3$ is corrosive” OR “poisonous/toxic gases produced”.
Accept “reaction is harmful/hazardous”.
b. Slope (gradient):
40
Equation:
absorbance $=40 \times$ concentration
OR
$$
y=40 x
$$
Accept any correct relationship for slope such as $\frac{1.00}{0.025}$.
Award [2] if equation in M2 is correct.
c. dilute $1.00 \mathrm{~cm}^3$ «of the standard solution with water» to $100 \mathrm{~cm}^3$
OR
dilute sample of standard solution «with water» 100 times
«graduated/volumetric» pipette/pipet
volumetric flask
Accept any $1: 100$ ratio for $M 1$.
Accept “mix $1 \mathrm{~cm}^3$ of the standard solution with $99 \mathrm{~cm}^3$ of water” for M1.
Do not accept “add $100 \mathrm{~cm}^3$ of water to $1.00 \mathrm{~cm}^3$ of standard solution” for M1.
Accept “burette/buret” for M2.
Accept “graduated/measuring flask” for M3 but not “graduated/measuring cylinder” or “conical/Erlenmeyer flask”.
d.i. concentration of copper $=0.0080 \ll \mathrm{mol} \mathrm{dm}^{-3} »$
mass of copper in $250.0 \mathrm{~cm}^3=« 0.0080 \mathrm{~mol} \mathrm{dm}^{-3} \times 0.2500 \mathrm{dm}^3 \times 63.55 \mathrm{~g} \mathrm{~mol}^{-1}=» 0.127$ «g
OR
mass of brass in $1 \mathrm{dm}^3=« 4 \times 0.200 \mathrm{~g}=» 0.800 \mathrm{~g} \mathrm{AND}$ [Cu2+] $=« 0.0080 \mathrm{~mol} \mathrm{dm}^{-3} \times 63.55 \mathrm{~g} \mathrm{~mol}^{-1}=» 0.5084 \mathrm{~g} \mathrm{dm}^{-3}$
« $\%$ copper in this sample of brass $=\frac{0.127}{0.200} \times 100=» 64$ «\%»
OR
$« \%$ copper in this sample of brass $=\frac{0.5084}{0.800} \times 100=» 64 « \% »$
Accept any value in range $0.0075-0.0085$ «mol dm${ }^{-3}$ ) for M1.
Accept annotation on graph for $M 1$.
Award [3] for correct final answer.
Accept “65 «\%»”.
d.ii.wo
Do not apply ECF from 1(d)(i).
e.i. «since it is greater than $60 \%$ » it will reduce the presence of bacteria «on door handles»
e.ii resistant to corrosion/oxidation/rusting
OR
low friction surface «so ideal for connected moving components»
Accept “hard/durable”, “«high tensile» strength”, “unreactive”, “malleable” or any reference to the appearance/colour of brass (eg “gold-like”, “looks nice” etc.).
Do not accept irrelevant properties, such as “high melting/boiling point”, “non-magnetic”, “good heat/electrical conductor”, “low volatility”, etc.
Do not accept “ductile”.
f.i. $2 \mathrm{I}^{-}(\mathrm{aq})+2 \mathrm{Cu}^{2+}(\mathrm{aq})+2 \mathrm{~S}_2 \mathrm{O}_3{ }^{2-}(\mathrm{aq}) \rightarrow 2 \mathrm{Cul}(\mathrm{s})+\mathrm{S}_4 \mathrm{O}_6{ }^{2-}(\mathrm{aq})$
correct reactants and products
balanced equation
M2 can only be awarded if M1 is correct.
f.ii. precipitate/copper(I) iodide/Cul makes colour change difficult to see
OR
release of $\mathrm{I}_2 /$ iodine from starch- $\mathrm{I}_2$ complex is slow so titration must be done slowly
Question
Physical properties of elements vary according to atomic number. Sections 6 to 9 of the data booklet list some of these properties.
a. Deduce, giving a reason, the group of elements in the periodic table most likely to undergo sublimation.
$b$ (i)Describe the density trend across periods 4 and 5 of the periodic table.
b(ii) Suggest, with a reason, whether the lanthanoids or actinoids of the f-block would have the higher density.
b(iiifompare the ease of oxidation of s-block and d-block metals to their melting points and densities. Use section 25 of the data booklet.
b(iVsketch how the first ionization energies of elements vary with their atomic radius.
▶️Answer/Explanation
Markscheme
a. group $18 /$ noble gases [ $[$ ]
smallest difference between melting and boiling points
OR
weakest intermolecular forces «in that period» $[\boldsymbol{V}]$
Note: Accept “group 17/halogens”.
b(i)density increases «to a maximum in the transition elements» AND then decreases $[\boldsymbol{V}]$
b(iiłctinoids $A N D$ density increases down all groups «due to large increase in atomic mass for small increase in atomic volume»
OR
actinoids AND «much» greater atomic mass with similar type of bonding
OR
actinoids $A N D$ density «of actinoids» atomic number 90 to 95 is greater than corresponding lanthanoids [ $\mathbf{V}]$
Note: Accept “actinoids AND on graph actinoids have “much» greater density than lanthanoids”.
b(iii) Alternative 1:
«metals with» low densities oxidize easier [ $\boldsymbol{C}$
«metals with» low melting points oxidize easier [ $\boldsymbol{c}$ ]
Alternative 2:
in s-block «metals with» high densities oxidize easier
OR
in s-block «metals with» low melting points oxidize easier [ $\boldsymbol{\swarrow}$ ]
in d-block «metals with» low densities oxidize easier
OR
in d-block «metals with» low melting points oxidize easier $[\boldsymbol{\sim}$ ]
Note: Award [1 max] for “s-block metals more easily oxidized” OR “s-block metals have lower melting points” OR “s-block metals have lower densities”.
Accept “have greater activity” for “oxidize easier”.
b(iv).
Note: Accept any negative sloping line.
Do not award mark if line touches either axis.
Question
A student investigated how the type of acid in acid deposition affects limestone, a building material mainly composed of calcium carbonate.
The student monitored the mass of six similarly sized pieces of limestone. Three were placed in beakers containing $200.0 \mathrm{~cm}^3$ of $0.100 \mathrm{~mol} \mathrm{dm}^{-3}$ nitric acid, $\mathrm{HNO}_3(\mathrm{aq})$, and the other three in $200.0 \mathrm{~cm}^3$ of $0.100 \mathrm{~mol} \mathrm{dm}{ }^{-3}$ sulfuric acid, $\mathrm{H}_2 \mathrm{SO}_4$ (aq).
The limestone was removed from the acid, washed, dried with a paper towel and weighed every day at the same time and then replaced in the beakers. The student plotted the mass of one of the pieces of limestone placed in nitric acid against time.
The student hypothesized that sulfuric acid would cause a larger mass loss than nitric acid.
a. Draw a best-fit line on the graph.
b(i)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.
$\mathrm{b}$ (ii) Explain why the rate of reaction of limestone with nitric acid decreases and reaches zero over the period of five days.
b(iii\$uggest a source of error in the procedure, assuming no human errors occurred and the balance was accurate.
c(i)Justify this hypothesis.
c(ii)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.
▶️Answer/Explanation
Markscheme
a. best-fit smooth curve
NOTE: Do not accept a series of connected lines that pass through all points OR any straight line representation.
b(i)tangent drawn at time zero
$\mathrm{g} \mathrm{day}^{-1}$
0.16
$M 3$ can only be awarded if the value corresponds to the correct unit given in $M 2$.
Accept values for the initial rate for M3 in the range: $0.13-0.20 \mathrm{~g} \mathrm{day}^{-1}$ OR $1.5 \times 10^{-6} \mathrm{~g} \mathrm{~s}^{-1}-2.3 \times 10^{-6} \mathrm{~g} \mathrm{~s}^{-1} \mathrm{OR}^{-5} .5 \times 10^{-8}-1.2 \times 10^{-7} \mathrm{~mol} \mathrm{dm}^{-3}$ $\mathrm{s}^{-1}$ OR $4.5 \times 10^{-6}-6.9 \times 10^{-6} \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{~min}^{-1}$ OR $9.0 \times 10^{-5}-1.4 \times 10^{-4} \mathrm{~g} \mathrm{~min}^{-1}$ OR a range based on any other reasonable unit for rate.
Ignore any negative rate value.
Award [ 2 max] for answers such as $0.12 / 0.11 \mathrm{~g} \mathrm{day}^{-1}$, 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.
,b(iigcid 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 $\mathrm{CaCO}_3$ is used up/acts as the limiting reactant” is conveyed for $\mathrm{M1}$.
Do not accept “reaction reaches equilibrium” for $M 2$.
b(iis)urface 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
c(i)sulfuric acid is diprotic/contains two $\mathrm{H}^{+}$«while nitric acid contains one $\mathrm{H}^{+}$»/releases more $\mathrm{H}^{+}$«so reacts with more limestone»
OR
higher concentration of protons $/ \mathrm{H}^{+}$
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”.
c(ii)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.
Question
The absorption of infrared (IR) radiation by molecules in the atmosphere affects global temperatures.
Using the graph, state, giving your reasons, whether or not oxygen and ozone are greenhouse gases.
▶️Answer/Explanation
Markscheme
Ozone: yes because it absorbs IR/is IR active
Oxygen: no because it does not absorb IR/is IR inactive
Award [1 max] for stating “ozone/O3 is a greenhouse gas but oxygen/O2 is not”.
Award [1 max] for stating “ozone/O3 absorbs IR/is IR active but oxygen/O2 does not/is IR inactive”.
Accept “oxygen/O2 is not a greenhouse gas because it absorbs UV”.
Examiners report
Question
In order to provide safe drinking water, a water supply is often treated with disinfectants, which aim to inactivate disease-causing bacteria in the water.
To compare the effectiveness of different disinfectants, a CT value is used as a measure of the dosage of disinfectant needed to achieve a certain level of inactivation of specific bacteria.
CT value (mg min dm−3) = C (mg dm−3) concentration of disinfectant × T (min) contact time with water
The table below compares the CT values of different disinfectants necessary to achieve 99% inactivation of two types of bacteria, listed as A and B.
(i) Deduce the oxidation state of chlorine in the following disinfectants.
(ii) From the data on CT values, justify the statement that bacterium B is generally more resistant to disinfection than bacterium A.
(iii) CT values can be used to determine whether a particular treatment process is adequate. Calculate the CT value, in mg min dm−3, when 1.50 × 10−5 g dm−3 of chlorine dioxide is added to a water supply with a contact time of 9.82 minutes.
(iv) From your answer to (a) (iii) and the data in the table, comment on whether this treatment will be sufficient to inactivate 99% of bacterium A.
CT values are influenced by temperature and by pH. The table below shows the CT values for chlorine needed to achieve 99% inactivation of a specific bacterium at stated values of pH and temperature.
(i) With reference to the temperature data in the table, suggest why it may be more difficult to treat water effectively with chlorine in cold climates.
(ii) Sketch a graph on the axes below to show how the CT value (at any temperature) varies with pH.
(iii) Comment on the relative CT values at pH 6.0 and pH 9.0 at each temperature.
(iv) Chlorine reacts with water as follows:
Cl2 (g) + H2O (l) \( \rightleftharpoons \) HOCl (aq) + HCl (aq)
HOCl (aq) \( \rightleftharpoons \) OCl− (aq) + H+ (aq)
Predict how the concentrations of each of the species HOCl (aq) and OCl− (aq) will change if the pH of the disinfected water increases.
Despite widespread improvements in the provision of safe drinking water, the sale of bottled water has increased dramatically in recent years. State one problem caused by this trend.
▶️Answer/Explanation
Markscheme
i
HOCl: +1
AND
ClO2: +4
Accept “I” and “IV” but not “1+/1” and “4+/4” notations.
ii
«most» CT values are higher for «bacterium» B
OR
«generally» higher dosage needed for «bacterium» B
Accept converse arguments. Accept “concentration” for “dosage”
iii
«CT = 1.50 × 10–5 × 103 mg dm–3 × 9.82 min =» 1.47 × 10–1 «mg min dm–3»
iv
lower than CT value/minimum dosage/1.8 × 10–1 «mg min dm–3»
AND
no/insufficient
Accept “concentration” for “dosage”.
i
higher CT value at lower temperature
OR
higher dosage «of chlorine» needed at low temperature
Accept “effectiveness decreases at lower temperature”.
Accept “concentration” for “dosage”.
Accept converse arguments.
ii
labeled axes ( y: CT and x: pH)
AND
curve with increasing gradient
Do not accept axes the wrong way round.
Accept a linear sketch.
iii
values at pH 9.0 approximately 3 times values at pH 6.0
OR
increase in CT values in same ratio
The exact ratio is 2.9 times
Do not accept just “increase in value”.
iv
[HOCl] decreases AND [OCl−] increases
plastic disposal/pollution
OR
plastic bottles use up petroleum/non-renewable raw material
OR
chemicals in plastic bottle can contaminate water
OR
«prolonged» storage in plastic can cause contamination of water
OR
plastic water bottles sometimes reused without proper hygiene considerations
Accept other valid answers.
Accept economic considerations such as “greater production costs”, “greater transport costs” or “bottled water more expensive than tap water”
Question
A class was determining the concentration of aqueous sodium hydroxide by titrating it with hydrochloric acid, whilst monitoring the pH of the solution. The sodium hydroxide solution was added into a glass beaker from a measuring cylinder and the hydrochloric acid added using a burette. One group of students accidentally used a temperature probe rather than a pH probe. Their results are given below.
Volume of aqueous NaOH = 25.0 ± 0.5 cm3
Concentration of HCl = 1.00 ± 0.01 mol dm−3
Calculate the percentage uncertainty of the volume of the aqueous sodium hydroxide.
Suggest how the precision of this measurement could be improved.
▶️Answer/Explanation
Markscheme
«\(\frac{{0.5}}{{25.0}} \times 100\)» = 2 «%»
[1 mark]
pipette/pipet «rather than a measuring cylinder»
Accept “using a burette/buret”.
Accept “using a volumetric/measuring flask”.
Do not accept “use of a more precise measuring cylinder”.
[1 mark]
Question
A class was determining the concentration of aqueous sodium hydroxide by titrating it with hydrochloric acid, whilst monitoring the pH of the solution. The sodium hydroxide solution was added into a glass beaker from a measuring cylinder and the hydrochloric acid added using a burette. One group of students accidentally used a temperature probe rather than a pH probe. Their results are given below.
Volume of aqueous NaOH = 25.0 ± 0.5 cm3
Concentration of HCl = 1.00 ± 0.01 mol dm−3
Suggest how the end point of the titration might be estimated from the graph.
▶️Answer/Explanation
Markscheme
volume «found by extrapolation of the two best fit lines» required to give the highest temperature
OR
extrapolate «two best fit» lines to the point where they meet
Accept “where lines through the points meet”.
Accept “at maximum temperature”.
Accept “at 35 cm3 of HCl”.
[1 mark]
Examiners report
Question
A class was determining the concentration of aqueous sodium hydroxide by titrating it with hydrochloric acid, whilst monitoring the pH of the solution. The sodium hydroxide solution was added into a glass beaker from a measuring cylinder and the hydrochloric acid added using a burette. One group of students accidentally used a temperature probe rather than a pH probe. Their results are given below.
Volume of aqueous NaOH = 25.0 ± 0.5 cm3
Concentration of HCl = 1.00 ± 0.01 mol dm−3
State and explain how the graph would differ if 1 mol\(\,\)dm−3 sulfuric acid had been used instead of 1 mol\(\,\)dm−3 hydrochloric acid.
Answer/Explanation
Markscheme
graph would peak/maximum at 17.5 cm3
OR
smaller volume of acid «needed to reach equivalence»
sulfuric acid is dibasic/diprotic
higher temperature would be reached
Accept “gradient/slope «of graph» is greater/steeper” for M1.
Accept “one mole of sulfuric acid neutralizes two moles of NaOH” for M2.
[2 marks]
Examiners report
Question
Medicines have a variety of different effects on the body and act at the molecular level.
Morphine and codeine are strong analgesics. Their structures are given in section 37 of the data booklet.
Dose response curves are determined for each drug.
Outline the significance of range “a”.
Suggest the type of reaction used to convert morphine to codeine.
State and explain the action of opiates as painkillers.
▶️Answer/Explanation
Markscheme
«measures» therapeutic window/margin «of a drug»
OR
range of doses that produce a therapeutic effect without causing toxic effects
Accept “difference between ED50/minimum effective/therapeutic dose «for 50% of population» AND TD50 /toxic dose «for 50% of population»”.
Do not accept “therapeutic index”.
Do not accept “lethal/fatal dose” as this is not LD50.
[1 mark]
«nucleophilic» substitution/SN
Accept “methylation”.
[1 mark]
work directly on opioid/pain receptors «in brain»
suppress pain impulses in brain/CNS
resemble endorphins/enkephalins/natural chemical painkillers «produced in the brain and spinal cord»
Do not award mark for “resemble hormones”.
[2 marks]
Question
There is a link between world energy consumption and carbon dioxide production.
Climate induced changes in the ocean can be studied using measurements such as the Atmospheric Potential Oxygen (APO). Trends in APO concentration from two stations, one in each hemisphere, are shown below.