Chemistry 9701/34 — October/November 2022

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 are to determine the enthalpy change for a metal displacement reaction using a known volume and concentration of copper(II) sulfate and an excess of powdered zinc.

\text{Cu}^{2+}(\text{aq}) + \text{Zn}(\text{s}) \rightarrow \text{Zn}^{2+}(\text{aq}) + \text{Cu}(\text{s})

FB 1 is $0.80\text{ mol dm}^{-3}$ copper(II) sulfate, $\text{CuSO}_4$ .
FB 2 is zinc, Zn.

(a)

Method

Weigh the container with FB 2. Record the mass.
Support the cup in the $250\text{ cm}^3$ beaker.
Use the $50\text{ cm}^3$ measuring cylinder to transfer $30.0\text{ cm}^3$ of FB 1 into the cup.
Place the thermometer in the solution. Measure and record the initial temperature of the solution. This is the temperature at time zero, $t = 0$ .
Start timing and do not stop the clock until the whole experiment has been completed.
Measure and record the temperature of the solution every half minute for 2 minutes.
At time $t = 2\frac{1}{2}$ minutes tip all the FB 2 into the solution and stir the mixture.
Measure and record the temperature of the mixture at $t = 3$ minutes and every half minute until $t = 9$ minutes. Stir the mixture between thermometer readings.
Weigh the container with any residue of FB 2. Record the mass.
Calculate and record the mass of FB 2 added to the solution.

Results

time / minutes	0	$\frac{1}{2}$	1	$1\frac{1}{2}$	2	$2\frac{1}{2}$	3	$3\frac{1}{2}$	4
temperature / $^\circ\text{C}$						[crossed]

time / minutes	$4\frac{1}{2}$	5	$5\frac{1}{2}$	6	$6\frac{1}{2}$	7	$7\frac{1}{2}$	8	$8\frac{1}{2}$	9
temperature / $^\circ\text{C}$

(b)

(i)

Plot a graph of temperature on the $y$ -axis against time on the $x$ -axis on the grid. The scale for temperature should extend $5\text{ }^\circ\text{C}$ above your highest recorded temperature. You will use this graph to determine the theoretical maximum temperature rise at $2\frac{1}{2}$ minutes.

Fig. 1.1

(ii)

Draw two lines of best fit through the points on your graph. The first line should be for the temperature before adding FB 2 and the second for the cooling of the mixture.

Extrapolate the two lines to $2\frac{1}{2}$ minutes. Draw a vertical line at $2\frac{1}{2}$ minutes. Determine the theoretical rise in temperature at this time.

theoretical rise in temperature at $2\frac{1}{2}$ minutes = .............................. $^\circ\text{C}$

(c)

Calculations

(i)

Calculate the amount, in mol, of copper(II) sulfate in FB 1 added to the cup in (a).

amount of $\text{CuSO}_4$ = .............................. mol

(ii)

Show that the amount of zinc you added was in excess.

(iii)

Use your answer to (b)(ii) to calculate the heat energy, in joules, given out when FB 2 is added to FB 1.

heat energy = .............................. J

(iv)

Calculate the enthalpy change of reaction, $\Delta H_r$ , for the reaction carried out in (a).
Show your working.

\Delta H_r = \text{.............................. kJ mol}^{-1}

\text{(sign) } \qquad \text{(value)}

(d)

A student carries out two further experiments using method (a). In the first experiment, FB 1 is made using a fresh sample of hydrated copper(II) sulfate, $\text{CuSO}_4\cdot5\text{H}_2\text{O}$ .
In the second experiment, FB 1 is made using an old sample of hydrated copper(II) sulfate.

The same mass of solid is used to make the solutions in these experiments.

In the second experiment, the student obtains a value for $\Delta H_r$ that is $10.7\text{ kJ mol}^{-1}$ greater in magnitude than the value from the first experiment.

Suggest what may be deduced about the formula of the old sample of hydrated copper(II) sulfate.


The water of crystallisation is greater than 5.
The formula, $\text{CuSO}_4\cdot5\text{H}_2\text{O}$ , is correct.
The water of crystallisation is less than 5.

Tick the appropriate box and explain your answer.

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2 more questions

Q2Presentation of Data and Observations · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Qualitative Analysis11M
Q3Qualitative Analysis · Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation14M

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