Chemistry 9701/33 — October/November 2018

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

questions

marks

120

minutes

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

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

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)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{Cu(s)}

FA 1 is a solution containing $151.9 \text{ g dm}^{-3}$ copper(II) sulfate, $\text{CuSO}_4$ .

FA 2 is magnesium powder, Mg.

(a)

Method

Weigh the container with FA 2 and record the mass in the space below.
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.
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.
Start timing and do not stop the clock until the whole experiment has been completed.
Record the temperature of the solution every half minute for 2 minutes.
At $2\frac{1}{2}$ minutes carefully transfer all of FA 2 into the solution in the cup and stir the mixture.
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.
Weigh the container with any residual FA 2 and record the mass below.
Calculate and record the mass of FA 2 used.

Keep FA 1 for use in Question 2.

Results

Table 1.1

(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 $10^\circ\text{C}$ 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.

Fig. 1.1

(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 = \dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots ^\circ\text{C}

(c)

Calculations

(i)

Use the information given to calculate the concentration of copper(II) sulfate in FA 1 in $\text{mol dm}^{-3}$ .

\text{concentration of FA 1} = \dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots \text{ mol dm}^{-3}

(ii)

Hence calculate the number of moles of copper(II) sulfate you used in (a).

\text{moles of CuSO}_4 = \dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots \text{ mol}

(iii)

Use your answer to (b)(ii) to calculate the heat energy evolved when FA 2 is added to FA 1.

(Assume $4.2\text{ J}$ of heat energy changes the temperature of $1.0\text{ cm}^3$ of the mixture by $1.0^\circ\text{C}$ .)

\text{heat energy evolved} = \dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots \text{ J}

(iv)

Calculate the enthalpy change, in $\text{kJ mol}^{-1}$ , when 1 mole of $\text{CuSO}_4$ reacts with magnesium.

\text{enthalpy change} = \underset{\text{(sign)}}{\dots\dots} \ \underset{\text{(value)}}{\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots\dots} \text{ kJ mol}^{-1}

(v)

Show by calculation that the magnesium was in excess.

(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.

(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.

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation9M
Q3Qualitative Analysis · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Presentation of Data and Observations15M

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