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You are to investigate how the rate of reaction is affected by changing the concentration of the reagents.
FB 1 is 0.0500\,mol\,dm^{-3} aqueous acidified iron(III) chloride, FeCl_{3}.
FB 2 is 0.0500\,mol\,dm^{-3} aqueous potassium iodide, KI.
FB 3 is 0.00500\,mol\,dm^{-3} aqueous sodium thiosulfate, Na_{2}S_{2}O_{3}.
FB 4 is starch indicator.
Read through the instructions carefully and prepare a table for your results on page 3 before starting any practical work.
(a) Method
Experiment 1
• Using a 25\,cm^{3} measuring cylinder add the following to a 100\,cm^{3} beaker:
o 10\,cm^{3} of FB 2
o 20\,cm^{3} of FB 3
o 10\,cm^{3} of FB 4
• Fill the burette labelled FB 1 with aqueous acidified iron(III) chloride, FB 1.
• Run 20.00\,cm^{3} of FB 1 into a second 100\,cm^{3} beaker.
• Add the contents of the first beaker to the second beaker and start timing immediately.
• Stir the mixture once and place the beaker on a white tile.
• Stop timing as soon as the solution turns blue-black.
• Record this reaction time to the nearest second in the table that you have prepared on page 3.
• Wash out both beakers.
Experiment 2
• Using a 25\,cm^{3} measuring cylinder add the following to a 100\,cm^{3} beaker:
o 10\,cm^{3} of FB 2
o 20\,cm^{3} of FB 3
o 10\,cm^{3} of FB 4
• Fill a second burette with distilled water.
• Run 10.00\,cm^{3} of FB 1 into a second 100\,cm^{3} beaker.
• Run 10.00\,cm^{3} of distilled water into the beaker containing FB 1.
• Add the contents of the first beaker to the second beaker and start timing immediately.
• Stir the mixture once and place the beaker on a white tile.
• Stop timing as soon as the solution turns blue-black.
• Record this reaction time to the nearest second in the table that you have prepared on page 3.
• Wash out both beakers.
Experiments 3 – 5
• Carry out three further experiments to investigate how the reaction time changes with different volumes of iron(III) chloride.
Remember that the combined volume of FB 1 and distilled water must always be 20.00\,cm^{3}.
Do not use a volume of FB 1 that is less than 6.00\,cm^{3}.
Record all your results in a single table. You should include the volume of iron(III) chloride, the volume of distilled water and the reaction time.
(b) In order to convert the times measured in the experiments into rates of reaction, it is necessary first to work out the change in the iron(III) ion concentration that occurs from the time when the mixtures are combined to the time when the solution turns blue-black. You must show your working.
(i) Calculate how many moles of thiosulfate ions, $S_{2}O_{3}^{2-}$, are added in each experiment.
moles of $S_{2}O_{3}^{2-}$ = ................ mol
(ii) Calculate how many moles of iodine, $I_{2}$, reacted with the number of moles of thiosulfate ions calculated in (i).
moles of $I_{2}$ = ................ mol
(iii) Calculate how many moles of iron(III) ions, $Fe^{3+}$, reacted to form the number of moles of iodine calculated in (ii).
moles of $Fe^{3+}$ = ................ mol
(iv) Use your answer from (iii) to calculate the decrease in concentration of iron(III) ions in the total reaction mixture (60\,cm^{3}) from the start of the experiment to the point when the solution turned blue-black.
decrease in $Fe^{3+}$ concentration = ................ mol\,dm^{-3}
(c) The rate of the reaction can be represented by the following formula.
'rate' = \( \frac{\text{decrease in Fe}^{3+} \text{ concentration from (b)(iv)}}{\text{reaction time}} \times 10^{6} \)
Use your experimental results to complete the following table. Include the volume of FB 1, the reaction time and the 'rate' with their units.
If you were unable to answer (b)(iv), you may assume that the decrease in Fe^{3+} concentration is 2.25 \times 10^{-3} mol\,dm^{-3} (This is not the correct value).
(d) On the grid opposite, plot 'rate' against the volume of FB 1. Draw a line of best fit.
(e) In your experiments, the volume of FB 1 represents the concentration of iron(III) chloride. From your results, what conclusion can you draw about the relationship between the rate of reaction and the concentration of iron(III) chloride?
.........................................................................................................................
(f) One source of error in this experiment arises from measuring the volumes of solutions.
(i) Calculate the maximum percentage error in the volume of FB 1 used in Experiment 1.
maximum percentage error = ..................
(ii) Other than errors involving measurements of volumes, suggest an additional source of error in these experiments.
.........................................................................................................................
(iii) In this experiment, thiosulfate ions reduce iodine while iron(III) ions oxidise iodide ions. What other reaction might take place that would affect your confidence in the conclusions you made in (e)?
.........................................................................................................................
(g) (i) Carry out one additional experiment using the following volumes of each reagent. Use the same method as in (a), mix FB 2, FB 3 and FB 4 together and start the reaction by adding this mixture to FB 1 and the distilled water.
o 10.00\,cm^{3} of FB 1
o 20.00\,cm^{3} of distilled water
o 10\,cm^{3} of FB 2
o 10\,cm^{3} of FB 3
o 10\,cm^{3} of FB 4
Record the time for the reaction to go blue-black.
(ii) Explain the relationship between this time and the one you recorded in Experiment 2.
.........................................................................................................................
570
523
530
FB 5, FB 6, FB 7 and FB 8 are aqueous solutions each of which contains a single cation and a single anion. Some of the ions present are listed below.
Pb^{2+} Ba^{2+} H^{+} CrO_4^{2-} SO_4^{2-} Cl^{-}
By observing the reactions that occur when pairs of the solutions are mixed together, you will be able to identify which solution contains which of these ions.
Use 1 cm depth of each solution in a test-tube and record your observations in the following table.
[Table_1]
(b) From your observations, deduce which solution contains each of the following ions.
[Table_2]
(c) The anion in FB 9 is either the nitrite ion, NO_2^{-}, or the nitrate ion, NO_3^{-}.
(i) Describe a test you could carry out that would give positive results for both of these ions.
........................................................................................................
........................................................................................................
(ii) Describe a test you could carry out that would distinguish between these two ions.
........................................................................................................
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(iii) Carry out both of these tests and record your results in an appropriate form in the space below.
(iv) Which anion is present in FB 9?
.................................................
541
593
596
