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A more reactive metal will displace a less reactive metal from a solution of its salt. This reaction is exothermic. If the same reaction is set up in an electrochemical cell then, instead of an enthalpy change, electrical energy is produced and a cell voltage can be measured.
You are to plan an investigation of the reaction of three different metals (magnesium, iron and zinc) with aqueous copper(II) sulfate. You will plan to investigate whether there is a relationship between their cell potential values, $E^\circ_{\text{cell}}$, and their enthalpy changes of reaction, $\Delta H_r$.
Mg(s) + Cu$^{2+}$(aq) $\rightarrow$ Mg$^{2+}$(aq) + Cu(s)
Fe(s) + Cu$^{2+}$(aq) $\rightarrow$ Fe$^{2+}$(aq) + Cu(s)
Zn(s) + Cu$^{2+}$(aq) $\rightarrow$ Zn$^{2+}$(aq) + Cu(s)
__table__
Copper(II) sulfate solution is classified as a moderate hazard.
Zinc sulfate solution is classified as corrosive.
Iron(II) sulfate solution is classified as a health hazard.
(a) Predict how $\Delta H_r$ may change as $E^\circ_{\text{cell}}$ increases. Give a reason for your prediction.
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[1]
(b) The first part of the investigation is to determine the enthalpy change, $\Delta H_r$, for the reaction of the same number of moles of three powdered metals with 0.500 moldm$^{-3}$ copper(II) sulfate.
When determining the $\Delta H_r$ for the reaction of the metals listed above with aqueous copper(II) sulfate,
the independent variable is, ...................................
the dependent variable is, ............................................................
[2]
(c) (i) Draw a fully labelled diagram to show how the apparatus should be set up to allow you to determine the increase in temperature of aqueous copper(II) sulfate. You should use apparatus normally found in a school or college laboratory.
[1]
(ii) State the measurements you would make in your experiment.
..........................
.........................
[2]
(iii) Other than eye protection, state one precaution you would take to make sure that the experiment proceeds safely.
......................
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[1]
(iv) For the reaction with magnesium, calculate the mass of magnesium, in g, you would use so that it is in a small excess. You must show your working.
[A_{r}: Mg, 24.3]
mass of Mg = ............................. g [2]
(v) Explain why the metal used should be in powdered form rather than in strips.
.....
[1]
(vi) The aqueous copper(II) sulfate and metal mixture should be stirred continuously. Explain why.
...
[1]
(d) In one experiment, the increase in temperature when excess magnesium powder is added to 50.0cm$^{3}$ of 0.500moldm$^{-3}$ aqueous copper(II) sulfate is 58.5°C.
Calculate the enthalpy change for this reaction, $\Delta H_r$, in kJmol$^{-1}$.
Assume the specific heat capacity, c, of the reaction mixture is 4.18 Jg$^{-1}$K$^{-1}$.
Assume 1.0 cm$^{3}$ of 0.500 mol dm$^{-3}$ aqueous copper(II) sulfate has a mass of 1.0 g.
Include a sign in your answer.
Mg(s) + Cu$^{2+}$(aq) $\rightarrow$ Mg$^{2+}$(aq) + Cu(s)
$\Delta H_r$ = .................... kJmol$^{-1}$ [2]
(e) The second part of the investigation involves determining the cell potential, $E^\circ_{\text{cell}}$ for the three electrochemical cells.
__table__
Draw a diagram of the apparatus you would use to measure the $E^{\circ}_{\text{cell}}$ for the magnesium/copper cell. Your labels should include the names of the metals and the names and concentrations of the solutions you would use.
[3]
(f) Explain why the enthalpy change determination and cell potential determination should be carried out at the same temperature as each other.
...
[1]
(g) Accepted $E^\circ_{\text{cell}}$ values are shown for the cell reactions.
__table__
Use your prediction in (a), your answer to (d) and data from the table to predict $\Delta H_{r}$ values for reactions 2 and 3. Complete the table with these values.
[1]
594
500
598
(a) Process the results in the table to calculate both the masses of volatile liquid Y used and the volumes of vaporised Y. [2]
(b) Plot a graph on the grid on page 9 to show the relationship between mass of liquid Y and volume of vapour Y.
Use a cross (×) to plot each data point.
Draw the line of best fit. [2]
(c) Liquid Y evaporates easily, even at room temperature. This can cause anomalous results giving points below the line of best fit.
(i) Explain how such anomalies occur. [1]
(ii) With reference to the experimental procedure, explain how this source of error could be minimised. [1]
(d) (i) Determine the gradient of your graph. State the co-ordinates of both points you used for your calculation. Record the value of the gradient to three significant figures.
co-ordinates 1 .................................................................................................................
co-ordinates 2 .................................................................................................................
gradient = ................................................... [2]
(ii) Use the gradient value in (i) and the mathematical relationship on page 7 to calculate the experimentally determined relative molecular mass of Y.
experimentally determined $M_r$ of Y = ............................................... [2]
(e) Compound Y is a hydrocarbon that contains 85.7% carbon by mass.
The diagram shows the mass spectrum of compound Y.
Use all the information given to determine the molecular formula of Y.
molecular formula of Y ......................................................... [2]
585
593
598
