Chemistry 9701/52 — May/June 2016

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

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Topics Analysis, Conclusions and Evaluation · Planning

Q1PlanningAnalysis, Conclusions and EvaluationFree sample

A more reactive metal will displace a less reactive metal from a solution of its salt. This reaction is exothermic. If the same reaction is set up in an electrochemical cell then, instead of an enthalpy change, electrical energy is produced and a cell voltage can be measured.

You are to plan an investigation of the reaction of three different metals (magnesium, iron and zinc) with aqueous copper(II) sulfate. You will plan to investigate whether there is a relationship between their cell potential values, $E^\ominus_{\text{cell}}$ , and their enthalpy changes of reaction, $\Delta H_r$ .

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

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

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

Copper(II) sulfate solution is classified as a moderate hazard.

Zinc sulfate solution is classified as corrosive.

Iron(II) sulfate solution is classified as a health hazard.

(a)

Predict how $\Delta H_r$ may change as $E^\ominus_{\text{cell}}$ increases. Give a reason for your prediction.

(b)

The first part of the investigation is to determine the enthalpy change, $\Delta H_r$ , for the reaction of the same number of moles of three powdered metals with $0.500\text{ mol dm}^{-3}$ copper(II) sulfate.

When determining the $\Delta H_r$ for the reaction of the metals listed above with aqueous copper(II) sulfate,

the independent variable is,

the dependent variable is.

(c)

You are provided with a sample of powdered metal and $50.0\text{ cm}^3$ of $0.500\text{ mol dm}^{-3}$ aqueous copper(II) sulfate.

(i)

Draw a fully labelled diagram to show how the apparatus should be set up to allow you to determine the increase in temperature of aqueous copper(II) sulfate.

You should use apparatus normally found in a school or college laboratory.

(ii)

State the measurements you would make in your experiment.

(iii)

Other than eye protection, state one precaution you would take to make sure that the experiment proceeds safely.

(iv)

For the reaction with magnesium, calculate the mass of magnesium, in g, you would use so that it is in a small excess. You must show your working.

[ $A_r$ : Mg, 24.3]

(v)

Explain why the metal used should be in powdered form rather than in strips.

(vi)

The aqueous copper(II) sulfate and metal mixture should be stirred continuously. Explain why.

(d)

In one experiment, the increase in temperature when excess magnesium powder is added to $50.0\text{ cm}^3$ of $0.500\text{ mol dm}^{-3}$ aqueous copper(II) sulfate is $58.5^\circ\text{C}$ .

Calculate the enthalpy change for this reaction, $\Delta H_r$ , in $\text{kJ mol}^{-1}$ .
Assume the specific heat capacity, $c$ , of the reaction mixture is $4.18\text{ J g}^{-1}\text{K}^{-1}$ .
Assume $1.0\text{ cm}^3$ of $0.500\text{ mol dm}^{-3}$ aqueous copper(II) sulfate has a mass of $1.0\text{ g}$ .
Include a sign in your answer.

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

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

(e)

The second part of the investigation involves determining the cell potential, $E^\ominus_{\text{cell}}$ , for the three electrochemical cells.

cell reaction
$\text{Mg(s)} + \text{Cu}^{2+}\text{(aq)} \rightarrow \text{Mg}^{2+}\text{(aq)} + \text{Cu(s)}$
$\text{Zn(s)} + \text{Cu}^{2+}\text{(aq)} \rightarrow \text{Zn}^{2+}\text{(aq)} + \text{Cu(s)}$
$\text{Fe(s)} + \text{Cu}^{2+}\text{(aq)} \rightarrow \text{Fe}^{2+}\text{(aq)} + \text{Cu(s)}$

Draw a diagram of the apparatus you would use to measure the $E^\ominus_{\text{cell}}$ for the magnesium/copper cell. Your labels should include the names of the metals and the names and concentrations of the solutions you would use.

(f)

Explain why the enthalpy change determination and cell potential determination should be carried out at the same temperature as each other.

(g)

Accepted $E^\ominus_{\text{cell}}$ values are shown for the cell reactions.

	cell reaction	$E^\ominus_{\text{cell}}/\text{V}$
1	$\text{Mg(s)} + \text{Cu}^{2+}\text{(aq)} \rightarrow \text{Mg}^{2+}\text{(aq)} + \text{Cu(s)}$	+2.72
2	$\text{Zn(s)} + \text{Cu}^{2+}\text{(aq)} \rightarrow \text{Zn}^{2+}\text{(aq)} + \text{Cu(s)}$	+1.10
3	$\text{Fe(s)} + \text{Cu}^{2+}\text{(aq)} \rightarrow \text{Fe}^{2+}\text{(aq)} + \text{Cu(s)}$	+0.78

Use your prediction in (a), your answer to (d) and data from the table to predict $\Delta H_r$ values for reactions 2 and 3.
Complete the table with these values.

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