AS & A Level Chemistry - 9701 Paper 5 2009 Winter Zone 1

(a) Using the results of the preliminary experiment predict the relationship between the concentration of potassium persulfate and the rate of reaction.

Explain your prediction in terms of the particles (ions) present in the solution.
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(b) (i) Identify the independent variable in the investigation.
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(ii) Identify the dependent variable in the investigation.
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(c) Explain why it is important that the iodine formed by oxidation reacts with the sodium thiosulfate and is converted back to iodide ions.
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(d) Design a laboratory experiment that you would use to investigate your prediction in (a).
The following materials are to be used in your plan.
0.60 mol dm^{-3} potassium iodide
0.20 mol dm^{-3} potassium persulfate
0.01 mol dm^{-3} sodium thiosulfate
distilled water
starch indicator solution
A first experiment is carried out using the following quantities.
20 cm^{3} potassium iodide
40 cm^{3} potassium persulfate
20 cm^{3} sodium thiosulfate
0 cm^{3} distilled water
10 cm^{3} starch indicator solution
Give a step-by-step description of the method you would use in further experiments.
Include the following in your plan.

• the range of concentrations
• the volume of each solution to be used
• the method of measuring the volume of each solution
• how and when the solutions are to be mixed
• the way in which the dependent variable will be measured
• the control of all other variables

A group of twelve students carried out the following experiment to confirm the formula of zinc iodide as $ZnI_2$. In the experiment an excess of powdered zinc reacts with a solution of iodine in ethanol.

The instructions for the experiment given to each student are as follows.

• Weigh a dry, hard-glass test-tube.
• Put approximately 0.5 g of zinc powder in the test-tube and reweigh the tube.
• Add approximately 1.0 g of iodine and weigh the tube again.
• Cautiously add ethanol, drop by drop, until no further visible reaction takes place, and then add a further $1 ext{cm}^3$ of ethanol, an excess.
• Shake the tube gently until the brown colour of the dissolved iodine disappears. This indicates that the reaction is complete.
• Place the tube in a centrifuge and spin the tube and its contents for 30 seconds. The excess zinc will “pack” at the bottom of the tube.
• Pour off and discard the liquid above the layer of zinc. This liquid contains the zinc iodide formed in the reaction.
• Wash the zinc powder with $1 ext{cm}^3$ of ethanol.
• Centrifuge the tube and pour off the liquid as before.
• Repeat the washing of the zinc several times to remove any remaining traces of zinc iodide.
• Dry the tube and its contents by laying the tube on a gauze, on top of a tripod, gently heated by a small Bunsen burner flame.
• Allow the tube to cool and reweigh the tube and its contents.

(a) The results of the experiment are shown in the table at the top of the next page. Process the results in the table to produce values that will enable you to calculate the number of moles of zinc and iodine reacting in each student’s experiment.
Record these values in the additional columns of the table. You may use some or all of the columns. Label the columns you use. For each column, include the units and an expression to show how your values are calculated. You may use the column headings A to G in the expressions e.g. C – B.

You are reminded that an excess of zinc was used in each experiment.



(b) (i) For each student calculate the number of moles of iodine atoms and number of moles of zinc that react together. $[A_r$: I, 127; Zn, 65.4]



(ii) Process the values from (i) to enable you to confirm or reject the suggested formula of zinc iodide, $ZnI_2$. Use the blank column in the table above to record your results.

(c) By inspection of the values in (b), identify any student(s) whose experimental results you consider to be anomalous.
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(d) In the light of your answer to (c) explain how you would process the values from (b) to find the formula of zinc iodide.
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(e) For each anomalous result identified in (c) refer to the instructions for the experiment and suggest a possible reason for the anomaly.
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(f) Does the range of experimental values and your answer to (d) support the suggested formula, $ZnI_2$? Explain your answer.
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(g) By referring to your calculated values in (a) identify any single weakness in the experimental method that could lead to errors in the results of the experiment.
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(h) A student suggests an alternative method with iodine in excess rather than zinc. Suggest a reagent that could be used, in a titration, to determine the amount of iodine remaining in solution after the reaction.
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