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You are required to investigate the effect of temperature on the rate of reaction of peroxydisulfate ions with iodide ions. Iodide ions are oxidised to iodine by peroxydisulfate ions.
$$S_2O_8^{2-}(aq) + 2I^{-}(aq) \rightarrow 2SO_4^{2-}(aq) + I_2(aq)$$
FA 1 is aqueous potassium peroxydisulfate, $K_2S_2O_8$.
FA 2 is an aqueous solution containing a mixture of potassium iodide, KI, sodium thiosulfate, $Na_2S_2O_3$, and starch.
When FA 1 and FA 2 are mixed together the potassium peroxydisulfate reacts with the potassium iodide to make iodine. As soon as this iodine is formed, it reacts with the sodium thiosulfate and is turned back into iodide ions. Only when all the sodium thiosulfate has reacted does iodine remain in the solution. The solution then turns blue-black because of the presence of the starch indicator. The rate of reaction can be determined by the time it takes for a blue-black colour to first appear in the colourless mixture.
(a) Method
Read through the method and prepare a table on page 3 to record the initial and final temperatures and the reaction time for each experiment, before starting any practical work.
• Half-fill a 250$cm^3$ beaker with water to act as a water bath.
• Place it on a tripod and gauze and heat it with a Bunsen burner to about 65°C then remove the Bunsen. While your water is being heated continue with the following steps of the method.
• Fill the burette, labelled FA 1, with the aqueous potassium peroxydisulfate, FA 1.
• Fill the burette, labelled FA 2, with the mixture of solutions, FA 2.
• Measure 10.0$cm^3$ of FA 1 into a boiling tube.
• Measure 10.0$cm^3$ of FA 2 into a second boiling tube.
• Place both boiling tubes in the water bath.
• Clamp one of the tubes and place a thermometer in this tube.
• When the temperature of this solution has reached about 60°C, pour the contents of the second tube into the clamped tube. Start timing immediately, note the temperature and stir the mixture.
• Record this initial temperature.
• Stop timing as soon as the blue-black colour appears. Record this reaction time to the nearest second and record the final temperature.
• Repeat the experiment at decreasing temperatures as many times as necessary to generate data for plotting a graph. The experiment should not be performed at a temperature below about 30°C. The temperature of the water bath may be adjusted by adding cold water or by reheating. (Boiling tubes may be rinsed and reused.)
(b) The rate of reaction for each experiment can be represented by the following.
$$\text{rate} = \frac{1000}{\text{reaction time in seconds}}$$
Complete the following table for each of your experiments. The mean temperature is the average of the initial and final temperature for the experiment.
[Table_1]
(c) (i) Using your values in (b), plot a graph of 'rate' (y-axis) against mean temperature (x-axis). Choose suitable scales to allow you to extrapolate the graph to include the 'rate' at 20°C.
[Graph_1]
(ii) Use your graph to obtain a value for the 'rate' at 20°C.
'rate' at 20°C is ........................................................ [4]
(d) It has been suggested that an increase in temperature of 10°C will double the rate of reaction. Use two pairs of temperatures from your graph to confirm or deny this statement.
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..................................................................................................................... [2]
(e) (i) Which of your experiments has the greatest percentage error in timing?
.....................................................................................................................
(ii) Calculate the percentage error in (i). You may assume that the error in measuring the time for a reaction is ±0.5 seconds.
$$\text{error in (i)} = ........................................ \%$$ [2]
(f) A student had difficulty in drawing a line of best fit. Identify a source of error in the experimental procedure. Do not include any errors involving the precision of apparatus.
.....................................................................................................................
..................................................................................................................... [1]
(g) Suggest a modification that could be used to reduce this error.
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..................................................................................................................... [1]
(h) Using FA 1, FA 2 and distilled water, describe how you could investigate the effect of concentration of potassium peroxydisulfate on the rate of reaction.
.....................................................................................................................
.....................................................................................................................
.....................................................................................................................
..................................................................................................................... [3]
530
500
518
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.
█ Reagents selected for use in a test the full name or correct formula of the reagents must be given.█
FA 3, FA 4 and FA 5 are aqueous solutions containing one cation and one anion. One of these solutions is a dilute acid and this is the only acid present.
(a) (i) Select a single chemical reagent from those supplied which would allow you to identify the dilute acid. You may not use indicator paper.
reagent ............................................................
(ii) Use this reagent to test all three solutions and record your results in an appropriate form in the space below.
(iii) From your observations in (ii), identify which solution is the dilute acid.
FA .......... is the dilute acid.
[5]
(b) The acid you have identified in (a)(iii) is dilute sulfuric acid. Complete the following table.
─────────────────────────
test | observations
─────────────────────────
To 1 cm depth of FA 3 in a test-tube, add 1 cm depth of FA 4, then
add excess hydrochloric acid.
To 1 cm depth of FA 4 in a test-tube, add 1 cm depth of FA 5, then
add excess hydrochloric acid.
To 1 cm depth of FA 5 in a test-tube, add 1 cm depth of FA 3, then
add excess hydrochloric acid.
[4]
(c) For the two unidentified solutions, complete the following table.
──────────────
| observations
test | FA ........ | FA ..........
──────────────
To 1 cm depth of unknown in a boiling tube, add NaOH(aq)
warm the tube carefully
[2]
(d) From your observations in (a), (b) and (c), identify the ions present in the two solutions tested in (c), giving the relevant evidence for each. If you have not been able to identify one or more of the ions, explain why the evidence obtained was insufficient.
FA ......... cation .............. evidence ................................
anion ........... evidence ..................................................
[4]
(e) If one of the aqueous anions was a bromide, what would be the minimum evidence needed for its identification?
[1]
511
574
502
