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(a) Method
Experiment 1
• Fill the burette labelled FA 1 with FA 1.
• Use the pen to label one of the 100 cm$^3$ beakers 'A' and the other 100 cm$^3$ beaker 'B'.
• Run 20.00 cm$^3$ of FA 1 from the burette into beaker A.
• Use the measuring cylinder to add 20.0 cm$^3$ of FA 2 into beaker B.
• Use the measuring cylinder to add 10.0 cm$^3$ of FA 3 to beaker B.
• Add 10 drops of starch indicator to beaker B.
• Add the contents of beaker A to beaker B 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 your results table.
• Wash out both beakers and shake to remove excess water.
Experiment 2
• Fill a second burette with distilled water.
• Run 10.00 cm$^3$ of FA 1 into beaker A.
• Run 10.00 cm$^3$ of distilled water into beaker A.
• Use the measuring cylinder to add 20.0 cm$^3$ of FA 2 into beaker B.
• Use the measuring cylinder to add 10.0 cm$^3$ of FA 3 to beaker B.
• Add 10 drops of starch indicator to beaker B.
• Add the contents of beaker A to beaker B 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 your results table.
• Wash out both beakers and shake to remove excess water.
Experiments 3-5
• Carry out three further experiments to investigate how the reaction time changes with different volumes of potassium peroxodisulfate, FA 1.
Note that the combined volume of FA 1 and distilled water must always be 20.00 cm$^3$. Do not use a volume of FA 1 that is less than 6.00 cm$^3$.
Keep FA 1, FA 2, FA 3 and the starch indicator for use in (e).
Calculating the rate of the reaction
The rate of the reaction can be represented by the formula shown.
\[ \text{rate} = \frac{500}{\text{reaction time in seconds}} \]
Use this formula to calculate the rate for each of your five experiments.
Record all your results in a single table. You should include the volume of FA 1, the volume of distilled water, the reaction time and the reaction rate for each of your five experiments.
(b) On the grid on page 5, plot the rate (y-axis) against the volume of FA 1 (x-axis). Include the origin in your plot. Draw a straight line of best fit and circle any clearly anomalous points.
(c) The volume of FA 1 is directly related to the concentration of potassium peroxodisulfate.
From your results, what can be stated about the relationship between the rate of reaction and the concentration of potassium peroxodisulfate?
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(d)
(i) Use your graph to calculate the reaction time you would expect to measure if you carried out an experiment using 5.00 cm$^3$ of FA 1. Show your working.
reaction time = .............................. s
(ii) Assume that the error in the time measured for each reaction was ±0.5 s in total. Calculate the maximum percentage error in the reaction time you measured in Experiment 1. Show your working.
maximum percentage error = ........................... %
(iii) A student suggested that this error could be reduced if 0.0100 mol dm$^{-3}$ sodium thiosulfate were used in place of FA 3.
Do you agree with this student? Explain your answer.
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(iv) A student repeated Experiment 1 but used 0.100 mol dm$^{-3}$ sodium thiosulfate in place of FA 3. The student found that the reaction mixture never turned blue-black.
Explain why.
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(e)
(i) Using the same method as in (a), carry out an additional experiment to record the reaction time to the nearest second when the following solutions are mixed together.
• 10.00 cm$^3$ of FA 1
• 20.0 cm$^3$ of FA 2
• 5.0 cm$^3$ of FA 3
• 15.00 cm$^3$ of distilled water
• 10 drops of starch indicator
reaction time = ..............................
(ii) Use your answer to (i) to estimate the reaction time that would be measured if the following solutions were mixed together.
DO NOT CARRY OUT THIS EXPERIMENT
• 10.00 cm$^3$ of FA 1
• 20.0 cm$^3$ of FA 2
• 20.0 cm$^3$ of FA 3
• 10 drops of starch indicator
Explain your answer.
estimated reaction time = ..............................
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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 reagents are selected for use in a test, the name or correct formula of the element or compound must be given.
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. 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.
(a) FA 4 and FA 5 are aqueous solutions. Each solution contains two different cations and the sulfate anion.
(i) Carry out the following tests and record your observations.
| test | observations |
| --- | --- | --- |
| | FA 4 | FA 5 |
| To a 1 cm depth of solution in a boiling tube, add aqueous sodium hydroxide, then | | |
| warm the tube carefully. | | |
| | | |
| To a 1 cm depth of solution in a test-tube, add aqueous ammonia. | | |
(ii) Identify as many as possible of the cations in FA 4 and FA 5.
FA 4 contains the cation(s) ...................................................... .
FA 5 contains the cation(s) ...................................................... .
The formula of the anion in FA 6 is ...................................... .
(b) FA 6 is a salt containing either the sulfate anion or sulfite anion. You will first make a solution of FA 6.
• Rinse one of the 100 cm^3 beakers with distilled water.
• Place all the sample of FA 6 into the beaker and add approximately 40 cm^3 of distilled water.
• Stir the mixture until the solid has dissolved.
Select reagent(s) and carry out tests to identify the anion in FA 6. Record your results in the space below.
The formula of the anion in FA 6 is ....................................... .
(c) FA 7 is a solution containing one cation and the sulfate anion.
FA 8 is a solution containing the sodium cation and one of the anions from those listed in the Qualitative Analysis Notes.
(i) Carry out the following tests to determine the formulae of FA 7 and FA 8.
| test | observations |
| --- | --- |
| To a 2 cm depth of FA 7 in a test-tube, add a 2 cm strip of magnesium. | |
| | |
| To a 1 cm depth of FA 7 in a test-tube add a 1 cm depth of FA 8 and shake the tube. | |
(ii) The formula of FA 7 is .......................................... .
The formula of FA 8 is ........................................... .
(iii) Give the ionic equation for the reaction that takes place when magnesium is added to FA 7. Include state symbols.
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