CAIE A Level 9701

Chemistry 2020 · Oct/Nov

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20 Questions Q&A Available
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This CAIE A Level Chemistry paper focuses mainly on practical chemistry, quantitative analysis, reaction kinetics, and data handling. Students can expect structured questions that test titration calculations, preparation of standard solutions, concentration and moles, experimental design, graph plotting, gradient and activation energy calculations, and evaluation of sources of error and anomalous results.…

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Questions & Model Answers 20 Qs

Q1a(ii)
Short Answer 1 mark
Suggest why the student leaves the solution for 24 hours in step 2.
To allow the calcium hydroxide to reach saturation / to ensure maximum dissolution and equilibrium is established.
Leaving the solution for 24 hours allows excess solid to dissolve until no more can dissolve.
Q1b(i)
Short Answer 1 mark
The student finds that the mean titre is 10.65 cm3. The equation for the reaction is shown. Ca(OH)2(aq) + 2HCl(aq) → CaCl2(aq) + 2H2O(l). Calculate the number of moles of calcium hydroxide that reacted.
moles of Ca(OH)2 = 5.325 × 10^-4 mol
Using the 2:1 ratio of HCl to Ca(OH)2 and the given titre gives 5.325 × 10^-4 mol of calcium hydroxide.
Q1b(ii)
Short Answer 2 marks
Calculate the concentration of the aqueous calcium hydroxide in g dm-3. [Ar: Ca, 40.1; O, 16.0; H, 1.0]
concentration of Ca(OH)2(aq) = 0.394 g dm^-3
Molar mass of Ca(OH)2 = 74.1 g mol^-1; concentration in mol dm^-3 is converted to g dm^-3.
Q1b(iii)
Short Answer 2 marks
In a chemistry data book, the student found the value for the concentration of a saturated solution of calcium hydroxide at 293 K to be higher than the value calculated in (b)(ii). Give two reasons why the data book value is higher than the value calculated in (b)(ii). (Assume the student recorded all volumes correctly, carried out the titration accurately and recorded concordant titres.)
The solution may not have been saturated; some calcium hydroxide may not have dissolved fully.
If the solution was not fully saturated, the concentration would be lower than the data book value.
The temperature may have been higher than 293 K, so more calcium hydroxide dissolved than at 293 K.
Solubility of calcium hydroxide changes with temperature, affecting the concentration.
Q1c
Short Answer 1 mark
A second student carried out the experiment but did not filter their solution correctly in step 3, leaving some undissolved calcium hydroxide suspended in the solution. Predict and explain how this would affect the titre obtained in their experiment.
The titre would be lower, because suspended solid would continue to dissolve during titration, reducing the amount of acid needed to reach the endpoint.
Extra dissolved Ca(OH)2 in the flask means less HCl is required.
Q1d
Structured 2 marks
Describe how to prepare exactly 250.0 cm3 of 0.100 mol dm-3 hydrochloric acid from 1.00 mol dm-3 hydrochloric acid. Include the names and capacities of any suitable apparatus used.
Use a 25.0 cm3 volumetric pipette to measure 25.0 cm3 of 1.00 mol dm^-3 HCl, transfer it to a 250.0 cm3 volumetric flask, make up to the mark with distilled water, stopper and invert to mix.
A tenfold dilution of 25.0 cm3 to 250.0 cm3 gives 0.100 mol dm^-3.
Q2a
Short Answer 1 mark
State one precaution that should be taken to reduce a hazard caused by one of the products of this reaction.
Carry out the reaction in a fume cupboard / wear a face mask / avoid inhaling the sulfur dioxide produced.
Sulfur dioxide is toxic and corrosive by inhalation.
Q2b
Short Answer 1 mark
Suggest appropriate equipment for measuring the required volumes of Na2S2O3(aq) and HCl(aq) stated in the method.
Na2S2O3(aq): 10.00 cm3 volumetric pipette; HCl(aq): 10 cm3 measuring cylinder or pipette.
The thiosulfate volume is given to 10.00 cm3, so a volumetric pipette is suitable; the acid volume is 10 cm3.
Q2c
Short Answer 1 mark
Complete column E in the table, giving your values to three significant figures.
Column E values: -2.00, -2.60, -3.05, -3.70, -3.90, -4.25
These are the values of 2.303 log(1/t) to three significant figures for the given times.
Q2d(i)
Structured 2 marks
Plot a graph of 2.303 log(1/t) on the y-axis and 1/T on the x-axis. Draw a line of best fit.
A straight line of best fit should be drawn through the plotted points on the graph of 2.303 log(1/t) against 1/T.
The relationship is linear according to the given equation.
Q2d(ii)
Short Answer 2 marks
Circle the point on the graph you consider to be most anomalous. Suggest two possible errors in how the procedure was carried out which could have caused this anomalous point.
Most anomalous point: the 41 °C / 314 K point (t = 21.12 s, 2.303 log(1/t) = -3.05).
This point is furthest from the line of best fit.
Possible error 1: the solutions were not at the correct temperature when mixed.
Incorrect temperature would change the rate and give an outlier.
Possible error 2: the timing was started or stopped incorrectly / the cross was judged inconsistently.
Human error in timing or endpoint judgement can produce an anomalous result.
Q2d(iii)
Structured 2 marks
Use the graph to determine the gradient of the line of best fit. State the coordinates of both points you used in your calculation. These must be selected from your line of best fit. Give the gradient to three significant figures.
Example coordinates: (0.00300, -2.00) and (0.00340, -4.25); gradient = -5.63 × 10^3 K
Any two points on the best-fit line giving a gradient close to this value are acceptable.
Q2d(iv)
Structured 3 marks
Use the gradient you have calculated and the equation to calculate Ea, in kJ mol-1, for the reaction. Give your answer to two significant figures. Include a sign.
Ea = +47 kJ mol^-1
From the slope of the graph and the equation, Ea is positive and about 47 kJ mol^-1.
Q2e
Short Answer 1 mark
A student repeats the experiment at 60 °C but with a different concentration of Na2S2O3(aq). The new solution is named solution B. The time taken until X can no longer be seen is 11.52 s. Assume no error was made in the timing. State whether the concentration of solution B is more or less than 0.10 mol dm-3. Explain your answer.
More than 0.10 mol dm^-3, because the cross disappeared in a shorter time, showing a faster reaction rate.
At the same temperature, a shorter time means a higher rate and therefore a higher thiosulfate concentration.
Q2f
Short Answer 1 mark
It is more appropriate for the values in column C of the table on page 6 to be recorded to the nearest second. Suggest one reason for this.
The timing method is not precise enough to justify recording to hundredths of a second / the endpoint is judged by eye, so the uncertainty is about 1 s.
The experimental uncertainty is larger than 0.01 s, so nearest second is more realistic.
Q3a(i)
Structured 2 marks
Complete the table so the student can use it to record the reaction time for each experiment. The table should include suggested volumes of acid and water for each experiment. Columns should be suitably labelled.
A suitable table would list experiment number, volume of 2.00 mol dm^-3 HCl, volume of water, and reaction time. Example volumes: 50, 40, 30, 20, 10, 0 cm3 acid with corresponding water volumes 0, 10, 20, 30, 40, 50 cm3.
These give a range of concentrations while keeping total volume constant at 50 cm3.
Q3a(ii)
Short Answer 1 mark
Identify the dependent variable in this experiment.
The time taken for the magnesium ribbon to disappear.
This is the measured outcome that depends on acid concentration.
Q3a(iii)
Short Answer 1 mark
Apart from the magnesium ribbon, identify one variable that should be controlled in these experiments.
Temperature of the reaction mixture / volume of acid used / size or surface area of magnesium ribbon.
A controlled variable is needed so only acid concentration affects the rate.
Q3b
Short Answer 1 mark
Explain why the student's proposed method of determining the reaction time may lead to inaccuracies in the recorded results.
The endpoint is judged by eye, so the exact moment the magnesium disappears is subjective and reaction time may be recorded inconsistently.
Visual judgement introduces human error.
Q3c
Short Answer 1 mark
Suggest one other variable that could be measured to determine the order of the reaction.
Volume of hydrogen gas produced / mass loss of the reaction mixture / change in concentration with time.
Any measurable quantity that changes with reaction progress can be used to determine rate and order.
Paper Details
Board CAIE
Level A Level
Subject Chemistry
Code 9701
Year 2020
Session Oct/Nov
Q&A 20 questions
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