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You will determine the enthalpy change, \( \Delta H \), for the reaction between magnesium and aqueous copper(II) sulfate. To do this you will measure the change in temperature when magnesium powder reacts with aqueous copper(II) sulfate. The magnesium is in excess.
\[ \text{Mg(s) + CuSO}_4\text{(aq) } \rightarrow \text{ MgSO}_4\text{(aq) } + \text{ Cu(s)} \]
FA 1 is a solution containing 151.9 g dm\(^{-3}\) copper(II) sulfate, CuSO\(_4\).
FA 2 is magnesium powder, Mg.
(a) Method
\(\bullet\) Weigh the container with FA 2 and record the mass in the space below.
\(\bullet\) Support the plastic cup in the 250 cm\(^3\) beaker.
\(\bullet\) Use the measuring cylinder to transfer 25 cm\(^3\) of FA 1 into the plastic cup.
\(\bullet\) Place the thermometer in the solution and tilt the cup if necessary so that the bulb of the thermometer is fully covered. Record the temperature at time zero in the table of results.
\(\bullet\) Start timing and do not stop the clock until the whole experiment has been completed.
\(\bullet\) Record the temperature of the solution every half minute for 2 minutes.
\(\bullet\) At 2\(\frac{1}{2}\) minutes carefully transfer all of FA 2 into the solution in the cup and stir the mixture.
\(\bullet\) Record the temperature of the mixture at 3 minutes and complete the table by recording the temperature every half minute. Stir the mixture continuously between thermometer readings.
\(\bullet\) Weigh the container with any residual FA 2 and record the mass below.
\(\bullet\) Calculate and record the mass of FA 2 used.
Keep FA 1 for use in Question 2.
Results
[Table_1]
(b) (i) 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 at least 10 \(^{\circ}\)C above your highest recorded temperature.
You will use the graph to determine the theoretical maximum temperature rise at 2\(\frac{1}{2}\) minutes.
Draw two lines of best fit through the points on your graph, the first for the temperature before adding FA 2 and the second for the cooling of the mixture. Label any points you consider anomalous.
(ii) Extrapolate the two lines to 2\(\frac{1}{2}\) minutes, draw a vertical line between the two lines of best fit and so determine the theoretical rise in temperature, \( \Delta T \), at this time.
\( \Delta T = \text{.............................} \ ^{\circ}\text{C} \) [2]
(c) Calculations
(i) Use the information given to calculate the concentration of copper(II) sulfate in FA 1 in mol dm\(^{-3}\).
concentration of FA 1 = \text{............................. mol dm}^{-3} \) [1]
(ii) Hence calculate the number of moles of copper(II) sulfate you used in (a).
moles of CuSO\(_4\) = \text{............................. mol} \) [1]
(iii) Use your answer to (b)(ii) to calculate the heat energy evolved when FA 2 is added to FA 1.
(Assume 4.2 J of heat energy changes the temperature of 1.0 cm\(^3\) of the mixture by 1.0 \(^{\circ}\)C.)
heat energy evolved = \text{............................. J} \) [1]
(iv) Calculate the enthalpy change, in kJ mol\(^{-1}\), when 1 mole of CuSO\(_4\) reacts with magnesium.
enthalpy change = ...... ............................. \text{ kJ mol}^{-1} \)
(\text{sign} \ \text{value}) [1]
(v) Show by calculation that the magnesium was in excess.
[1]
(d) (i) A student suggested that the calculated enthalpy change would be more accurate if magnesium turnings were used instead of magnesium powder.
State whether you agree with the student and give a reason for your answer.
\text{.................................................................................................................................................................}
\text{.................................................................................................................................................................}
\text{.................................................................................................................................................................} [1]
(ii) The enthalpy change determined in (c)(iv) is only an approximation of the actual value.
Suggest and explain one improvement to the method in (a) you would make to increase the accuracy of the experiment.
\text{.................................................................................................................................................................}
\text{.................................................................................................................................................................}
\text{.................................................................................................................................................................} [1]
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In this experiment you will determine the enthalpy change, $\Delta H$, for the reaction between aqueous copper(II) sulfate and zinc. The zinc is in excess.
FA 1 is aqueous copper(II) sulfate solution.
FA 3 is zinc powder.
(a) You are provided with FA 1 and FA 3. Carry out the following procedure:
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Weigh a clean, dry plastic cup and record its mass.
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Add between 1.80 g and 2.00 g of zinc powder (FA 3) into the cup and record the new mass.
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Place the plastic cup in a 250 cm³ beaker for insulation.
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Measure 25.0 cm³ of FA 1 using a measuring cylinder.
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Place a thermometer in FA 1 and record the initial temperature to the nearest 0.5 °C.
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Pour FA 1 into the plastic cup containing zinc powder.
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Stir the mixture gently. Record the highest temperature reached to the nearest 0.5 °C.
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Calculate and record:
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the mass of FA 3 (zinc) used
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the temperature rise (ΔT)
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(i) Present your data in a clear table with headings.
(ii) Keep the remaining zinc powder for use in Question 3.
(b)
(i) Use the temperature change you recorded in (a) and the value of ΔH to calculate the enthalpy change, in kJ mol⁻¹, for the reaction of 1 mole of copper(II) sulfate with excess zinc.
(Use your answer from Question 1(c)(ii) for number of moles of CuSO₄ used in 25.0 cm³ solution)
Answer to minimum 2 significant figures.
Also state the correct sign of the enthalpy change.
ΔH = ………… kJ mol⁻¹
(ii) The maximum error in a single thermometer reading is ±0.5 °C.
Calculate the percentage error in the temperature rise using the formula:
Give your answer to at least 2 significant figures.
(c)
A student repeats the experiment using 50.0 cm³ of FA 1 and double the mass of FA 3.
Tick (✓) the correct prediction about the new temperature rise compared to the original experiment.
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The temperature rise would be approximately half that for the first experiment.
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The temperature rise would be approximately the same as that for the first experiment.
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The temperature rise would be approximately double that for the first experiment.
Explain your choice:
(d)
You have previously calculated:
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ΔH for the reaction:
Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s) in Question 1(c)(iv) -
ΔH for the reaction:
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s) in part 2(b)(i)
Use these values to construct a Hess’s Cycle and determine the enthalpy change for:
Mg(s)+ZnSO4(aq)→MgSO4(aq)+Zn(s)\text{Mg(s)} + \text{ZnSO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{Zn(s)}
(i) Write the equation and show the direction of energy change arrows clearly.
(ii) Use the Hess’s to calculate ΔH for the Mg + ZnSO₄ reaction
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(a) FA 4 is a solution of a transition metal compound dissolved in acid.
Carry out the following tests on FA 4 and record your observations.
Keep the mixture of FA 4 and FA 3 for use in (c).
The solution formed from the mixture is FA 6.
Complete part (b) while FA 4 and FA 3 continue reacting.
(b) (i) FA 5 is the acid used to make FA 4. The anion in FA 5 is listed in the Qualitative Analysis Notes.
List the reagents you would use to identify this anion and state which acid the reagents are testing for.
....................................................................................................................................
Give the formula of the anion present in FA 5. Write 'unknown' if your test could not positively identify this anion.
.................................................................................................................... [2]
(ii) Carry out one of your tests in (i) and record your observations.
.................................................................................................................................... [2]
(c) Carry out the following tests on FA 6 (the solution formed in (a)).
(d) (i) State the type of reaction occurring in (c) when acidified potassium manganate(VII) was added to FA 6.
Give a reason for your answer.
....................................................................................................................................
.................................................................................................................................... [2]
(ii) The ion present in FA 4 when in acidic solution is usually written as MO_2^+. where M represents the transition metal.
Calculate the oxidation state of M in MO_2^+.
oxidation state of M = ........................................ [1]
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