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You are to determine the concentration of aqueous copper(II) sulfate by titration. The concentration of $\text{Cu}^{2+}$ ions in a solution can be found by reaction with an excess of aqueous iodide ions to produce iodine. The amount of iodine formed can be found by titration with thiosulfate ions, $\text{S}_2\text{O}_3^{2-}$.
FB 1 is aqueous copper(II) sulfate, $\text{CuSO}_4$.
FB 2 is $0.100$ $\text{mol dm}^{-3}$ sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3$.
FB 3 is aqueous potassium iodide, $\text{KI}$.
starch indicator
Read through the instructions carefully before starting any practical work.
(a) Method
- Fill the burette with FB 2.
- Pipette $25.0 \text{cm}^3$ of FB 1 into a conical flask.
- Use a measuring cylinder to add $10 \text{cm}^3$ of FB 3 into the conical flask.
- Titrate this mixture with FB 2 until the colour of the mixture changes from brown to yellow-brown. An off-white precipitate will also be present in the flask throughout the titration.
- Add approximately $1 \text{cm}^3$ of starch indicator.
- Continue the titration until the blue-black colour of the starch-iodine complex just disappears leaving the off-white precipitate.
- Perform a rough titration and record your burette readings in the space below.
The rough titre is ............................... $\text{cm}^3$. - Carry out as many accurate titrations as you think necessary to obtain consistent results.
- Make certain any recorded results show the precision of your practical work.
- Record in a suitable form below all of your burette readings and the volume of FB 2 added in each accurate titration.
(b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you obtained this value.
$25.0 \text{cm}^3$ of FB 1 required ........................ $\text{cm}^3$ of FB 2 [1]
(c) Calculations
The equations for the formation of iodine and its reaction with thiosulfate ions are given below.
$$2\text{Cu}^{2+} + 4\text{I}^- \rightarrow 2\text{CuI} + \text{I}_2$$
$$\text{I}_2 + 2\text{S}_2\text{O}_3^{2-} \rightarrow \text{S}_4\text{O}_6^{2-} + 2\text{I}^-$$
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, $\text{S}_2\text{O}_3^{2-}$, present in the volume of FB 2 in (b).
moles of $\text{S}_2\text{O}_3^{2-}$ = ................... mol
(ii) Using the equations above, deduce the number of moles of $\text{Cu}^{2+}$ ions present in each $25.0 \text{cm}^3$.
moles of $\text{Cu}^{2+}$ = .......................... mol
(iii) Calculate the concentration, in $\text{mol dm}^{-3}$, of copper(II) sulfate in FB 1.
concentration of $\text{CuSO}_4$ = ................ $\text{mol dm}^{-3}$
(d) Three students repeated the experiment but each obtained different values for the concentration of $\text{CuSO}_4$.
The students each suggested possible improvements.
Student 1 suggested that a larger quantity of starch should be added.
Student 2 suggested that a larger volume of potassium iodide, FB 3, should be added.
Student 3 suggested that the contents of the conical flask should be filtered before titration.
Comment on the effectiveness of each of these possible improvements. Explain your answers.
Student 1
........................................................................................................................
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Student 2
........................................................................................................................
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Student 3
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557
529
505
You are to determine the enthalpy change for the reaction between aqueous copper(II) sulfate and zinc. The enthalpy change of reaction can be found by measuring the temperature change when powdered zinc is added to aqueous copper(II) sulfate.
FB 4 is 1.10 mol dm$^{-3}$ aqueous copper(II) sulfate, CuSO$_4$. powdered zinc
(a) Method
- Weigh a 100 cm$^3$ beaker.
- In the beaker weigh out between 2.1 g and 2.3 g of powdered zinc.
- Record the weighings and the mass of zinc in the space below.
mass of zinc used = ............. g
- Support the plastic cup in a 250 cm$^3$ beaker.
- Use a measuring cylinder to transfer 50 cm$^3$ of FB 4 into the plastic cup.
- Measure and record in the table below, the initial temperature of FB 4 in the cup.
- Start the stop watch. Measure and record the temperature of FB 4 in the cup after 1 minute, 2 minutes and 3 minutes.
- At time 3½ minutes, add the weighed zinc to FB 4 in the cup and stir the mixture.
- From time 4 minutes, continue to stir the mixture and measure the temperature of the contents of the cup to complete the table.
Results
[Table]
time / min | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
temperature / °C | | | | | | | | | | | | |
(b)
(i) On the axes opposite, plot the temperature (y-axis) against time (x-axis). The temperature axis should allow you to include a point at least 5 °C greater than the maximum temperature recorded.
(ii) Complete the graph to show how the temperature of the contents of the cup varies with time.
- Draw one straight line through the points between time 0 minutes and 3 minutes.
- Draw one straight line through the points after the maximum was reached.
- Extrapolate these two lines and draw a vertical line at time 3½ minutes.
(c) Calculation
(i) Use your graph to determine the change in temperature at 3½ minutes.
change in temperature = ............ °C
(ii) Calculate the heat energy produced in the reaction. (You may assume that 4.3 J are required to raise the temperature of 1.0 cm$^3$ of any solution by 1.0 °C.)
heat energy produced = .......................... J
(iii) The reaction between zinc and aqueous copper(II) sulfate is a displacement reaction shown in the equation below.
Zn(s) + Cu$^{2+}$(aq) → Cu(s) + Zn$^{2+}$(aq)
From the mass of zinc added and the concentration of FB 4, show that the copper(II) sulfate was in excess in your reaction.
[A$_r$: Zn, 65.4]
(iv) Assuming that the copper(II) sulfate was in excess, use your answer to (ii) to calculate the enthalpy change of the reaction between Zn(s) and Cu$^{2+}$(aq).
Give your answer in kJ mol$^{-1}$ and include the relevant sign.
enthalpy change of reaction = ........ sign .............. value kJ mol$^{-1}$
(d) One source of error in this experiment is due to the accuracy to which the thermometer can be read.
What is the maximum error in a single temperature reading on a thermometer with graduations at 1°C?
maximum error = ........... °C
Calculate the maximum percentage error when measuring a temperature rise of 12.0 °C.
maximum percentage error = ............................ %
524
510
522
(a) (i) Carry out the following tests on FB 5.
[Table_1]
(ii) From these tests, what conclusions, if any, can you reach about the identity of FB 5?
(b) (i) Carry out the following tests on FB 6.
[Table_2]
(ii) From these tests suggest identities for the following.
The anion in FB 6 is ..............................................................
The cation in FB 7 is .............................................................
The cation in FB 8 could be .............................. or ..............................
(iii) Suggest a test to determine which of the two possible cations is present in FB 8.
Do not carry out this test.
(c) Using your conclusions about the possible identities of FB 5 and FB 8, predict the result of mixing solutions of each.
Do not carry out this test.
Prediction ...............................................................................................................................................................
(d) Suggest what happened to the ethanol when it was warmed with the mixture of FB 6 and FB 7.
...............................................................................................................................................................
(e) You are to devise and carry out a test to confirm the identity of the cation in FB 7.
Record the test you use and the results of the test in the space below.
533
574
510
