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(a) Method
Read the whole method before starting any practical work.
- Weigh the container with FB 2 and record the mass in the space below.
- Support the plastic cup in the 250 cm³ beaker.
- Use the measuring cylinder to transfer 25 cm³ of FB 1 into the plastic cup.
- Place the thermometer in the solution and record the initial temperature in a suitable table of results. Tilt the cup if necessary so that the bulb of the thermometer is fully covered. This is the temperature at time zero $(t = 0)$.
- Start timing and do not stop the clock until the whole experiment has been completed at $t = 8$ minutes.
- Measure and record the temperature of the FB 1 in the cup every half minute up to and including $t = 2$ minutes.
- At $t = 2\frac{1}{2}$ minutes add all the FB 2 into the cup and stir the contents until $t$ is nearly 3 minutes.
- Measure and record the temperature of the mixture in the cup every half minute from $t = 3$ minutes until $t = 8$ minutes. Stir occasionally throughout this time.
- Weigh the container and any residual FB 2. Record this mass and calculate the mass of FB 2 added.
Results
Mass
Temperature
(b) Plot a graph of temperature on the $y$-axis against time on the $x$-axis on the grid below. The scale for temperature should extend 3°C above your highest recorded temperature. You will use the graph to determine the theoretical maximum temperature rise at $t = 2\frac{1}{2}$ minutes.
Draw two lines of best fit through the points on your graph, the first for the temperature before adding FB 2 and the second for the temperature of the mixture after addition of FB 2. Extrapolate the lines to $t = 2\frac{1}{2}$ minutes and determine the theoretical maximum temperature rise, $\Delta T$.
Theoretical maximum temperature rise at $t = 2\frac{1}{2}$ minutes, $\Delta T = \text{.........................} \text{°C}$
(c) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Use your answer to (b) to calculate the heat energy, in J, given out when FB 2 was added to the FB 1 in the cup. (Assume that 4.2 J of heat energy raises the temperature of 1.0 cm³ of the mixture by 1.0°C.)
Heat energy given out = ................................ J
(ii) Use your answer to (i) and the Periodic Table on page 12 to calculate the enthalpy change, in $\text{kJ mol}^{-1}$, for the displacement reaction.
$$\text{Zn(s)} + \text{CuSO}_4(\text{aq}) \rightarrow \text{Cu(s)} + \text{ZnSO}_4(\text{aq})$$
You should assume that FB 2 was pure zinc for this calculation.
Enthalpy change, $\Delta H = \text{...... ..........................} \, \text{kJ mol}^{-1}$
(d) The accepted value for the enthalpy change of this reaction is $-217 \, \text{kJ mol}^{-1}$.
Assuming no heat loss and that the other metals present in FB 2 do not react with aqueous copper(II) sulfate, calculate the percentage of zinc present in FB 2.
Percentage of Zn = ............................ %
(e) A student carried out the same experiment but used pieces of zinc instead of zinc powder. All quantities and the initial temperature of the aqueous copper(II) sulfate remained the same.
State and explain what effects this change would have on the graph plotted.
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Solid hydrated copper(II) sulfate has the formula CuSO₄·xH₂O where x is the number of moles of water of crystallisation present in 1 mole of compound.
You will determine the equation for the reaction that occurs when hydrated copper(II) sulfate is heated to remove the water of crystallisation producing anhydrous copper(II) sulfate.
FB 3 is hydrated copper(II) sulfate CuSO₄·xH₂O.
(a) Method
Record all masses in the space below.
• Weigh the crucible and add 2.2–2.4 g of FB 3.
• Weigh the crucible plus FB 3.
• Place the crucible on the pipe-clay triangle and heat it gently for approximately 4 minutes.
• Leave the crucible to cool and reweigh the crucible plus residue.
Keep the crucible and residue for test (c).
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(b) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Calculate the mass of anhydrous copper(II) sulfate, CuSO₄, produced after heating.
mass of CuSO₄ = ......................... g
(ii) Calculate the mass of water lost by heating.
mass of water = ......................... g
(iii) Use your answers to (i) and (ii) and the Periodic Table on page 12 to deduce the value of x in the formula CuSO₄·xH₂O.
x is .........................
(iv) Use your answer to (iii) to complete the equation for the reaction that occurs when hydrated copper(II) sulfate is heated. You should include state symbols.
CuSO₄·........H₂O(......) → CuSO₄(.......) + ........H₂O(......)
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(c) Place the cooled crucible, with the residue, on a heatproof mat and carefully add a few drops of water.
(i) Note your observations.
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(ii) Explain your observations in (i) in terms of the reaction occurring.
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(d) Two students carried out the experiment in (a) and obtained values for x that did not agree with the accepted value. One student calculated a value that was less than the accepted value and the other student calculated a value that was more than the accepted value.
In each case, suggest a reason for the error and an improvement that could be made to minimise it. You can assume that the calculations were correctly carried out.
Value less than accepted value
error ..................................................................................................................
improvement .....................................................................................................
Value more than accepted value
error ..................................................................................................................
improvement .....................................................................................................
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500
(a) FB 4 and FB 5 are aqueous solutions of equal concentrations, in mol dm^-3. Each contains one anion and one cation. The cation is the same in both FB 4 and FB 5.
Half fill the 250 cm^3 beaker with water. Heat the water to about 80°C and then turn off the Bunsen burner. This is the hot water bath needed in the tests below.
To about a 2 cm depth of aqueous silver nitrate in a test-tube, add a few drops of aqueous sodium hydroxide to give a grey/brown precipitate. Then add aqueous ammonia dropwise until the precipitate just disappears. This solution is Tollens' reagent and is needed in a test below.
(i) Carry out the tests on separate samples of FB 4 and FB 5 and complete the table.
| test | observations |
|------|-------------|
| | FB 4 | FB 5 |
|-------------------|-------------|
| To a 1 cm depth of solution in a test-tube in a test-tube rack, add a spatula measure of sodium carbonate. | | |
| To a 1 cm depth of solution in a test-tube, add a few drops of acidified potassium manganate(VII). Place the test-tube in the hot water bath. | | |
| To a 1 cm depth of Tollens' reagent in a test-tube, add a few drops of solution. Place the test-tube in the hot water bath and leave for several minutes. | | |
(ii) From your observations in (i), identify the cation present in both FB 4 and FB 5.
cation ..........................
(iii) From your observations in (i), what can be deduced about the anion present in FB 4?
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(iv) Place a 1 cm depth of FB 4 and FB 5 separately in two test-tubes.
Measure and record the temperature of the two solutions.
FB 4 .................................... °C FB 5 .................................... °C
To each solution, add an approximately 2 cm length of magnesium ribbon. Measure and record the maximum temperature reached in each test-tube.
FB 4 + Mg .................................... °C FB 5 + Mg .................................... °C
(v) Explain why there is a difference in the temperature rise for the reactions of magnesium with solutions FB 4 and FB 5.
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(b) FB 6 is a solid that contains two cations from those listed on page 10.
You are to plan a series of experiments that will enable you to identify the cations present. You should then carry out your plan, record all the observations you made in a suitable table and identify the cations present.
cations present are .......................... and ..........................
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