[Maximum number: 3]

This question is about copper and some of its compounds.

(a)

An electrochemical cell was made from the electrode systems represented by these half-equations:

\begin{array}{ll} \mathrm{Cu}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightleftharpoons \mathrm{Cu}(\mathrm{~s}) & E^{\ominus}=+0.34 \mathrm{~V} \\ \mathrm{Fe}^{3+}(\mathrm{aq})+\mathrm{e}^{-} \rightleftharpoons \mathrm{Fe}^{2+}(\mathrm{aq}) & E^{\ominus}=+0.77 \mathrm{~V} \end{array}

[ 1 ]

(i)

Calculate $E_{\text {cell }}^{\ominus}$ for the electrochemical cell.

[ 1 ]

(ii)

A student drew a diagram of an experiment to measure the standard emf of the cell.

Identify three mistakes in this diagram and the changes needed to correct them.

Assume that standard conditions were used.

(b)

Brass is an alloy of copper and zinc.

A student determined the percentage of copper in a sample of brass.

Procedure
- weigh the sample of brass
- place the brass in a beaker and add concentrated nitric acid until all the brass dissolves
- transfer the solution and washings to a $250.0 \mathrm{~cm}^{3}$ volumetric flask
- make the solution up to the mark with distilled water and mix well
- pipette $25.0 \mathrm{~cm}^{3}$ of the solution into a conical flask
- neutralise the excess nitric acid in the solution
- add $10 \mathrm{~cm}^{3}$ of potassium iodide solution (an excess) to the conical flask
- titrate the iodine produced with $0.100 \mathrm{~mol} \mathrm{dm}^{-3}$ sodium thiosulfate solution using starch indicator
- repeat the titration until concordant titres are obtained.

[ 2 ]

(i)

Copper and zinc both react with concentrated nitric acid to form the metal nitrates, nitrogen dioxide and water.

Write the balanced equation for the reaction of zinc with concentrated nitric acid.
State symbols are not required.

[ 1 ]

(ii)

State at what point in the titration the starch solution should be added.

[ 1 ]

(iii)

Only $\mathrm{Cu}^{2+}$ ions in the solution react with the aqueous potassium iodide.

2 \mathrm{Cu}^{2+}+4 \mathrm{I}^{-} \rightarrow 2 \mathrm{CuI}+\mathrm{I}_{2}

The iodine reacts with sodium thiosulfate solution.

Mass of brass sample $=3.90 \mathrm{~g}$
Mean titre of $0.100 \mathrm{~mol} \mathrm{dm}^{-3}$ sodium thiosulfate solution $=28.60 \mathrm{~cm}^{3}$
Calculate the percentage, by mass, of copper in this sample of brass.
Give your answer to an appropriate number of significant figures.

2 \mathrm{~S}_{2} \mathrm{O}_{3}^{2-}+\mathrm{I}_{2} \rightarrow \mathrm{~S}_{4} \mathrm{O}_{6}^{2-}+2 \mathrm{I}^{-}

\section*{Results \\ Results}
Mass of brass sample $=3.90 \mathrm{~g}$
Mean titre of $0.100 \mathrm{~mol} \mathrm{dm}^{-3}$ sodium thiosulfate solution $=28.60 \mathrm{~cm}^{3}$
Calculate the percentage, by mass, of copper in this sample of brass.
Give your answer to an appropriate number of significant figures.

[Maximum number: 3]

The apparatus shown was used to measure the standard electrode potential for the reduction of $\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}$ ions to $\mathrm{Cr}^{3+}$ ions in acid solution:

\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}(\mathrm{aq})+14 \mathrm{H}^{+}(\mathrm{aq})+6 \mathrm{e}^{-} \rightarrow 2 \mathrm{Cr}^{3+}(\mathrm{aq})+7 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})

(a)

Which material should be used for each electrode?

(b)

Solution 1 is

DO NOT WRITE IN THIS AREA	DO NOT WRITE IN THIS AREA	DO NOT WRITE IN THIS AREA

□ A $0.33 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{H}_{3} \mathrm{PO}_{4}(\mathrm{aq})$
□ B $0.50 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq})$
□ C $1.00 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{HCl}(\mathrm{aq})$
□ D $1.00 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{CH}_{3} \mathrm{COOH}(\mathrm{aq})$

[ 1 ]

(c)

Solution 2 contains 14.71 g of $\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}$ .

What mass of $\mathrm{Cr}_{2}\left(\mathrm{SO}_{4}\right)_{3} \cdot 18 \mathrm{H}_{2} \mathrm{O}$ should also be used?
$\left[M_{\mathrm{r}}\right.$ values: $\left.\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}=294.2 \quad \mathrm{Cr}_{2}\left(\mathrm{SO}_{4}\right)_{3} \cdot 18 \mathrm{H}_{2} \mathrm{O}=716.3\right]$

A

$8.95 \mathrm{~g}$

B

17.91 g

C

19.62 g

D

35.82 g

[ 1 ]

(d)

Solution 2 is best acidified with

A

$\mathrm{H}_{2} \mathrm{SO}_{4}$

B

HCl

C

HBr

D

$\mathrm{H}_{2} \mathrm{CrO}_{4}$

[ 1 ]

[Maximum number: 3]

This question is about the reaction

2 \mathrm{Fe}^{3+}(\mathrm{aq})+\mathrm{Ti}(\mathrm{~s}) \rightarrow 2 \mathrm{Fe}^{2+}(\mathrm{aq})+\mathrm{Ti}^{2+}(\mathrm{aq}) \quad E_{\text {cell }}^{\ominus}=+2.40 \mathrm{~V}

(a)

The electrode potential for the $\mathrm{Fe}^{3+} / \mathrm{Fe}^{2+}$ electrode system is +0.77 V .

What is the electrode potential for the $\mathrm{Ti}^{2+} / \mathrm{Ti}$ electrode system?

A

-3.17 V

B

-1.63 V

C

+1.63 V

D

+3.17 V

[ 1 ]

(b)

What metals should be used for the electrodes in the cell for this reaction?
□ A
□ B
□ C
□ D

[ 1 ]

(c)

The half-cell for the $\mathrm{Fe}^{3+} / \mathrm{Fe}^{2+}$ electrode system is prepared by mixing equal volumes of solutions of iron(II) sulfate, $\mathrm{FeSO}_{4}$ , and iron(III) sulfate, $\mathrm{Fe}_{2}\left(\mathrm{SO}_{4}\right)_{3}$ .

What concentrations of the original solutions are needed for the resulting mixture to be standard?
□ A
□ B
□ C
□ D

Use this space for any rough working. Anything you write in this space will gain no credit.

[ 1 ]

[Maximum number: 2]

A hydrogen-oxygen fuel cell is used to provide electrical energy for an electric motor in a car.

(a)

The electrolyte in the fuel cell is acidic. What is the half-equation at the anode?
□

A $1 / 2 \mathrm{O}_{2}(\mathrm{~g})+2 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{l})$

□

B $\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow 1 / 2 \mathrm{O}_{2}(\mathrm{~g})+2 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{e}^{-}$

□

C $\mathrm{H}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{e}^{-}$

□

D $2 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightarrow \mathrm{H}_{2}(\mathrm{~g})$

[ 1 ]

(b)

Hydrogen-oxygen fuel cells have advantages over methanol-oxygen fuel cells in vehicles.

Which of these is an advantage of the hydrogen-oxygen fuel cell?

A

more energy is released per mole of fuel used

B

emissions do not contribute to climate change

C

hydrogen is easier to store than methanol

D

only hydrogen can be obtained from renewable resources

[ 1 ]

[Maximum number: 5]

An electrochemical cell is made from the electrode systems shown by these half-equations.

\begin{aligned} \mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{e}^{-} & \rightleftharpoons \mathrm{Ag}(\mathrm{~s}) & E^{\ominus}=+0.80 \mathrm{~V} \\ \mathrm{VO}_{2}^{+}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})+\mathrm{e}^{-} & \rightleftharpoons \mathrm{VO}^{2+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) & E^{\ominus}=+1.00 \mathrm{~V} \end{aligned}

The apparatus used to measure the value for $E_{\text {cell }}^{\ominus}$ under standard conditions is shown.

(a)

Which silver compound could be used as the electrolyte in the left-hand half-cell?

A

silver nitrate

B

silver hydroxide

C

silver chloride

D

silver carbonate

[ 1 ]

(b)

The electrolyte in the right-hand half-cell is prepared using equal volumes of

A

$1 \mathrm{~mol} \mathrm{dm}^{-3}$ acidified $\mathrm{VO}_{2}^{+}(\mathrm{aq})$ and $1 \mathrm{~mol} \mathrm{dm}^{-3}$ acidified $\mathrm{VO}^{2+}(\mathrm{aq})$

B

$2 \mathrm{~mol} \mathrm{dm}^{-3}$ acidified $\mathrm{VO}_{2}^{+}(\mathrm{aq})$ and $2 \mathrm{~mol} \mathrm{dm}^{-3}$ acidified $\mathrm{VO}^{2+}(\mathrm{aq})$

C

$1 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{VO}_{2}^{+}(\mathrm{aq})$ and $1 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{HCl}(\mathrm{aq})$

D

$1 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{VO}^{2+}(\mathrm{aq})$ and $1 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{HCl}(\mathrm{aq})$

[ 1 ]

(c)

Which is the equation for the overall cell reaction under standard conditions?
□

A $\mathrm{VO}^{2+}(\mathrm{aq})+\mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{VO}_{2}^{+}(\mathrm{aq})+\mathrm{Ag}(\mathrm{s})+2 \mathrm{H}^{+}(\mathrm{aq})$

□

B $\mathrm{VO}_{2}^{+}(\mathrm{aq})+\mathrm{Ag}(\mathrm{s})+2 \mathrm{H}^{+}(\mathrm{aq}) \rightarrow \mathrm{VO}^{2+}(\mathrm{aq})+\mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})$

□

C $\mathrm{VO}^{2+}(\mathrm{aq})+3 \mathrm{Ag}(\mathrm{s})+2 \mathrm{H}^{+}(\mathrm{aq}) \rightarrow \mathrm{VO}_{2}^{+}(\mathrm{aq})+3 \mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})$

□

D $\mathrm{VO}_{2}^{+}(\mathrm{aq})+3 \mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{VO}^{2+}(\mathrm{aq})+3 \mathrm{Ag}(\mathrm{s})+2 \mathrm{H}^{+}(\mathrm{aq})$

[ 1 ]

(d)

Which is the value of $E_{\text {cell }}^{\ominus}$ in volts?

A

-1.80

B

-0.20

C

+0.20

D

+1.80

[ 1 ]

(e)

Which is the cell diagram for this cell, using the conventional representation of half-cells?

A

$\mathrm{Ag}(\mathrm{s})\left|\mathrm{Ag}^{+}(\mathrm{aq})\right|\left|\left[\mathrm{VO}_{2}^{+}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})\right]\right|\left[\mathrm{VO}^{2+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})\right] \mid \mathrm{Pt}(\mathrm{s})$

B

$\mathrm{Ag}(\mathrm{s})\left|\mathrm{Ag}^{+}(\mathrm{aq})\right|\left|\left[\mathrm{VO}_{2}^{+}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})\right],\left[\mathrm{VO}^{2+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})\right]\right| \mathrm{Pt}(\mathrm{s})$

C

$\mathrm{Ag}(\mathrm{s})\left|\mathrm{Ag}^{+}(\mathrm{aq})\right|\left|\left[\mathrm{VO}^{2+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})\right]\right|\left[\mathrm{VO}_{2}^{+}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})\right] \mid \mathrm{Pt}(\mathrm{s})$

D

$\mathrm{Ag}(\mathrm{s})\left|\mathrm{Ag}^{+}(\mathrm{aq})\right|\left|\left[\mathrm{VO}^{2+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})\right],\left[\mathrm{VO}_{2}^{+}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})\right]\right| \mathrm{Pt}(\mathrm{s})$

[ 1 ]

[Maximum number: 2]

A group of students carry out an experiment to find the concentration of chlorine, $\mathrm{Cl}_{2}(\mathrm{aq})$ , in a solution.

Excess potassium iodide solution is added to a $10.0 \mathrm{~cm}^{3}$ sample of the chlorine solution.

\mathrm{Cl}_{2}(\mathrm{aq})+2 \mathrm{I}^{-}(\mathrm{aq}) \rightarrow 2 \mathrm{Cl}^{-}(\mathrm{aq})+\mathrm{I}_{2}(\mathrm{aq})

The iodine produced is titrated with a solution of thiosulfate ions of known concentration, using starch indicator.

2 \mathrm{~S}_{2} \mathrm{O}_{3}^{2-}(\mathrm{aq})+\mathrm{I}_{2}(\mathrm{aq}) \rightarrow \mathrm{S}_{4} \mathrm{O}_{6}^{2-}(\mathrm{aq})+2 \mathrm{I}^{-}(\mathrm{aq})

The concentration of the $\mathrm{Cl}_{2}(\mathrm{aq})$ is between 0.038 and $0.042 \mathrm{~mol} \mathrm{dm}^{-3}$ .

(a)

What concentration of thiosulfate ions, in $\mathrm{mol} \mathrm{dm}^{-3}$ , is required to give a titre of approximately $20 \mathrm{~cm}^{3}$ ?

A

0.010

B

0.020

C

0.040

D

0.080

[ 1 ]

(b)

What is the colour change at the end-point of the titration?
□ A colourless to pale yellow
□ B pale yellow to colourless
□ C colourless to blue-black
□ D blue-black to colourless

VIIV SIHI NI JIIYM ION OC	VIAV SIHI NI JIIIM ION OC	VIIIV SIHI NI IIIYM ION OC

[ 1 ]

[Maximum number: 21]

This question is about manganese compounds. Some data are given below.

(a)

Write the ionic equation for the disproportionation of manganate( VI ) ions, $\mathrm{MnO}_{4}^{2-}$ , in acidic conditions, using relevant half-equations from the table.
State symbols are not required.

[ 2 ]

(b)

Calculate $E_{\text {cell }}^{\ominus}$ for the disproportionation of manganate(VI) ions in acidic conditions, stating whether or not the reaction is thermodynamically feasible.
(2)

[ 2 ]

(c)

Using the standard electrode potentials in the table, assess the thermodynamic feasibility of preparing manganate(VI) by reacting manganate(VII) and manganese(IV) oxide in alkaline conditions.

[ 4 ]

(d)

Steel is an alloy of iron and carbon. A group of students determined the iron content of a sample of steel wire by a titration method.

A known mass of the wire was dissolved in dilute sulfuric acid and the resulting solution made up to $250.0 \mathrm{~cm}^{3}$ with more dilute sulfuric acid and mixed thoroughly.

\mathrm{Fe}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{FeSO}_{4}+\mathrm{H}_{2}

$25.0 \mathrm{~cm}^{3}$ samples of the resulting solution were titrated with $0.0195 \mathrm{~mol} \mathrm{dm}^{-3}$ potassium manganate(VII) solution.

[ 9 ]

(i)

State the colour change at the end-point of the titration.

[ 1 ]

(ii)

One student used 1.53 g of the wire (weighed directly on the balance pan) and obtained a mean titre of $27.35 \mathrm{~cm}^{3}$ .

Using half-equations 3 and 5 from the table, calculate the percentage of iron in the steel wire. Give your answer to three significant figures.

[ 5 ]

(iii)

A second student carried out the same experiment but used distilled water to make up the solution in the volumetric flask.

A brown suspension formed during the titration.
Explain how, if at all, the titre value would be affected by this student's error.
(3)

[ 3 ]

(e)

The uncertainties of the apparatus used in the experiment in (b) are shown.

[ 4 ]

(i)

Complete the table.

[ 2 ]

(ii)

A third student obtained a value of 95.863% for the proportion of iron in the wire. State whether or not this student has given their answer to an appropriate number of significant figures. Justify your answer in terms of the total percentage uncertainty of the experiment.

[ 2 ]

Edexcel IAL Chemistry A2 Topic 16 Redox Equilibria

Exam points

Exam points

Question 1

Question 1(c)

Question 1(c)(i)

Question 1(c)(ii)

Question 1(d)

Question 1(d)(i)

Question 1(d)(iii)

Question 1(d)(iv)

Question 1

Question 1(a)

Question 1(b)

Question 1(c)

Question 1(d)

Question 2

Question 2(a)

Question 2(b)

Question 2(c)

Question 2

Question 2(a)

Question 2(b)

Question 3

Question 3(a)

Question 3(b)

Question 3(c)

Question 3(d)

Question 3(e)

Question 8

Question 8(a)

Question 8(c)

Question 18

Question 18(a)(i)

Question 18(a)(ii)

Question 18(a)(iii)

Question 18(b)

Question 18(b)(i)

Question 18(b)(ii)

Question 18(b)(iii)

Question 18(c)

Question 18(c)(i)

Question 18(c)(ii)