CAIE Chemistry (9701)

2020 Oct/Nov
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Questions & Answers

Q1a(ii).
Short Answer 1 mark

Suggest why the student leaves the solution for 24 hours in step 2.

Model Answer:
To allow the calcium hydroxide to reach saturation / to ensure maximum dissolution and equilibrium is established.
Explanation: 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.

Model Answer:
moles of Ca(OH)2 = 5.325 × 10^-4 mol
Explanation: 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]

Model Answer:
concentration of Ca(OH)2(aq) = 0.394 g dm^-3
Explanation: 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.)

Model Answer:
The solution may not have been saturated; some calcium hydroxide may not have dissolved fully.
Explanation: If the solution was not fully saturated, the concentration would be lower than the data book value.
Model Answer:
The temperature may have been higher than 293 K, so more calcium hydroxide dissolved than at 293 K.
Explanation: 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.

Model Answer:
The titre would be lower, because suspended solid would continue to dissolve during titration, reducing the amount of acid needed to reach the endpoint.
Explanation: 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.

Model Answer:
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.
Explanation: 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.

Model Answer:
Carry out the reaction in a fume cupboard / wear a face mask / avoid inhaling the sulfur dioxide produced.
Explanation: 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.

Model Answer:
Na2S2O3(aq): 10.00 cm3 volumetric pipette; HCl(aq): 10 cm3 measuring cylinder or pipette.
Explanation: 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.

Model Answer:
Column E values: -2.00, -2.60, -3.05, -3.70, -3.90, -4.25
Explanation: 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.

Model Answer:
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.
Explanation: 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.

Model Answer:
Most anomalous point: the 41 °C / 314 K point (t = 21.12 s, 2.303 log(1/t) = -3.05).
Explanation: This point is furthest from the line of best fit.
Model Answer:
Possible error 1: the solutions were not at the correct temperature when mixed.
Explanation: Incorrect temperature would change the rate and give an outlier.
Model Answer:
Possible error 2: the timing was started or stopped incorrectly / the cross was judged inconsistently.
Explanation: 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.

Model Answer:
Example coordinates: (0.00300, -2.00) and (0.00340, -4.25); gradient = -5.63 × 10^3 K
Explanation: 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.

Model Answer:
Ea = +47 kJ mol^-1
Explanation: 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.

Model Answer:
More than 0.10 mol dm^-3, because the cross disappeared in a shorter time, showing a faster reaction rate.
Explanation: 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.

Model Answer:
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.
Explanation: 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.

Model Answer:
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.
Explanation: 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.

Model Answer:
The time taken for the magnesium ribbon to disappear.
Explanation: 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.

Model Answer:
Temperature of the reaction mixture / volume of acid used / size or surface area of magnesium ribbon.
Explanation: 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.

Model Answer:
The endpoint is judged by eye, so the exact moment the magnesium disappears is subjective and reaction time may be recorded inconsistently.
Explanation: 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.

Model Answer:
Volume of hydrogen gas produced / mass loss of the reaction mixture / change in concentration with time.
Explanation: Any measurable quantity that changes with reaction progress can be used to determine rate and order.
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