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You are to investigate the temperature change when a piece of magnesium ribbon is added to hydrochloric acid. You will measure the temperature at regular intervals during the reaction.
FA 1 is approximately 2 mol\text{dm}^{-3} hydrochloric acid, HCl. You are also provided with magnesium ribbon.
Read through the instructions carefully before starting any practical work.
(a) Method
• Curl or fold the magnesium ribbon so that it will just fit in the bottom of the plastic cup. Remove the magnesium from the cup.
• Support the plastic cup in a 250 cm³ beaker.
• Use a measuring cylinder to transfer 50 cm³ of FA 1 into the empty plastic cup.
• Measure and record in the table below, the initial temperature of the acid in the cup.
• Start the stop watch. Measure and record the temperature of FA 1 in the cup after 1 minute, 2 minutes and 3 minutes.
• At time 3½ minutes, add the curled or folded magnesium to the FA 1 in the cup and stir the mixture. Make sure the magnesium is submerged in the acid.
• From time 4 minutes, continue to measure the temperature of the contents of the cup to complete the table.
Results
| time/min | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
|----------|---|---|---|---|---|---|---|---|---|---|----|
| temperature/°C | | | | | | | | | | | | [1]
(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. [4]
(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 between time 5 minutes and 10 minutes.
• Extrapolate these two lines and draw a vertical line at time 3½ minutes. [4]
(c) Calculation
(i) Use your graph to determine the change in temperature at 3½ minutes.
change in temperature = ………….. °C
(ii) In the experiment you have just carried out, explain how you know that the hydrochloric acid was in excess.
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(iii) 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 7.5°C.
maximum percentage error = ................................. %
(iv) Apart from errors associated with the thermometer, suggest one significant source of error in the procedure used in this experiment. Suggest an improvement that could be made to reduce this error.
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(v) The experiment was repeated on another day when the temperature of the room was much higher than when the original experiment was carried out. Discuss the effect of this higher room temperature on the results of the experiment.
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502
522
586
The concentration of the acid, FA 2, can be found by titrating it against aqueous sodium carbonate of known concentration.
FA 2 is hydrochloric acid, $HCl$.
FA 3 is sodium carbonate, $Na_2CO_3$.
methyl orange indicator
You are to determine the concentration of FA 2.
(a) Method
• Weigh a 100 cm3 beaker.
• Weigh out between 1.3 g and 1.5 g of FA 3 into this beaker.
• Record the weighings and the mass of FA 3 added.
mass of FA 3 = ...................... g
• Add about 50 cm3 of distilled water to the beaker.
• Stir with a glass rod until all the solid has dissolved.
• Pour the solution from the beaker into the 250 cm3 graduated (volumetric) flask.
• Wash the beaker with distilled water and add the washings to the flask.
• Make up the contents of the graduated flask to the 250 cm3 mark with distilled water.
• Invert the flask as many times as you think necessary to mix the solution of FA 3 thoroughly.
• Fill the burette with FA 2.
• Pipette 25.0 cm3 of the solution of FA 3 into a conical flask.
• Add methyl orange indicator.
• Titrate the solution of FA 3 with FA 2.
• Perform a rough titration and record your burette readings in the space below.
The rough titre is .............................. cm3.
• 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 FA 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 cm3 of FA 3 required .................. cm3 of FA 2.
(c) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Calculate how many moles of sodium carbonate were present in 25.0 cm3 of the solution you pipetted for the titration.
[Ar: C, 12.0; O, 16.0; Na, 23.0]
moles of $Na_2CO_3$ = ................ mol
(ii) The equation for the reaction between hydrochloric acid and sodium carbonate is shown below.
$$Na_2CO_3 + 2HCl \rightarrow 2NaCl + CO_2 + H_2O$$
Use your answers to (b) and (c)(i) to calculate the concentration, in mol dm-3, of the hydrochloric acid, FA 2.
concentration of HCl, FA 2 = ............ mol dm-3
(iii) FA 2 is hydrochloric acid, HCl, made using 50.0 cm3 of FA 1 diluted to 1.00 dm3 with distilled water. Calculate the concentration, in mol dm-3, of the hydrochloric acid, FA 1.
concentration of HCl, FA 1 = ............ mol dm-3 [5]
573
542
558
(a) You are provided with solutions FA 4, FA 5 and FA 6 and solid FA 7. Each of these contain a transition metal ion.
(i) Carry out the following tests on FA 4.
| test | observations |
|---|---|
| To 1 cm depth of FA 4 in a test-tube, add an equal depth of concentrated hydrochloric acid with care. | |
| To 1 cm depth of FA 4 in a test-tube, add aqueous ammonia. | |
| To 1 cm depth of FA 4 in a test-tube, add aqueous silver nitrate. | |
| To 1 cm depth of FA 4 in a test-tube, add an equal depth of aqueous potassium iodide then, | |
| add a few drops of starch solution. |
(ii) From these tests identify FA 4.
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(iii) Suggest what happened to the potassium iodide in its reaction with FA 4. Use your practical results to explain your answer.
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(b) FA 5, FA 6 and FA 7 each contain compounds of the same transition element.
(i) Complete the following table.
| test | observations |
|---|---|
| To 1 cm depth of aqueous iron(II) sulfate in a test-tube, add a few drops of FA 5. | |
| To 1 cm depth of FA 6 in a test-tube, add an equal depth of aqueous sodium hydroxide then, | |
| add 1 cm depth of hydrogen peroxide. | |
| To 1 cm depth of FA 6 in a test-tube, add aqueous barium chloride or aqueous barium nitrate then, | |
| add an excess of either hydrochloric acid or nitric acid. | |
| To 1 cm depth of hydrogen peroxide in a boiling tube, add a small spatula measure of FA 7. |
(ii) From these tests identify FA 6. Explain how your observations support this conclusion.
identity ......................................................................
evidence for cation ............................................................
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evidence for anion ............................................................
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(iii) It was suggested that $\text{Fe}^{2+}$ was oxidised when iron(II) sulfate reacted with FA 5.
To 1 cm depth of aqueous iron(II) sulfate in a test-tube add FA 5 dropwise until a pale permanent pink colour persists.
Devise and carry out a test to show that the $\text{Fe}^{2+}$ ions in this solution have been oxidised.
Record the test used and the result obtained in the space below.
590
545
533
