Chemistry 9701/52 — February/March 2021

Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme

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Topics Planning · Analysis, Conclusions and Evaluation

Q1PlanningAnalysis, Conclusions and EvaluationFree sample

Zinc metal reacts with aqueous copper(II) sulfate.

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

The enthalpy change of this reaction, $\Delta H$ , can be determined by adding excess zinc powder to a measured volume of $0.500\text{ mol dm}^{-3}$ aqueous copper(II) sulfate.

The temperature of $25.0\text{ cm}^3$ of $0.500\text{ mol dm}^{-3}$ aqueous copper(II) sulfate is recorded for three minutes. At four minutes $3\text{ g}$ , an excess, of zinc powder is added and the mixture is continuously stirred. The temperature is recorded at times shown in the table.

time/min	0	1	2	3	$4\frac{1}{2}$	5	$5\frac{1}{2}$	6	$6\frac{1}{2}$	7	8	9	10
temperature / °C	18	19.5	19.5	19.5	32.5	38	36	34	33	32.5	31.5	31	31

(a)

Use the results table to deduce the graduations on the thermometer that is used to record these temperature readings.

(b)

Draw a labelled diagram of the apparatus set up at one minute.

(c)

Plot a graph of temperature ( $y$ -axis) against time ( $x$ -axis). Use a cross ( $\times$ ) to plot each data point. Draw a line of best fit during cooling.

Extrapolate the cooling curve back to four minutes and determine the temperature change during the reaction.

Fig. 1.1

temperature change = .............................. °C

(d)

Use the formula $\Delta H = -mc\Delta T$ to determine the enthalpy change of reaction, $\Delta H$ , in $\text{kJ mol}^{-1}$ .

Assume:

mass of $1.00\text{ cm}^3$ of solution = $1.00\text{ g}$
$c = 4.18\text{ J g}^{-1}\text{ K}^{-1}$

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

(e)

Heat loss is a major source of error in the results of this experiment.

Suggest how the following changes would affect the amount of heat loss, if at all.

Explain your answer in each case.

(i)

The mass of zinc is doubled.

effect on heat loss ...............................................................................................................

explanation .........................................................................................................................

(ii)

The concentration of $25.0\text{ cm}^3$ of aqueous copper(II) sulfate is doubled. The amount of zinc used is still an excess.

effect on heat loss ...............................................................................................................

explanation .........................................................................................................................

(iii)

The volume of $0.500\text{ mol dm}^{-3}$ aqueous copper(II) sulfate is doubled. The amount of zinc used is still an excess.

effect on heat loss ...............................................................................................................

explanation .........................................................................................................................

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