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A-Level · Practical Skills / Required Practicals

ARP-Chem A-Level Chemistry: Required Practicals and Practical Skills

AQA 7405 · Calculator allowed · about 200 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

A student is given an unknown white crystalline solid X and instructed to identify the cation and anion present using standard qualitative tests. The results are shown below.

Test 1: A small sample of X was dissolved in distilled water, then dilute nitric acid was added, followed by aqueous silver nitrate. Observation: a cream precipitate formed. The precipitate did not dissolve in dilute ammonia solution but dissolved slowly in concentrated ammonia solution.

Test 2: A separate sample of X was dissolved in water. Aqueous barium chloride, acidified with dilute hydrochloric acid, was added. Observation: no precipitate formed.

Test 3: A sample of solid X was heated gently with aqueous sodium hydroxide. Observation: a pungent gas was produced, which turned damp red litmus paper blue.

(a)Identify the halide ion present in X, using the result of Test 1. Give the ionic equation for the reaction between this ion and silver ions.(3)

(b)Explain why dilute nitric acid is added before the silver nitrate solution in Test 1.(2)

(c)State the conclusion that can be drawn from the result of Test 2, and explain why the hydrochloric acid is added before the barium chloride solution.(2)

(d)Identify the gas produced in Test 3 and write an ionic equation for its formation. Explain the result observed with the damp litmus paper.(4)

(e)Using your answers to Tests 1-3, deduce the identity of solid X, giving its name and formula.(2)

(Total for Question 1 is 13 marks)

An unknown colourless organic liquid Y was analysed using the tests below.

Test 1: A few drops of Y were added to Brady's reagent (2,4-dinitrophenylhydrazine dissolved in methanol and sulfuric acid). Observation: an orange precipitate formed immediately.

Test 2: A sample of Y was warmed gently with Tollens' reagent (ammoniacal silver nitrate) in a test tube. Observation: a silver mirror formed on the inside of the tube.

Test 3: A separate sample of Y was warmed with acidified potassium dichromate(VI) solution. Observation: the solution changed from orange to green.

Test 4: A few drops of Y were added to bromine water and shaken. Observation: no change was seen; the bromine water remained orange.

(a)Identify the functional group shown to be present by the result of Test 1, and state the type(s) of compound this test alone cannot distinguish between.(2)

(b)Use the result of Test 2 to identify whether Y is an aldehyde or a ketone. Write a balanced equation for the oxidation of Y (represented as RCHO) by the diamminesilver(I) ion, [Ag(NH3)2]+, in Tollens' reagent, giving the organic product formed.(3)

(c)State the colour change observed in Test 3 and explain this observation in terms of the change in oxidation state of chromium.(3)

(d)Explain what the result of Test 4 indicates about the structure of Y.(2)

(e)Using all four test results, identify the class of organic compound Y belongs to, and suggest a molecular formula consistent with a compound of this class containing three carbon atoms.(2)

(f)Describe an alternative test, using Fehling's solution, that could be used instead of Test 2 to confirm that Y is an aldehyde. State the reagent's colour before the test and the observation that would confirm a positive result.(2)

(Total for Question 2 is 14 marks)

A student prepared a standard solution of anhydrous sodium carbonate by dissolving 1.325 g of the solid in distilled water and making the solution up to exactly 250 cm3 in a volumetric flask.

25.0 cm3 portions of this solution were pipetted into a conical flask and titrated against a solution of hydrochloric acid of unknown concentration, using methyl orange indicator. The equation for the reaction is:

Na2CO3(aq) + 2HCl(aq) -> 2NaCl(aq) + H2O(l) + CO2(g)

The titration results are shown below.

Rough titre: 24.75 cm3
Titre 1: 24.65 cm3
Titre 2: 24.60 cm3
Titre 3: 24.70 cm3

(Mr of Na2CO3 = 106.0)

(a)Describe how the student should accurately prepare the 250 cm3 standard solution from the solid sodium carbonate, to ensure the concentration is known precisely.(4)

(b)Calculate the concentration, in mol dm-3, of the sodium carbonate solution.(2)

(c)Identify which of the three titres are concordant (within 0.10 cm3 of each other) and calculate the mean titre to be used in further calculations.(2)

(d)Use your answers to calculate the concentration of the hydrochloric acid, in mol dm-3.(3)

(e)The burette used has an uncertainty of ±0.05 cm3 for each reading. Calculate the percentage uncertainty in the mean titre, and suggest one modification to the method that would reduce this percentage uncertainty.(3)

(Total for Question 3 is 14 marks)

A student determined the enthalpy of combustion of ethanol using simple calorimetry. A spirit burner containing ethanol was weighed, used to heat 100 g of water in a copper calorimeter, and then reweighed.

Mass of water heated = 100 g
Specific heat capacity of water = 4.18 J g-1 K-1
Initial temperature of water = 19.0 degC
Final temperature of water = 40.5 degC
Mass of ethanol burned = 0.68 g
(Mr of ethanol, C2H5OH = 46.0)

The data book (Hess's law cycle) value for the enthalpy of combustion of ethanol is -1367 kJ mol-1.

(a)Calculate the temperature rise, dT, of the water.(1)

(b)Calculate the heat energy transferred to the water, q, in joules.(2)

(c)Calculate the amount, in mol, of ethanol burned.(1)

(d)Calculate the experimental enthalpy of combustion of ethanol, in kJ mol-1, giving your answer to 3 significant figures and including the correct sign.(2)

(e)Calculate the percentage difference between the magnitude of the experimental value and the magnitude of the data book value of -1367 kJ mol-1.(2)

(f)Evaluate why the student's experimental value differs so significantly from the data book value, and suggest how the experimental method could be improved. In your answer, refer to specific sources of error and their effect on the calculated enthalpy change.(6)

(Total for Question 4 is 14 marks)

A student set up an electrochemical cell to measure its EMF, using a zinc half-cell (a zinc electrode in 1.00 mol dm-3 ZnSO4(aq)) connected via a salt bridge to a copper half-cell (a copper electrode in 1.00 mol dm-3 CuSO4(aq)). The two electrodes were connected through a high-resistance voltmeter.

Standard electrode potentials:
Cu2+(aq) + 2e- -> Cu(s) E DEG = +0.34 V
Zn2+(aq) + 2e- -> Zn(s) E DEG = -0.76 V

(a)State the half-equation for the reaction occurring at the zinc electrode and identify whether it is oxidation or reduction.(2)

(b)Calculate the standard EMF of the cell.(2)

(c)Write the conventional cell diagram (cell notation) for this cell, including state symbols and the given concentrations.(2)

(d)State two conditions that must be met for the values of E DEG given above to be valid standard electrode potentials.(2)

(e)Explain the function of the salt bridge in this cell.(2)

(f)When the student measured the EMF experimentally, they obtained a value of +1.05 V rather than the +1.10 V calculated in part (b). Suggest two reasons for this difference.(2)

(Total for Question 5 is 12 marks)

A student prepared 1-bromobutane by refluxing butan-1-ol with a mixture of sodium bromide and concentrated sulfuric acid, which generates hydrogen bromide in situ:

NaBr(s) + H2SO4(aq) -> NaHSO4(aq) + HBr

The hydrogen bromide then reacts with the butan-1-ol:

C4H9OH + HBr -> C4H9Br + H2O

Mass of butan-1-ol used = 7.40 g (Mr = 74.0)
Mass of 1-bromobutane obtained after purification = 9.25 g (Mr of C4H9Br = 137.0)

(a)Explain the purpose of heating the reaction mixture under reflux, rather than simply heating it in an open flask.(2)

(b)After reflux, the crude 1-bromobutane was separated by distillation and then purified further. Describe how the distillate would be purified, and explain the purpose of each step.(4)

(c)Calculate the theoretical yield of 1-bromobutane, in grams.(2)

(d)Calculate the percentage yield of 1-bromobutane obtained.(2)

(e)Suggest one reason, other than losses during purification/transfer, why the percentage yield is less than 100%.(1)

(Total for Question 6 is 11 marks)

A student synthesised a solid organic product, Z, with a literature melting point of 122-124 degC. The crude solid obtained directly from the reaction had a melting point range of 110-119 degC. The student purified the crude product by recrystallisation, after which the melting point was found to be 121-123 degC.

Theoretical yield of Z = 3.05 g
Mass of crude product obtained = 2.85 g
Mass of purified (recrystallised) product obtained = 2.10 g

(a)Describe how the student would purify the crude solid Z by recrystallisation.(4)

(b)Explain why the solvent used for recrystallisation must dissolve Z when hot but only sparingly when cold.(2)

(c)Explain what the melting point data indicate about the purity of the crude product and the purified product.(3)

(d)Calculate the percentage yield of the purified (recrystallised) product.(2)

(e)Suggest one reason why the percentage yield of the purified product is lower than the percentage yield of the crude product (2.85 g).(1)

(Total for Question 7 is 12 marks)

A student investigated the effect of concentration on the rate of the reaction between sodium thiosulfate solution and dilute hydrochloric acid:

Na2S2O3(aq) + 2HCl(aq) -> 2NaCl(aq) + SO2(g) + S(s) + H2O(l)

A conical flask was placed over a piece of paper marked with a cross. Different volumes of 0.150 mol dm-3 sodium thiosulfate solution and distilled water were mixed in the flask, then dilute hydrochloric acid was added and a stopclock started. The time taken, t, for the sulfur precipitate formed to make the cross invisible from above was recorded for each mixture. The total volume of the reaction mixture (thiosulfate + water + acid) was kept constant at 45.0 cm3 in every experiment.

Experiment	Vol. Na2S2O3(aq) / cm3	Vol. H2O / cm3	Vol. HCl(aq) / cm3	t / s
1	10.0	30.0	5.0	150
2	20.0	20.0	5.0	75
3	30.0	10.0	5.0	50
4	40.0	0.0	5.0	37.5

(a)Explain why the total volume of the reaction mixture is kept constant at 45.0 cm3 in every experiment.(2)

(b)Calculate the concentration of sodium thiosulfate, in mol dm-3, in the reaction mixture in Experiment 1.(1)

(c)Complete the table by calculating the relative rate (1/t) for each experiment, and state the shape of the graph you would expect if relative rate were plotted against thiosulfate concentration.(3)

(d)Deduce, with reference to the data, the order of reaction with respect to sodium thiosulfate.(2)

(e)Explain, using collision theory, why repeating the experiments at a higher temperature (with the same concentrations) would decrease the time taken for the cross to disappear in each case.(4)

(f)Suggest one limitation of using the 'disappearing cross' method to measure the rate of this reaction.(1)

(Total for Question 8 is 13 marks)

The acid-catalysed reaction between propanone and iodine was studied:

CH3COCH3(aq) + I2(aq) -[H+ catalyst]-> CH3COCH2I(aq) + HI(aq)

A colorimeter was used to follow the concentration of iodine during the reaction, since iodine is coloured orange-brown while the other species involved are colourless. Four experiments were carried out at the same temperature, each starting with different initial concentrations of propanone, hydrochloric acid (H+) and iodine, and the initial rate was determined from the initial gradient of a concentration-time graph.

Experiment	[CH3COCH3] / mol dm-3	[H+(aq)] / mol dm-3	[I2(aq)] / mol dm-3	Initial rate / mol dm-3 s-1
1	0.500	0.500	0.0100	1.15 x 10^-5
2	1.000	0.500	0.0100	2.30 x 10^-5
3	0.500	1.000	0.0100	2.30 x 10^-5
4	0.500	0.500	0.0200	1.15 x 10^-5

(a)Explain why a colorimeter can be used to follow the rate of this reaction.(2)

(b)In this experiment, a graph of [I2] against time is a straight line (constant negative gradient) rather than a curve. Explain what this observation shows about the order of reaction with respect to iodine.(2)

(c)Using the data in the table, deduce the order of reaction with respect to (i) propanone, (ii) H+(aq), and (iii) I2(aq), explaining your reasoning in each case by comparing pairs of experiments.(6)

(d)Write the overall rate equation for this reaction.(1)

(e)Calculate the rate constant, k, for this reaction using the data from Experiment 1, including its units.(3)

(f)Predict and explain the effect on (i) the initial rate, and (ii) the total time taken for all the iodine to react, if the initial concentration of iodine in Experiment 1 were doubled to 0.0200 mol dm-3 (as in Experiment 4), while [propanone] and [H+] remain as in Experiment 1.(2)

(Total for Question 9 is 16 marks)

A 0.100 mol dm-3 solution of NaOH(aq) was added from a burette to 25.0 cm3 of a solution of ethanoic acid, CH3COOH(aq), of unknown concentration, with the pH recorded continuously using a pH meter. Selected data points from the titration curve are shown below.

Volume of NaOH added / cm3	pH
0.0	2.87
12.5	4.76
24.0	6.24
24.9	7.50
25.0 (equivalence point)	8.72
25.1	9.90
26.0	11.40
30.0	12.10

Indicator	pH range over which colour change occurs
Methyl orange	3.1 - 4.4
Phenolphthalein	8.2 - 10.0

(a)Calculate the concentration, in mol dm-3, of the original ethanoic acid solution.(3)

(b)Using the pH data given, identify a suitable indicator for this titration and explain your choice.(2)

(c)State the pH at the half-equivalence point (12.5 cm3) and use this value to calculate the acid dissociation constant, Ka, for ethanoic acid.(3)

(d)Explain, in terms of the relative concentrations of CH3COOH and CH3COO- present, why the pH at the half-equivalence point is equal to pKa.(2)

(e)Explain why the initial pH of the ethanoic acid solution (2.87) is higher than the pH of a strong monoprotic acid of the same concentration (0.100 mol dm-3) would be.(2)

(Total for Question 10 is 12 marks)

A student carried out an experiment to determine the enthalpy change of combustion of a fuel, using the following apparatus and measurements:

- A digital thermometer, reading to the nearest 0.1 degC, was used to measure the temperature rise of the water, dT = 18.5 degC (found from an initial and a final reading).
- A two-decimal-place balance (resolution 0.01 g) was used to find the mass of fuel burned, by weighing the burner before and after burning, giving a mass burned of 0.83 g.
- A measuring cylinder, with a resolution uncertainty of ±0.5 cm3, was used to measure out 100 cm3 of water.

(a)Calculate the percentage uncertainty in the temperature rise, dT. (The uncertainty in a single thermometer reading is half the smallest scale division.)(2)

(b)Calculate the percentage uncertainty in the mass of fuel burned.(2)

(c)Calculate the percentage uncertainty in the volume of water measured.(1)

(d)State which single measurement contributes the greatest percentage uncertainty to the final result, and suggest a change to the method or apparatus that would reduce this uncertainty.(2)

(e)Calculate the total percentage uncertainty in the calculated enthalpy change, by summing the percentage uncertainties from parts (a) to (c). Hence calculate the absolute uncertainty, in kJ mol-1, in an experimental enthalpy value of -650 kJ mol-1.(3)

(Total for Question 11 is 10 marks)

A group of students carried out several of the required practicals covered in this pack (titration, calorimetry, and rates). Their teacher asked them to review their methods and identify the type of error involved in each scenario below, and to propose an overall strategy for improving the reliability and accuracy of practical measurements.

Scenario 1: In the titration (Question 3), the burette used had not been rinsed with the hydrochloric acid before use, and had been rinsed with distilled water only, leaving a thin film of water inside.

Scenario 2: In the enthalpy experiment (Question 4), the same heat loss occurred in every repeat run, because the same unshielded apparatus was used each time.

Scenario 3: In the rates experiment (Question 8), the student's judgement of exactly when the cross became invisible varied slightly from one repeat to the next.

(a)For each of Scenarios 1-3, state whether the error described is a systematic error or a random error, giving a reason for your answer in each case.(6)

(b)Explain why repeating an experiment several times and taking a mean reduces the effect of random errors but does not reduce the effect of systematic errors.(3)

(c)Evaluate the overall reliability of the practical techniques used across the titration, calorimetry and rates experiments in this pack, and recommend an overall strategy the students could adopt to improve the accuracy and reliability of their results across all three types of experiment. In your answer, refer to specific improvements for each type of error you identified in part (a).(6)

(Total for Question 12 is 15 marks)

Mark scheme · ARP-Chem A-Level Chemistry: Required Practicals and Practical Skills

Question 1

(a) B1 identifies bromide ion, Br-(aq) (cream precipitate, dissolves only in concentrated ammonia - distinguishes from chloride and iodide)
(a) B1 correct ionic equation Ag+(aq) + Br-(aq) -> AgBr(s)
(a) B1 fully balanced with correct state symbols (cao)
(a) Answer: Bromide ion, Br-(aq); Ag+(aq) + Br-(aq) -> AgBr(s)
(b) B1 removes/reacts with any carbonate (or hydroxide/sulfite) ions present as impurities
(b) B1 which would otherwise also form a precipitate with Ag+ (e.g. Ag2CO3), giving a false positive result
(b) Answer: To remove carbonate (or similar) ions that would otherwise also precipitate with silver ions and give a misleading (false positive) result.
(c) B1 no sulfate (SO4²-) ions are present in X
(c) B1 the dilute HCl prevents carbonate/sulfite ions from also giving a precipitate with Ba2+, so the test is specific for sulfate
(c) Answer: No sulfate ions present; acidifying first removes carbonate/sulfite so only sulfate would give a precipitate.
(d) B1 ammonia, NH3
(d) B1 correct ionic equation NH4+(aq) + OH-(aq) -> NH3(g) + H2O(l)
(d) B1 ammonia gas dissolves in the water present on the damp litmus paper
(d) B1 forming an alkaline solution (ammonium hydroxide/OH-(aq)), which turns red litmus blue
(d) Answer: Ammonia, NH3; NH4+(aq) + OH-(aq) -> NH3(g) + H2O(l); ammonia dissolves in the damp litmus paper forming an alkaline solution, turning it blue.
(e) B1 ammonium bromide
(e) B1 correct formula NH4Br (cao)
(e) Answer: Ammonium bromide, NH4Br

Question 2

(a) B1 a carbonyl group, C=O, is present
(a) B1 cannot distinguish between an aldehyde and a ketone (2,4-DNPH/Brady's reagent gives a positive result with both)
(a) Answer: Carbonyl group (C=O); does not distinguish aldehyde from ketone.
(b) B1 Y is an aldehyde (only aldehydes are oxidised by Tollens' reagent to give a silver mirror; ketones give no reaction)
(b) M1 attempt at a balanced equation showing RCHO oxidised and Ag+ reduced to Ag
(b) A1 RCHO + 2[Ag(NH3)2]+ + 3OH- -> RCOO- + 2Ag + 4NH3 + 2H2O (oe; allow RCOOH as product with adjusted balancing)
(b) Answer: Aldehyde; RCHO + 2[Ag(NH3)2]+ + 3OH- -> RCOO- + 2Ag + 4NH3 + 2H2O
(c) B1 orange (dichromate(VI), Cr2O7²-) changes to green (Cr3+)
(c) B1 chromium is reduced from oxidation state +6 (in Cr2O7²-) to +3 (in Cr3+)
(c) B1 as it oxidises the aldehyde, RCHO, to a carboxylic acid, RCOOH
(c) Answer: Orange to green; Cr is reduced from +6 to +3 as it oxidises RCHO to RCOOH.
(d) B1 bromine water is not decolourised, so Y contains no C=C double bond (no alkene present)
(d) B1 Y is a saturated compound (oe)
(d) Answer: No C=C bond present; Y is saturated.
(e) B1 Y is a saturated aliphatic aldehyde
(e) B1 C3H6O (propanal, CH3CH2CHO) or any valid saturated aldehyde formula CnH2nO
(e) Answer: Saturated aliphatic aldehyde; C3H6O (propanal).
(f) B1 Fehling's solution is blue before the test
(f) B1 on warming with an aldehyde, a brick-red (orange-red) precipitate of copper(I) oxide, Cu2O, forms; no change occurs with a ketone
(f) Answer: Blue Fehling's solution turns to a brick-red precipitate (Cu2O) on warming with an aldehyde.

Question 3

(a) B1 weigh the solid accurately, e.g. by difference, using a balance reading to at least 2 decimal places
(a) B1 dissolve the solid completely in a small volume of distilled water in a beaker
(a) B1 transfer the solution quantitatively to a 250 cm3 volumetric flask, rinsing the beaker and glass rod with distilled water and adding the washings to the flask
(a) B1 make up to the 250 cm3 graduation mark with distilled water, adding the final few drops with a dropping pipette until the bottom of the meniscus sits on the mark at eye level, then stopper and invert to mix thoroughly
(a) Answer: Weigh accurately, dissolve, quantitatively transfer with washings to a volumetric flask, make up to the mark reading the meniscus at eye level, then mix.
(b) M1 moles Na2CO3 = 1.325 / 106.0 (= 0.01250 mol)
(b) A1 concentration = 0.01250 / 0.250 = 0.0500 mol dm-3 (cao)
(b) Answer: 0.0500 mol dm-3
(c) B1 titres 1, 2 and 3 (24.65, 24.60, 24.70) are all concordant, within 0.10 cm3 of one another; the rough titre is excluded
(c) A1 mean = (24.65 + 24.60 + 24.70) / 3 = 24.65 cm3 (cao)
(c) Answer: 24.65 cm3
(d) M1 moles Na2CO3 in 25.0 cm3 = 0.0500 x (25.0/1000) = 0.001250 mol (ecf from (b))
(d) M1 moles HCl = 2 x 0.001250 = 0.002500 mol (1:2 ratio from the equation)
(d) A1 concentration HCl = 0.002500 / 0.02465 = 0.1014 mol dm-3 (awrt 0.101, ecf from mean titre in (c))
(d) Answer: 0.1014 mol dm-3 (3 s.f.)
(e) M1 total uncertainty in titre = 2 x 0.05 = ±0.10 cm3 (two readings, initial and final)
(e) A1 % uncertainty = (0.10/24.65) x 100 = 0.41% (awrt 0.4%, ecf from mean titre in (c))
(e) B1 e.g. use a greater volume of Na2CO3 solution (scaling up the HCl accordingly) so the same absolute uncertainty is a smaller percentage of a larger titre; or repeat to obtain more concordant results
(e) Answer: 0.41% (2 s.f.); e.g. use a larger titre volume to reduce the percentage uncertainty.

Question 4

(a) B1 dT = 40.5 - 19.0 = 21.5 degC (cao)
(a) Answer: 21.5 degC
(b) M1 q = m x c x dT = 100 x 4.18 x 21.5
(b) A1 q = 8987 J (awrt 8990 J, cao, ecf from (a))
(b) Answer: 8987 J (8.987 kJ)
(c) B1 0.68 / 46.0 = 0.0148 mol (awrt 0.0148)
(c) Answer: 0.0148 mol
(d) M1 dH = -q / moles = -8.987 / 0.014783 (ecf from (b) and (c))
(d) A1 dH = -608 kJ mol-1 (awrt -608, cao, negative sign required)
(d) Answer: -608 kJ mol-1
(e) M1 difference = 1367 - 608 = 759 (using magnitudes, ecf from (d))
(e) A1 % difference = (759/1367) x 100 = 55.5% (awrt 55-56%)
(e) Answer: 55.5%
(f) L3 (5-6): A detailed, logically structured evaluation identifying multiple relevant sources of error, correctly explaining the direction of their effect on the calculated (less negative/less exothermic) enthalpy value, and proposing at least two specific, workable improvements linked to the errors identified.
(f) L2 (3-4): Several relevant sources of error identified with some explanation of their effect, and at least one relevant improvement suggested, but the evaluation lacks full detail or logical structure.
(f) L1 (1-2): One or two sources of error or improvements identified, with limited or no explanation of their effect on the result.
(f) L0 (0): No relevant content.

Question 5

(a) B1 Zn(s) -> Zn2+(aq) + 2e-
(a) B1 oxidation (loss of electrons)
(a) Answer: Zn(s) -> Zn2+(aq) + 2e-; oxidation.
(b) M1 E(cell) = E(more positive/reduced electrode) - E(more negative/oxidised electrode) = 0.34 - (-0.76)
(b) A1 +1.10 V (cao)
(b) Answer: +1.10 V
(c) B1 Zn(s) | Zn2+(aq, 1.00 mol dm-3) || Cu2+(aq, 1.00 mol dm-3) | Cu(s)
(c) B1 correct convention: single line = phase boundary, double line = salt bridge, more negative electrode (Zn) on the left (oe correct equivalent format)
(c) Answer: Zn(s) | Zn2+(aq, 1.00 mol dm-3) || Cu2+(aq, 1.00 mol dm-3) | Cu(s)
(d) B1 temperature of 298 K (25 degC)
(d) B1 ion concentration(s) of 1.00 mol dm-3 (and, for any gases, a pressure of 100 kPa); accept: platinum electrode used where no solid metal is present in the half-cell
(d) Answer: 298 K; 1.00 mol dm-3 ion concentrations (and 100 kPa for any gases).
(e) B1 allows ions to flow between the two half-cells, completing the circuit
(e) B1 maintains electrical neutrality in each half-cell as electrons flow through the external circuit, without allowing the two solutions to mix directly
(e) Answer: Completes the circuit by allowing ion flow while maintaining charge neutrality and keeping the solutions from mixing.
(f) B1 the concentrations of ZnSO4/CuSO4 used were not exactly 1.00 mol dm-3, or the temperature was not exactly 298 K
(f) B1 resistance in the wires/salt bridge/connections causes a small voltage drop (the voltmeter is not perfectly infinite resistance); accept: oxide layer/impurities on the electrode surfaces affecting the electrode potential
(f) Answer: Non-standard concentrations/temperature; resistance in the circuit/salt bridge causing a voltage drop.

Question 6

(a) B1 reflux allows the mixture to be heated strongly/continuously (to increase the rate of reaction) for an extended period
(a) B1 the condenser continuously condenses and returns any volatile reactants/products (e.g. HBr, butan-1-ol) to the flask, preventing their loss by evaporation
(a) Answer: Reflux allows prolonged strong heating while the condenser returns volatile reactants/products, preventing losses.
(b) B1 shake the distillate with sodium hydrogencarbonate solution in a separating funnel (releasing the tap to allow CO2 gas to escape), to neutralise/remove any remaining acid (H2SO4/HBr)
(b) B1 run off and discard the aqueous layer, keeping the organic layer of 1-bromobutane
(b) B1 wash the organic layer with distilled water to remove remaining traces of inorganic/aqueous impurities
(b) B1 dry the organic layer using an anhydrous drying agent (e.g. anhydrous calcium chloride or magnesium sulfate), then redistil, collecting the fraction boiling at the known boiling point of 1-bromobutane (approx. 101 degC), to remove impurities with a different boiling point
(b) Answer: Wash with NaHCO3(aq) to neutralise acid, then with water; dry with an anhydrous drying agent; redistil, collecting the correct boiling-point fraction.
(c) M1 moles butan-1-ol = 7.40/74.0 = 0.100 mol (1:1 ratio, so moles product = 0.100 mol)
(c) A1 theoretical mass = 0.100 x 137.0 = 13.70 g (cao)
(c) Answer: 13.70 g
(d) M1 % yield = (9.25/13.70) x 100 (ecf from (c))
(d) A1 = 67.5% (awrt 67-68%)
(d) Answer: 67.5%
(e) B1 a competing elimination reaction can occur, forming but-1-ene as a side product (oe: the reaction is an equilibrium/does not go to completion, so not all the alcohol is converted)
(e) Answer: Elimination side reaction forms but-1-ene (oe: reaction does not go to completion).

Question 7

(a) B1 dissolve the crude solid in a minimum volume of a hot, appropriate solvent (one in which Z is soluble when hot but only sparingly soluble when cold)
(a) B1 filter the hot solution (using a fluted/preheated filter funnel) to remove any insoluble impurities
(a) B1 allow the filtrate to cool slowly (e.g. to room temperature, then in ice) so crystals of pure Z form, leaving soluble impurities in solution
(a) B1 filter off the crystals under reduced pressure (Buchner funnel/flask), wash with a small volume of cold solvent, and allow to dry (e.g. in a desiccator or a warm oven below the melting point)
(a) Answer: Dissolve in minimum hot solvent, hot filter, cool slowly to crystallise, filter under reduced pressure, wash and dry.
(b) B1 so a saturated (or near-saturated) solution can be made using a minimum volume of hot solvent, maximising the amount of Z that crystallises out (and hence the yield) on cooling
(b) B1 impurities, present in small amounts, remain dissolved in the small volume of cold solvent even as Z crystallises out, so they are removed by filtration, purifying Z
(b) Answer: Maximises recovery on cooling while impurities stay dissolved and are removed by filtration.
(c) B1 the crude product has a wide melting range (110-119 degC), well below the literature value, showing it is impure - impurities disrupt the regular crystal lattice, lowering the melting point and broadening the melting range
(c) B1 the purified product has a much narrower melting range (121-123 degC), very close to the literature range (122-124 degC)
(c) B1 showing that the purified product is of much higher purity than the crude product
(c) Answer: Crude product is impure (wide, depressed range); purified product is close to the literature value and much narrower, so highly pure.
(d) M1 % yield = (2.10/3.05) x 100
(d) A1 = 68.9% (awrt 68-69%)
(d) Answer: 68.9%
(e) B1 some product remains dissolved in the cold solvent (the mother liquor) and/or is lost during transfers/filtration during recrystallisation, reducing the mass recovered
(e) Answer: Product lost in the mother liquor / during transfers and filtration.

Question 8

(a) B1 so that [thiosulfate] is the only variable being changed between experiments (the depth/path length of liquid the cross is viewed through stays the same)
(a) B1 this makes it a fair test/valid comparison, so any change in rate can be attributed only to the change in thiosulfate concentration
(a) Answer: Keeps all other variables (depth of solution, viewing path) constant so it is a fair test of concentration alone.
(b) B1 0.150 x (10.0/45.0) = 0.0333 mol dm-3 (awrt 0.033)
(b) Answer: 0.0333 mol dm-3
(c) B1 1/t values (s-1): Exp 1 = 0.00667, Exp 2 = 0.01333, Exp 3 = 0.0200, Exp 4 = 0.0267 (accept awrt to 3 s.f.)
(c) B1 a graph of 1/t (relative rate) against [Na2S2O3] would be a straight line through the origin
(c) B1 confirming that rate is directly proportional to [Na2S2O3]
(c) Answer: 1/t = 0.00667, 0.01333, 0.0200, 0.0267 s-1; straight line through the origin.
(d) B1 order = 1 (first order)
(d) B1 e.g. doubling [Na2S2O3] from Experiment 1 to Experiment 2 doubles the relative rate (1/t doubles from 0.00667 to 0.01333); the same direct proportionality holds across all four experiments
(d) Answer: First order with respect to thiosulfate.
(e) B1 at a higher temperature, particles have more kinetic energy, so move faster
(e) B1 this increases the frequency of collisions between reacting particles
(e) B1 more importantly, a greater proportion of particles have energy greater than or equal to the activation energy (Maxwell-Boltzmann distribution shifts/broadens)
(e) B1 so a greater proportion of collisions are successful/effective, increasing the rate of reaction and decreasing the time taken
(e) Answer: Higher temperature increases collision frequency and, more significantly, the proportion of particles with energy ≥ activation energy, increasing the rate.
(f) B1 judging the exact point at which the cross is no longer visible is subjective (depends on the observer's eyesight/judgement), introducing random error/reducing precision (accept: reaction time in starting/stopping the stopclock adds error)
(f) Answer: The endpoint judgement is subjective, introducing random error.

Question 9

(a) B1 iodine is the only coloured species in the reaction mixture (the other reagents and products are colourless)
(a) B1 so the absorbance measured is directly proportional to the concentration of iodine remaining, allowing [I2] to be monitored continuously over time
(a) Answer: Iodine is the only coloured species, so absorbance tracks [I2] directly.
(b) B1 the reaction is zero order with respect to I2 (rate is independent of [I2])
(b) B1 the rate (gradient of the graph) stays constant even as [I2] decreases, whereas a reaction with a positive order in I2 would give a curved (e.g. exponential) concentration-time graph
(b) Answer: Zero order with respect to iodine, since the rate (gradient) is constant regardless of [I2].
(c) B1 (i) order with respect to propanone = 1 (first order)
(c) B1 (i) comparing Exp 1 and Exp 2: doubling [propanone] (0.500 to 1.000), with [H+] and [I2] unchanged, doubles the initial rate (1.15 to 2.30 x 10^-5) - rate is directly proportional to [propanone]
(c) B1 (ii) order with respect to H+ = 1 (first order)
(c) B1 (ii) comparing Exp 1 and Exp 3: doubling [H+] (0.500 to 1.000), with [propanone] and [I2] unchanged, doubles the initial rate - rate is directly proportional to [H+]
(c) B1 (iii) order with respect to I2 = 0 (zero order)
(c) B1 (iii) comparing Exp 1 and Exp 4: doubling [I2] (0.0100 to 0.0200), with [propanone] and [H+] unchanged, has no effect on the initial rate (both 1.15 x 10^-5) - rate is independent of [I2]
(c) Answer: Order 1 with respect to propanone; order 1 with respect to H+; order 0 with respect to I2.
(d) B1 rate = k[CH3COCH3][H+] (I2 does not appear, as it is zero order; cao, ecf from (c))
(d) Answer: rate = k[CH3COCH3][H+]
(e) M1 k = rate / ([propanone][H+]) = 1.15 x 10^-5 / (0.500 x 0.500)
(e) A1 k = 4.60 x 10^-5 (awrt)
(e) B1 units: dm3 mol-1 s-1 (since rate = mol dm-3 s-1 and [propanone][H+] = mol2 dm-6, so k = mol dm-3 s-1 / mol2 dm-6 = dm3 mol-1 s-1)
(e) Answer: k = 4.60 x 10^-5 dm3 mol-1 s-1
(f) B1 (i) no effect on the initial rate (remains 1.15 x 10^-5 mol dm-3 s-1) because the reaction is zero order with respect to I2, so rate does not depend on [I2]
(f) B1 (ii) the total time taken would double, because there is twice the amount of I2 to be used up but it is consumed at the same constant rate
(f) Answer: Initial rate unchanged; time taken for all iodine to react would double.

Question 10

(a) M1 moles NaOH at equivalence = 0.100 x (25.0/1000) = 0.00250 mol
(a) M1 moles CH3COOH = moles NaOH = 0.00250 mol (1:1 stoichiometry, CH3COOH + NaOH -> CH3COONa + H2O)
(a) A1 concentration = 0.00250/0.0250 = 0.100 mol dm-3 (cao)
(a) Answer: 0.100 mol dm-3
(b) B1 phenolphthalein
(b) B1 its colour-change range (pH 8.2-10.0) lies within the steep part of the titration curve around the equivalence point (pH rises sharply from about 7.50 to 11.40 between 24.9 and 26.0 cm3), close to the equivalence point pH of 8.72; methyl orange (pH 3.1-4.4) would change colour far too early and is unsuitable
(b) Answer: Phenolphthalein, since its colour-change range coincides with the steep part of the curve near the equivalence point (pH 8.72).
(c) B1 pH = 4.76
(c) M1 [H+] = 10^-4.76
(c) A1 Ka = 1.74 x 10^-5 mol dm-3 (awrt 1.7-1.8 x 10^-5)
(c) Answer: Ka = 1.74 x 10^-5 mol dm-3
(d) B1 at the half-equivalence point, exactly half of the original CH3COOH has been converted to its conjugate base CH3COO- (as sodium ethanoate), so [CH3COOH] = [CH3COO-]
(d) B1 substituting into Ka = [H+][CH3COO-]/[CH3COOH]: since [CH3COO-] = [CH3COOH], these cancel, giving Ka = [H+], so pKa = -log(Ka) = -log[H+] = pH
(d) Answer: At half-equivalence [CH3COOH] = [CH3COO-], so Ka = [H+] and hence pKa = pH.
(e) B1 ethanoic acid is a weak acid and only partially dissociates in water (CH3COOH ≤> CH3COO- + H+), an equilibrium that lies well to the left
(e) B1 so [H+] produced is much lower than the initial acid concentration, unlike a strong acid which fully dissociates and would give [H+] = 0.100 mol dm-3 (pH = 1.00), resulting in a higher pH for the weak acid
(e) Answer: Ethanoic acid only partially dissociates, giving a much lower [H+] than a fully dissociated strong acid of the same concentration.

Question 11

(a) M1 uncertainty per reading = ±0.05 degC; since dT is found from two readings (initial and final), total uncertainty = 2 x 0.05 = ±0.1 degC
(a) A1 % uncertainty = (0.1/18.5) x 100 = 0.54% (awrt 0.5%)
(a) Answer: 0.54%
(b) M1 uncertainty per reading = ±0.01 g; mass burned is found by difference (two readings), so total uncertainty = 2 x 0.01 = ±0.02 g
(b) A1 % uncertainty = (0.02/0.83) x 100 = 2.41% (awrt 2.4%)
(b) Answer: 2.41%
(c) B1 (0.5/100) x 100 = 0.5%
(c) Answer: 0.5%
(d) B1 the mass of fuel burned (2.41%) contributes the greatest percentage uncertainty (ecf from (a)-(c))
(d) B1 e.g. use a balance that reads to 3 or more decimal places (0.001 g resolution), or burn a greater mass of fuel (achieve a larger temperature rise) so the fixed absolute uncertainty becomes a smaller percentage of a larger mass
(d) Answer: Mass of fuel burned; use a more precise balance or burn a larger mass of fuel.
(e) M1 total % uncertainty = 0.54 + 2.41 + 0.5 = 3.45% (awrt 3.4-3.5%, ecf from (a)-(c))
(e) M1 absolute uncertainty = 3.45% of 650 = (3.45/100) x 650
(e) A1 = 22 kJ mol-1 (awrt 22-22.5), so the enthalpy value can be quoted as -650 ± 22 kJ mol-1
(e) Answer: ±22 kJ mol-1 (total % uncertainty = 3.45%)

Question 12

(a) B1 Scenario 1: systematic error
(a) B1 because the residual water dilutes the HCl in the same way every time the burette is used, shifting the recorded titre in the same direction on every measurement (not randomly)
(a) B1 Scenario 2: systematic error
(a) B1 because the same heat loss occurs consistently in every repeat (same apparatus/method each time), shifting all results in the same direction by a similar amount, rather than scattering them
(a) B1 Scenario 3: random error
(a) B1 because the student's judgement varies unpredictably from one repeat to another (sometimes stopping slightly early, sometimes slightly late), scattering results both above and below the true value
(a) Answer: Scenario 1: systematic; Scenario 2: systematic; Scenario 3: random.
(b) B1 random errors cause results to be scattered unpredictably both above and below the true value, so taking a mean of several repeats allows these random variations to (largely) average/cancel out
(b) B1 systematic errors consistently shift every measurement in the same direction by a similar amount
(b) B1 so averaging repeated results does not reduce a systematic error - the mean itself remains shifted away from the true value by (approximately) the same amount
(b) Answer: Random errors average out over repeats; systematic errors shift every reading the same way, so the mean is still offset.
(c) L3 (5-6): A well-structured evaluation that clearly distinguishes between improving precision (reducing random error, e.g. by repeating readings/using more precise apparatus and averaging concordant results) and improving accuracy (reducing/eliminating systematic error, e.g. by correct rinsing, calibration, or a more efficient/insulated setup), with specific, correctly-linked recommendations for at least two of the three experiment types.
(c) L2 (3-4): Relevant strategies to reduce random and/or systematic error are identified, with some correct links to specific experiments, but the distinction between the two error types or the range of experiments covered is incomplete.
(c) L1 (1-2): A general improvement (e.g. 'repeat the experiment' or 'use better apparatus') is identified, with little or no reference to the specific type of error or to more than one experiment.
(c) L0 (0): No relevant content.

A-Level Chemistry: Required Practicals and Practical Skills - Worksheets, Questions and Revision