AS & A Level Chemistry - 9701 Paper 3 2014 Winter Zone 1

Determining the enthalpy change for Reaction 1
\[ \text{Mg(s) + H}_2\text{SO}_4\text{(aq) \rightarrow MgSO}_4\text{(aq) + H}_2\text{(g)} \] Reaction 1
(a) Method
FA 1 is 1.00 mol dm$^{-3}$ sulfuric acid, \text{H}_2\text{SO}_4.
FA 2 is magnesium powder, \text{Mg}.
Read through the method before you start any practical work and prepare a suitable table for your results.
\begin{itemize}
\item Weigh the stoppered tube containing FA 2. Record the mass.
\item Support the plastic cup in the 250 cm$^3$ beaker.
\item Use the measuring cylinder to transfer 25 cm$^3$ of FA 1 into the plastic cup.
\item Measure the temperature of FA 1 in the plastic cup and start the stop clock. Record this temperature as being the temperature at time = 0.
\item Measure, and record, the temperature of this FA 1 every half minute for 2 minutes.
\item At time = 2 1/2 minutes add the FA 2 to the acid and stir carefully to reduce acid spray.
\item Measure the temperature of the mixture in the cup at time = 3 minutes and then every half minute up to time = 7 minutes.
\item Continue stirring occasionally throughout this time.
\item Weigh the stoppered tube that had contained FA 2. Record the mass.
\item Calculate and record the mass of FA 2 added to the sulfuric acid.
\item Rinse the plastic cup with water and shake to dry.\end{itemize}

(b) (i) On the grid below plot a graph of temperature (y-axis) against time (x-axis).
(ii) Complete the graph by inserting two, straight lines of best fit:
\begin{itemize}
\item one to show the temperature up to time = 2 \(\frac{1}{2} \) minutes,
\item one to show the temperature after time = 2 \(\frac{1}{2} \) minutes.\end{itemize}

(iii) From your graph, use the two straight lines of best fit to calculate the change in temperature at time = 2 \(\frac{1}{2} \) minutes.
temperature change = .................. $^{\circ}$C [4]

(c) Calculations
(i) In the reaction in (a), the sulfuric acid was in excess. Without carrying out any additional tests, what observation could you have made during your experiment to confirm this?
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(ii) Calculate the energy change that occurred during the reaction in (a).
[Assume that 4.2 J is needed to raise the temperature of 1.0 cm$^3$ of solution by 1.0$^{\circ}$C.]
energy change = .................. J
(iii) Use your answer to (ii) to calculate the enthalpy change, in kJ mol$^{-1}$, for the reaction between sulfuric acid and magnesium.
\[ \text{Mg(s) + H}_2\text{SO}_4\text{(aq) \rightarrow MgSO}_4\text{(aq) + H}_2\text{(g)} \]
Reaction 1
enthalpy change for Reaction 1 = .......... .............. kJ mol$^{-1}$ [4]

Determining the enthalpy change for Reaction 2
\[ \text{Mg(s) + CuSO}_4\text{(aq) \rightarrow MgSO}_4\text{(aq) + Cu(s)} \] Reaction 2
(d) Method
FA 3 is 1.00 mol dm$^{-3}$ copper(II) sulfate, CuSO$_4$.
FA 4 and FA 5 are magnesium powder, Mg.
Read through the method before you start any practical work and prepare a suitable table for your results.
\begin{itemize}
\item Weigh the stoppered tube containing FA 4. Record the mass.
\item Support the plastic cup in the 250 cm$^3$ beaker.
\item Use the measuring cylinder to transfer 25 cm$^3$ of FA 3 into the plastic cup.
\item Measure the temperature of FA 3 in the plastic cup and record the temperature.
\item Add the FA 4 to the FA 3 in the cup and stir the mixture constantly.
\item Measure and record the maximum temperature reached during the reaction.
\item Calculate and record the maximum temperature change that occurred during the reaction.
\item Weigh the stoppered tube that had contained FA 4. Record the mass.
\item Calculate and record the mass of FA 4 added to the copper(II) sulfate.
\item Empty the contents of the plastic cup into the 100 cm$^3$ beaker labelled waste.
\item Rinse the plastic cup and shake to dry.
\item Repeat this experiment using FA 5 in place of FA 4.\end{itemize} [2]

(e) Calculations
(i) Using your results from (d), calculate the mean temperature rise.
mean temperature rise = .................. $^{\circ}$C
(ii) Using your results from (d), calculate the mean mass of magnesium used.
mean mass = .................. g
(iii) Show, using a suitable calculation, that the copper(II) sulfate was in excess in these reactions.
(iv) Using your values from (i) and (ii), calculate the enthalpy change, in kJ mol$^{-1}$, for the reaction between magnesium and copper(II) sulfate.
[Assume that 4.2 J is needed to raise the temperature of 1.0 cm$^3$ of solution by 1.0$^{\circ}$C.]
\[ \text{Mg(s) + CuSO}_4\text{(aq) \rightarrow MgSO}_4\text{(aq) + Cu(s)} \] Reaction 2
enthalpy change for Reaction 2 = .......... .............. kJ mol$^{-1}$ [4]

Enthalpy change for Reaction 3
Reaction 3 is shown below.
\[ \text{Cu(s) + H}_2\text{SO}_4\text{(aq) \rightarrow CuSO}_4\text{(aq) + H}_2\text{(g)} \] Reaction 3
(f) Use your values for the enthalpy changes for Reactions 1 and 2 to calculate the enthalpy change for Reaction 3.
\[ \text{Mg(s) + H}_2\text{SO}_4\text{(aq) \rightarrow MgSO}_4\text{(aq) + H}_2\text{(g)} \]
Reaction 1
\[ \text{Mg(s) + CuSO}_4\text{(aq) \rightarrow MgSO}_4\text{(aq) + Cu(s)} \] Reaction 2
Show clearly how you obtained your answer.
(If you were unable to calculate the enthalpy changes for Reactions 1 and 2, you should assume that the value for Reaction 1 is −444 kJ mol$^{-1}$ and that the value for Reaction 2 is −504 kJ mol$^{-1}$. Note: these are not the correct values.)
enthalpy change for Reaction 3 = .......... .............. kJ mol$^{-1}$ [2]

(g) (i) The method you used to determine the enthalpy change for Reaction 1 was more accurate than the method you used to determine the enthalpy change for Reaction 2. Suggest two reasons why the method used for Reaction 2 was less accurate. Explain your answers.
1 ......................................................................................................................................................
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2 ......................................................................................................................................................
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(ii) A student suggested that the accuracy of the method used for Reaction 2 could be improved by using a larger volume of copper(II) sulfate. Is this a correct suggestion? Give a reason for your answer.
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[3]

(a) FA 6 is a solid that contains one cation and one anion. One of the ions present is included in the lists on pages 14 and 15. This ion contains the element nitrogen.
(i) State which nitrogen-containing ions could be present. Select reagents for use in tests that would distinguish between them.
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(ii) Carry out tests on FA 6 using the reagents selected in (i) to identify the nitrogen-containing ion. Record your tests and observations in the space below.
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(iii) Identify the nitrogen-containing ion in FA 6.
Ion present is .................................................................

(b) Half fill the 250cm³ beaker with water and heat the water to about 60°C. This is the water bath to be used in one of the following tests.
Carry out the following tests on FA 7 and complete the table below.