Chemistry 9701/31 — October/November 2014

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

questions

marks

120

minutes

Topics Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Qualitative Analysis · Presentation of Data and Observations

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationQualitative AnalysisFree sample

You are to determine the enthalpy change of reaction, $\Delta H$ , for the reaction shown below.

\text{Cu(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{CuSO}_4\text{(aq)} + \text{H}_2\text{(g)}

Since copper is an unreactive metal it does not react directly with dilute acids. You will therefore need to find the enthalpy change of reaction for two reactions that do occur. The equations for these two reactions are below.

\begin{aligned} \text{Mg(s)} + \text{H}_2\text{SO}_4\text{(aq)} &\rightarrow \text{MgSO}_4\text{(aq)} + \text{H}_2\text{(g)} && \text{Reaction 1} \\ \text{Mg(s)} + \text{CuSO}_4\text{(aq)} &\rightarrow \text{MgSO}_4\text{(aq)} + \text{Cu(s)} && \text{Reaction 2} \end{aligned}

You will carry out experiments to find the enthalpy changes for each of Reaction 1 and Reaction 2 and use these values to calculate the enthalpy change for the reaction of copper with sulfuric acid.

TURN OVER FOR EXPERIMENTAL METHOD

(a)

Determining the enthalpy change for Reaction 1

\text{Mg(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{H}_2\text{(g)} \quad \text{Reaction 1}

Method

FA 1 is $1.00\text{ mol dm}^{-3}$ sulfuric acid, $\text{H}_2\text{SO}_4$ .
FA 2 is magnesium powder, Mg.

Read through the method before you start any practical work and prepare a suitable table for your results.

Weigh the stoppered tube containing FA 2. Record the mass.
Support the plastic cup in the $250\text{ cm}^3$ beaker.
Use the measuring cylinder to transfer $25\text{ cm}^3$ of FA 1 into the plastic cup.
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.
Measure, and record, the temperature of this FA 1 every half minute for 2 minutes.
At time = $2\frac{1}{2}$ minutes add the FA 2 to the acid and stir carefully to reduce acid spray.
Measure the temperature of the mixture in the cup at time = 3 minutes and then every half minute up to time = 7 minutes.
Continue stirring occasionally throughout this time.
Weigh the stoppered tube that had contained FA 2. Record the mass.
Calculate and record the mass of FA 2 added to the sulfuric acid.
Rinse the plastic cup with water and shake to dry.

(b)

(i)

On the grid below plot a graph of temperature ( $y$ -axis) against time ( $x$ -axis).

Grid 1

(ii)

Complete the graph by inserting two, straight lines of best fit:

one to show the temperature up to time = $2\frac{1}{2}$ minutes,
one to show the temperature after time = $2\frac{1}{2}$ minutes.

(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 = ................. $^\circ\text{C}$

(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?

(ii)

Calculate the energy change that occurred during the reaction in (a).
[Assume that $4.2\text{ J}$ is needed to raise the temperature of $1.0\text{ cm}^3$ of solution by $1.0\text{ }^\circ\text{C}$ .]

energy change = ................. J

(iii)

Use your answer to (ii) to calculate the enthalpy change, in $\text{kJ mol}^{-1}$ , for the reaction between sulfuric acid and magnesium.
[ $A_r\text{: Mg, 24.3}$ ]

\text{Mg(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{H}_2\text{(g)} \quad \text{Reaction 1}

enthalpy change for Reaction 1 = $\underset{\text{(sign)}}{\text{.................}}$ $\underset{\text{(value)}}{\text{.................}}$ $\text{kJ mol}^{-1}$

(d)

Determining the enthalpy change for Reaction 2

\text{Mg(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{Cu(s)} \quad \text{Reaction 2}

Method

FA 3 is $1.00\text{ mol dm}^{-3}$ copper(II) sulfate, $\text{CuSO}_4$ .
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.

Weigh the stoppered tube containing FA 4. Record the mass.
Support the plastic cup in the $250\text{ cm}^3$ beaker.
Use the measuring cylinder to transfer $25\text{ cm}^3$ of FA 3 into the plastic cup.
Measure the temperature of FA 3 in the plastic cup and record the temperature.
Add the FA 4 to the FA 3 in the cup and stir the mixture constantly.
Measure and record the maximum temperature reached during the reaction.
Calculate and record the maximum temperature change that occurred during the reaction.
Weigh the stoppered tube that had contained FA 4. Record the mass.
Calculate and record the mass of FA 4 added to the copper(II) sulfate.
Empty the contents of the plastic cup into the $100\text{ cm}^3$ beaker labelled waste.
Rinse the plastic cup and shake to dry.
Repeat this experiment using FA 5 in place of FA 4.

(e)

Calculations

(i)

Using your results from (d), calculate the mean temperature rise.

mean temperature rise = ................. $^\circ\text{C}$

(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 $\text{kJ mol}^{-1}$ , for the reaction between magnesium and copper(II) sulfate.
[Assume that $4.2\text{ J}$ is needed to raise the temperature of $1.0\text{ cm}^3$ of solution by $1.0\text{ }^\circ\text{C}$ .]
[ $A_r\text{: Mg, 24.3}$ ]

\text{Mg(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{Cu(s)} \quad \text{Reaction 2}

enthalpy change for Reaction 2 = $\underset{\text{(sign)}}{\text{.................}}$ $\underset{\text{(value)}}{\text{.................}}$ $\text{kJ mol}^{-1}$

(f)

Enthalpy change for Reaction 3

Reaction 3 is shown below.

\text{Cu(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{CuSO}_4\text{(aq)} + \text{H}_2\text{(g)} \quad \text{Reaction 3}

Use your values for the enthalpy changes for Reactions 1 and 2 to calculate the enthalpy change for Reaction 3.

\begin{aligned} \text{Mg(s)} + \text{H}_2\text{SO}_4\text{(aq)} &\rightarrow \text{MgSO}_4\text{(aq)} + \text{H}_2\text{(g)} && \text{Reaction 1} \\ \text{Mg(s)} + \text{CuSO}_4\text{(aq)} &\rightarrow \text{MgSO}_4\text{(aq)} + \text{Cu(s)} && \text{Reaction 2} \end{aligned}

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 $-444\text{ kJ mol}^{-1}$ and that the value for Reaction 2 is $-504\text{ kJ mol}^{-1}$ . Note: these are not the correct values.)

enthalpy change for Reaction 3 = $\underset{\text{(sign)}}{\text{.................}}$ $\underset{\text{(value)}}{\text{.................}}$ $\text{kJ mol}^{-1}$

(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.

..........................................................................................................................................
..........................................................................................................................................

(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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Q2Qualitative Analysis · Analysis, Conclusions and Evaluation · Manipulation, Measurement and Observation15M

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