Chemistry 9701/36 — October/November 2016

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

Zinc metal is extracted from ores that, in addition to compounds of zinc, can also contain copper and silver. This means zinc metal is often impure.

You will investigate the percentage purity of a sample of zinc by reacting it with excess aqueous copper(II) sulfate and comparing the enthalpy change of the displacement reaction with the accepted value.

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

FB 1 is $1.00\text{ mol dm}^{-3}$ copper(II) sulfate, $\text{CuSO}_4$ .
FB 2 is powdered impure zinc, $\text{Zn}$ .

(a)

Method

Read the whole method before starting any practical work.

Weigh the container with FB 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 FB 1 into the plastic cup.
Place the thermometer in the solution and record the initial temperature in a suitable table of results. Tilt the cup if necessary so that the bulb of the thermometer is fully covered. This is the temperature at time zero ( $t = 0$ ).
Start timing and do not stop the clock until the whole experiment has been completed at $t = 8$ minutes.
Measure and record the temperature of the FB 1 in the cup every half minute up to and including $t = 2$ minutes.
At $t = 2\frac{1}{2}$ minutes add all the FB 2 into the cup and stir the contents until $t$ is nearly 3 minutes.
Measure and record the temperature of the mixture in the cup every half minute from $t = 3$ minutes until $t = 8$ minutes. Stir occasionally throughout this time.
Weigh the container and any residual FB 2. Record this mass and calculate the mass of FB 2 added.

Results

Mass

Temperature

(b)

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 $3\text{ }^\circ\text{C}$ above your highest recorded temperature.
You will use the graph to determine the theoretical maximum temperature rise at $t = 2\frac{1}{2}$ minutes.

Graph 1

Draw two lines of best fit through the points on your graph, the first for the temperature before adding FB 2 and the second for the temperature of the mixture after addition of FB 2.
Extrapolate the lines to $t = 2\frac{1}{2}$ minutes and determine the theoretical maximum temperature rise, $\Delta T$ .

theoretical maximum temperature rise at $t = 2\frac{1}{2}$ minutes, $\Delta T$ = ............................ $^\circ\text{C}$

(c)

Calculations

Show your working and appropriate significant figures in the final answer to each step of your calculations.

(i)

Use your answer to (b) to calculate the heat energy, in J, given out when FB 2 was added to the FB 1 in the cup.
(Assume that $4.2\text{ J}$ of heat energy raises the temperature of $1.0\text{ cm}^3$ of the mixture by $1.0\text{ }^\circ\text{C}$ .)

heat energy given out = ............................ J

(ii)

Use your answer to (i) and the Periodic Table on page 12 to calculate the enthalpy change, in $\text{kJ mol}^{-1}$ , for the displacement reaction.

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

You should assume that FB 2 was pure zinc for this calculation.

enthalpy change, $\Delta H$ = ............................ $\text{kJ mol}^{-1}$

(d)

The accepted value for the enthalpy change of this reaction is $-217\text{ kJ mol}^{-1}$ .

Assuming no heat loss and that the other metals present in FB 2 do not react with aqueous copper(II) sulfate, calculate the percentage of zinc present in FB 2.

percentage of Zn = ............................ %

(e)

A student carried out the same experiment but used pieces of zinc instead of zinc powder. All quantities and the initial temperature of the aqueous copper(II) sulfate remained the same.

State and explain what effects this change would have on the graph plotted.

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation13M
Q3Qualitative Analysis · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Presentation of Data and Observations13M

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