AS & A Level Chemistry - 9701 Paper 3 2015 Winter Zone 6

You will investigate the rate of reaction between iron(III) ions, Fe^{3+}, and iodide ions, I^{-}.
$$2Fe^{3+}(aq) + 2I^{-}(aq) \rightarrow 2Fe^{2+}(aq) + I_{2}(aq)$$
The iodine, I_{2}, produced can be reacted immediately with thiosulfate ions, S_{2}O_{3}^{2-}.
$$I_{2}(aq) + 2S_{2}O_{3}^{2-}(aq) \rightarrow 2I^{-}(aq) + S_{4}O_{6}^{2-}(aq)$$
When all the thiosulfate has been used, the iodine produced will turn starch indicator blue-black. The rate of the reaction can therefore be measured by finding the time for the blue-black colour to appear.

FB 1 is aqueous iron(III) chloride, FeCl_{3}.
FB 2 is aqueous potassium iodide, KI.
FB 3 is 0.0060 mol dm^{-3} sodium thiosulfate, Na_{2}S_{2}O_{3}.
starch indicator

You are advised to read the instructions before starting any practical work and draw a table for your results in the space on page 3.

(a) Method

Experiment 1

• Fill a burette with FB 1.
• Run 20.00 cm^{3} of FB 1 into a 100 cm^{3} beaker.
• Use the measuring cylinder to place the following in a second 100 cm^{3} beaker.
◦ 10 cm^{3} of FB 2
◦ 20 cm^{3} of FB 3
◦ 10 cm^{3} of starch indicator
• Add the contents of the second beaker to the first beaker and start timing.
• Stir the mixture once and place the beaker on the white tile.
• The mixture turns brown and then yellow before turning a blue-black colour. Stop timing when this blue-black colour appears.
• Record in your table the volume of FB 1 used, the volume of distilled water used and the time to the extbf{nearest second} for the blue-black colour to appear.
• Wash both beakers.

For each of extbf{Experiments 2-6} you should complete your results table to show the volume of FB 1 used, the volume of distilled water used and the time taken to the nearest second for the blue-black colour to appear.

Experiment 2

• Fill the other burette with distilled water.
• Run 10.00 cm^{3} of FB 1 into a 100 cm^{3} beaker.
• Run 10.00 cm^{3} of distilled water into the same beaker.
• Use the measuring cylinder to place the following in a second 100 cm^{3} beaker.
◦ 10 cm^{3} of FB 2
◦ 20 cm^{3} of FB 3
◦ 10 cm^{3} of starch indicator
• Add the contents of the second beaker to the first beaker and start timing.
• Stir the mixture once and place the beaker on the white tile.
• Stop timing when a blue-black colour appears.
• Wash both beakers.

Experiments 3-6

Carry out extbf{four} further experiments to investigate the effect of changing the concentration of Fe^{3+}(aq) by altering the volume of aqueous FeCl_{3}, FB 1, used.

You should not use a volume of FB 1 that is less than 6.00 cm^{3} and the total volume of the reaction mixture must always be 60 cm^{3}.

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(b) Calculations

The rate of reaction can be found by calculating the change in concentration of Fe^{3+}(aq) that occurred when enough iodine was produced to change the colour of the indicator to blue-black.

Use your data and the equations on page 2 to carry out the following calculations.

Show your working and appropriate significant figures in the final answer to each step of your calculations.

(i) Calculate the number of moles of thiosulfate ions, S_{2}O_{3}^{2-} used in each experiment in (a).

moles S_{2}O_{3}^{2-} = .................... mol

(ii) Calculate the number of moles of iodine, I_{2}, that react with the number of moles of S_{2}O_{3}^{2-}
in (i).

moles I_{2} = .................... mol

(iii) Calculate the number of moles of iron(III) ions, Fe^{3+}, that were used to produce the number
of moles of iodine in (ii).

moles Fe^{3+} = .................... mol

(iv) When the moles of Fe^{3+} that you calculated in (iii) reacted, a change in the concentration
of moles of Fe^{3+} occurred. Calculate this change in concentration.

change in concentration of Fe^{3+}(aq) = .................... mol dm^{-3}

(v) The following formula can be used as a measure of the ‘rate of reaction’.

$$ 'rate of reaction' = \frac{ \text{change in concentration of Fe}^{3+}(aq)}{\text{reaction time}} \times 10^{6} $$

Complete the table to show the volume of FB 1, the reaction time and the rate in Experiments 1-6. You should include units.

If you were unable to calculate a value for the change in concentration of Fe^{3+}(aq) in (iv), you should assume it is $2.50 \times 10^{-3} \text{ mol dm}^{-3}$. (Note: this is not the correct value.)

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(c) On the grid, plot the rate (y-axis) against the volume of FB 1 (x-axis). Draw a line of best fit through the points. You should identify any points you consider anomalous.



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(d) Using your graph, what conclusion can you reach about the effect of changing the concentration of FeCl_{3} on the rate of the reaction between Fe^{3+}(aq) and I^{-}(aq)?

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(e) A student wanted to investigate how changing the concentration of I^{-} would affect the rate of reaction. Explain how this investigation could be carried out.

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(f) It was found, by carrying out experiments similar to those used in (a), that increasing the concentration of I^{-} increased the rate of the reaction.
The student suggested modifications to the method as used in (a). In each case, state what the effect would be on the reaction time in Experiment 1 and explain how these changes would affect the possible errors in the measurements.

Suggested modification 1
The reaction was carried out using the same volumes of all reagents but with the concentrations of FB 1 and FB 2 being double their original values.
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Suggested modification 2
The reaction was carried out using half the volume of all reagents.
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(g) (i) Which of the experiments you carried out in (a) had the greatest percentage error in the reaction time?

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(ii) Calculate this percentage error. Assume that the error in measuring the reaction time is $\pm$ 0.5 s.

percentage error = .................... %

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Qualitative Analysis
At each stage of any test you are to record details of the following.
● colour changes seen
● the formation of any precipitate
● the solubility of such precipitates in an excess of the reagent added
Where gases are released they should be identified by a test, described in the appropriate place in your observations.
You should indicate clearly at what stage in a test a change occurs. Marks are not given for chemical equations. No additional tests for ions present should be attempted.
If any solution is warmed, a boiling tube MUST be used.
Rinse and reuse test-tubes and boiling tubes where possible.
Where reagents are selected for use in a test, the name or correct formula of the element or compound must be given.
Half fill the 250 cm³ beaker with water and heat it to about 80°C and then switch off the burner. This will be used as a water bath in (b).
Keep two clean, dry test-tubes for use in (b).
(a) FB 4 and FB 5 each contain two cations and one anion. One of the cations in FB 4 is the same as one of the cations in FB 5. The anion in FB 4 is the same as the anion in FB 5.
(i) Separately dissolve about half of each of your samples of FB 4 and FB 5 in about 5 cm depth of distilled water in a boiling tube.
Carry out the following tests and record your observations in the table.