AS & A Level Chemistry - 9701 Paper 3 2009 Winter Zone 1

You are provided with the following reagents.

• two weighing bottles labelled FA 1, each containing between 2.90 g and 3.00 g of zinc powder
• FA 2, 0.80 mol dm$^{-3}$ copper sulfate, CuSO$_4$

You are to determine the enthalpy change, $\Delta H$, for the following reaction.
$$\text{Zn}(s) + \text{CuSO}_4(aq) \longrightarrow \text{Cu}(s) + \text{ZnSO}_4(aq)$$
You will carry out the experimental procedure twice.
Read through the instructions below before starting the experiment.
(a) You will weigh each bottle and later in the experiment weigh it again after the zinc powder has been tipped into copper sulfate solution. In the space below prepare a table to record the weighings and the mass of zinc powder used in each experiment.
Weigh accurately, to at least one decimal place, one of the weighing bottles labelled FA 1. Record this mass in the table you have prepared. [1]
(b) Procedure

• Support the plastic cup in the 250 cm$^3$ beaker and, using a pipette, place 25.0 cm$^3$ of FA 2 into the plastic cup.
• Stir gently, taking a temperature reading every $\frac{1}{2}$ minute until a steady temperature has been obtained for a period of at least 2 minutes. You may need to tilt the beaker in order to cover the bulb of the thermometer with solution.
• On a precise minute reading tip the zinc powder from the weighing bottle into the plastic cup. Do not read the temperature at this time or at the following $\frac{1}{2}$ minute.
• Continue to stir the mixture thoroughly. Starting 1 minute after the addition of the zinc powder, record the temperature every $\frac{1}{2}$ minute until the temperature has reached a maximum value and then decreased steadily for at least 5 minutes.
• Reweigh the empty weighing bottle. Record the mass of the bottle + any residual zinc powder and the mass of zinc powder used in the experiment in the table you prepared in (a).
• Record your results in an appropriate form in the space on the following page.
• Repeat the experiment using the contents of the second weighing bottle and 25.0 cm$^3$ copper sulfate solution pipetted into a clean plastic cup.

(b) continued
Results Make certain your readings of temperature display the precision of the apparatus used.
(c) Plot your temperature and time readings separately for each experiment on the grids on the next page. Your temperature axis should extend 10 °C above the highest temperature you recorded. Draw lines as instructed below.
On each graph draw a horizontal straight line through the steady initial temperature.
Extrapolate the cooling section of each graph back to the time when you added the zinc powder. Draw construction lines on the graphs to deduce the “theoretical” temperature rise at the moment of mixing the reagents. [4]
(d) The “theoretical” temperature rises are ................ °C and ................ °C.
The mean “theoretical” temperature rise is ................ °C. [1]
Calculations
Show working and appropriate significant figures in all of your calculations. [2]
(e) Calculate how many moles of copper sulfate, CuSO$_4$, were pipetted into the plastic cup.
................ mol of CuSO$_4$ were pipetted into the cup
For each experiment calculate how many moles of zinc powder were added to the plastic cup.
[$A_r$: Zn, 65.4]

1$^{st}$ experiment	2$^{nd}$ experiment
In the 1$^{st}$ experiment ................. mol of zinc powder were added to the plastic cup. In the 2$^{nd}$ experiment ................. mol of zinc powder were added to the plastic cup.

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(f) Use your answers to (e) and the equation for the reaction to determine which reagent was in excess and which was the limiting reagent. Explain your answer.
$$\text{Zn}(s) + \text{CuSO}_4(aq) \longrightarrow \text{Cu}(s) + \text{ZnSO}_4(aq)$$
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(g) From your mean “theoretical” temperature rise at the time of mixing, calculate the heat energy released in the plastic cup by the reaction of zinc powder with copper sulfate solution.
[You may assume that 4.3 J are required to raise the temperature of 1 cm$^3$ of any solution by 1 °C and that the mass of any solid may be ignored.]
.................. ...... of heat energy are released. [1]
(h) Calculate, correct to 3 significant figures, the enthalpy change in kJ mol$^{-1}$ for the following reaction.
$$\text{Zn}(s) + \text{CuSO}_4(aq) \longrightarrow \text{Cu}(s) + \text{ZnSO}_4(aq)$$
$$\Delta H = .................................. \text{ kJ mol}^{-1}$$ [2]
(i) Identify and explain one source of error in the experiment you have carried out.
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(j) Suggest a way in which the experimental method you used could be improved in a school or college laboratory in order to minimise this error.
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(a) Heat the boiling-tube containing FA 5 gently at first then more strongly.
Record your observations in the space below.
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(b) In their boiling-tubes, dissolve FA 3, FA 4 and the cold residue after heating FA 5 in a minimum of dilute nitric acid and then add distilled water so that each boiling-tube is approximately 2/3 full. Warm to dissolve if necessary.
Record your observations in the space below.
Use these solutions for tests (d), (e) and (f).
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(c) Which anion can be identified from your observations in (a) and (b)?
Explain your answer.
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(d) The cations present in FA 3, FA 4 and FA 5 can be identified by reaction of each solution with aqueous sodium hydroxide and with aqueous ammonia.
React 1 cm depth of each of the solutions prepared in (b) with each of these two reagents.

Record, in an appropriate form in the space below, your observations for these reactions.

Conclusions
Using your observations you should be able to identify the cation present in two of the solutions. For the remaining solution you should be able to identify two possible cations.

FA 3 contains the cation(s) ...................................... .
FA 4 contains the cation(s) ...................................... .
FA 5 contains the cation(s) ...................................... .
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(e) Use the information on pages 11 and 12 to select a reagent to distinguish between the two possible cations identified as present in one of the solutions in (d).

Carry out the test with the selected reagent.

reagent ..................................................................................
observation ..................................................................................
conclusion ..................................................................................
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(f) Carry out the following tests.

``` | test | observations | | | FA 3 | FA 4 | FA 5 | |---------------------------------------------------------------|------|------|------| | To 1 cm depth of solution in a test-tube, add 1 cm depth | | | | | of aqueous barium nitrate, | | | | | then | | | | | add 2 cm depth of dilute nitric acid. | | | | | To 1 cm depth of solution in a test-tube, add 1 cm depth | | | | | of aqueous silver nitrate, | | | | | then | | | | | allow any precipitate formed to settle, pour off the solution | | | | | and add aqueous ammonia to the precipitate. | | | | ```
What conclusions can be made from the observations above?
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