A student investigates the charge ($z+$) carried by aqueous manganese ions, $\text{Mn}^{z+}\text{(aq)}$. The electrochemical cell shown is set up for this investigation with the following two half-cells: a standard copper(II) ion/copper half-cell ($E^\ominus = +0.340\text{ V}$) a half-cell made from manganese and $0.500\text{ mol dm}^{-3}$ $\text{Mn}^{z+}\text{(aq)}$. (a) Label the items P and Q and state the concentration of the copper(II) ion solution in the copper…

Chemistry 9701/51 — May/June 2019

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

questions

marks

minutes

Topics Planning · Analysis, Conclusions and Evaluation

Q1PlanningAnalysis, Conclusions and EvaluationFree sample

A student investigates the charge ( $z+$ ) carried by aqueous manganese ions, $\text{Mn}^{z+}\text{(aq)}$ . The electrochemical cell shown is set up for this investigation with the following two half-cells:

a standard copper(II) ion/copper half-cell ( $E^\ominus = +0.340\text{ V}$ )
a half-cell made from manganese and $0.500\text{ mol dm}^{-3}$ $\text{Mn}^{z+}\text{(aq)}$ .

(a)

Label the items P and Q and state the concentration of the copper(II) ion solution in the copper half-cell.

Fig. 1.1

concentration of the copper(II) ion solution in the copper half-cell = ..............................

(b)

During the investigation the student plans to use solutions of $\text{Mn}^{z+}\text{(aq)}$ of lower concentration than $0.500\text{ mol dm}^{-3}$ .

(i)

Calculate the volume of $0.500\text{ mol dm}^{-3}$ $\text{Mn}^{z+}\text{(aq)}$ needed to prepare $100.0\text{ cm}^3$ of $0.200\text{ mol dm}^{-3}$ $\text{Mn}^{z+}\text{(aq)}$ .

volume = .............................. $\text{cm}^3$

(ii)

Describe how, using a $100\text{ cm}^3$ volumetric flask, the student should prepare exactly $100.0\text{ cm}^3$ of $0.200\text{ mol dm}^{-3}$ $\text{Mn}^{z+}\text{(aq)}$ using the volume of $0.500\text{ mol dm}^{-3}$ $\text{Mn}^{z+}\text{(aq)}$ calculated in (b)(i) and standard school or college apparatus.

(iii)

The cell potential of the electrochemical cell in (a) is measured. The $0.500\text{ mol dm}^{-3}$ $\text{Mn}^{z+}\text{(aq)}$ is then replaced by the $0.200\text{ mol dm}^{-3}$ solution and the cell potential is measured again. This is repeated for other lower concentrations of $\text{Mn}^{z+}\text{(aq)}$ . All measurements are made at $25^\circ\text{C}$ .

The results of the experiment are shown in the table.

Complete column three of the table, calculating $\log[\text{Mn}^{z+}]$ to two decimal places.
Complete column four of the table, calculating $E$ , the electrode potential of each manganese half-cell, to three decimal places, using the equation shown.

E(\text{manganese half-cell}) = E_{\text{cell}} + 0.340\text{ V}

$[\text{Mn}^{z+}]$ / $\text{mol dm}^{-3}$	cell potential, $E_{\text{cell}}$ / $\text{V}$	$\log[\text{Mn}^{z+}]$	electrode potential of each manganese half-cell, $E$ / $\text{V}$
$5.0 \times 10^{-1}$	$-1.529$	$-0.30$	$-1.189$
$2.0 \times 10^{-1}$	$-1.541$
$1.0 \times 10^{-1}$	$-1.550$
$7.5 \times 10^{-2}$	$-1.553$
$2.5 \times 10^{-2}$	$-1.567$
$8.0 \times 10^{-3}$	$-1.582$
$6.0 \times 10^{-3}$	$-1.590$
$4.0 \times 10^{-3}$	$-1.591$
$3.0 \times 10^{-3}$	$-1.594$
$5.0 \times 10^{-4}$	$-1.617$

(c)

Plot a graph of electrode potential of manganese half-cell ( $y$ -axis) against $\log[\text{Mn}^{z+}]$ ( $x$ -axis). Use a cross ( $\times$ ) to plot each data point. Draw a line of best fit.

Fig. 1.2

(d)

(i)

Circle the most anomalous point on your graph.

(ii)

The student is careful to ensure that all solutions used are at the same temperature in all experiments.

Suggest a possible explanation for the position of the anomalous point circled in (d)(i) relative to the line of best fit.

(e)

Your graph is a plot of $E$ against $\log[\text{Mn}^{z+}]$ and can be analysed using the Nernst equation at $25^\circ\text{C}$ .

E = E^\ominus + \frac{0.059}{z} \log[\text{Mn}^{z+}]

where:

$z$ is the value of the charge carried by the manganese ion
$E$ is the electrode potential/V
$E^\ominus$ is the standard electrode potential/V

Use the Nernst equation and your graph to find the standard electrode potential, $E^\ominus$ , of the manganese half-cell.

$E^\ominus$ = .............................. V

(f)

(i)

Determine the gradient of the graph.
State the co-ordinates of both points you used for your calculation.
Record the value of the gradient to three significant figures.

co-ordinates 1 .............................................. co-ordinates 2 ..............................................

gradient = ..............................

(ii)

Use your answer to (f)(i) and the Nernst equation to calculate the value of $z$ to three significant figures and give the formula of the manganese ion.
Your calculation must show the use of the Nernst equation.

(If you were unable to calculate an answer to (f)(i) you may use the value 0.0197. This is not the correct value.)

$z$ = ..............................

formula of manganese ion = ..............................

(g)

Lowering $[\text{Mn}^{z+}]$ causes the value of the electrode potential of the manganese half-cell to become more negative.

Suggest why this happens.

The rest of this paper

1 more questions

Q2Analysis, Conclusions and Evaluation · Planning14M

Loading the full paper…