[Maximum number: 6]

An electrochemical cell is constructed using two half-cells.
- a $\mathrm{Br}_{2} / \mathrm{Br}^{-}$ half-cell
- an $\mathrm{Mn}^{3+} / \mathrm{Mn}^{2+}$ half-cell

(a)

An aqueous solution of copper(II) sulfate is electrolysed using copper electrodes. A current of 1.50 A is passed for 3.00 hours. 5.09 g of copper is deposited on the cathode.

The charge on one electron is $-1.60 \times 10^{-19} \mathrm{C}$ .
The relative atomic mass of copper is 63.5.
Use these data to calculate an experimentally determined value for the Avogadro constant, L. Give your answer to three significant figures.

\begin{aligned} & L= \\ & \mathrm{mol}^{-1} \end{aligned}

[ 4 ]

(b)

Explain why magnesium metal cannot be obtained by the electrolysis of dilute aqueous magnesium sulfate. Your answer should include data from the Data Booklet.

[ 2 ]

[Maximum number: 6]

An electrochemical cell is constructed using two half-cells.
- an $\mathrm{Sn}^{4+} / \mathrm{Sn}^{2+}$ half-cell
- an $\mathrm{A} l^{3+} / \mathrm{A} l$ half-cell

(a)

Aluminium is produced industrially by electrolysis of a melt containing large amounts of $\mathrm{A} l^{3+}$ ions.

Calculate the mass of aluminium that is obtained when a current of 300000 A is passed for 24 hours. Give your answer to three significant figures.
mass = units =

[ 4 ]

(b)

Explain why chromium metal cannot be obtained by the electrolysis of dilute aqueous chromium(II) sulfate. Your answer should include data from the Data Booklet.

[ 2 ]

(a)

The main ore of manganese, pyrolusite, is mainly $\mathrm{MnO}_{2}$ . A solution of $\mathrm{SnCl}_{2}$ can be used to estimate the percentage of $\mathrm{MnO}_{2}$ in a sample of pyrolusite, using the following method.
- A known mass of pyrolusite is warmed with an acidified solution containing a known amount of $\mathrm{SnCl}_{2}$ .
- The excess $\mathrm{Sn}^{2+}(\mathrm{aq})$ ions are titrated with a standard solution of $\mathrm{KMnO}_{4}$ .

In one such experiment, 0.100 g of pyrolusite was warmed with an acidified solution containing $2.00 \times 10^{-3} \mathrm{molSn}^{2+}$ . After the reaction was complete, the mixture was titrated with $0.0200 \mathrm{moldm}^{-3} \mathrm{KMnO}_{4}$ , and required $18.1 \mathrm{~cm}^{3}$ of this solution to reach the end point.

The equation for the reaction between $\mathrm{Sn}^{2+}(\mathrm{aq})$ and $\mathrm{MnO}_{4}^{-}(\mathrm{aq})$ is as follows.

2 \mathrm{MnO}_{4}^{-}+5 \mathrm{Sn}^{2+}+16 \mathrm{H}^{+} \rightarrow 2 \mathrm{Mn}^{2+}+5 \mathrm{Sn}^{4+}+8 \mathrm{H}_{2} \mathrm{O}

[ 6 ]

(i)

Calculate the percentage of $\mathrm{MnO}_{2}$ in this sample of pyrolusite by the following steps.
- number of moles of $\mathrm{MnO}_{4}{ }^{-}$ used in the titration
- number of moles of $\mathrm{Sn}^{2+}$ this $\mathrm{MnO}_{4}^{-}$ reacted with
- number of moles of $\mathrm{Sn}^{2+}$ that reacted with the 0.100 g sample of pyrolusite
- number of moles of $\mathrm{MnO}_{2}$ in 0.100 g pyrolusite. Use your equation in (i).
- mass of $\mathrm{MnO}_{2}$ in 0.100 g pyrolusite
- percentage of $\mathrm{MnO}_{2}$ in pyrolusite

[ 6 ]

[Maximum number: 5]

When dilute sulfuric acid is electrolysed, water is split into hydrogen and oxygen.

2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow 2 \mathrm{H}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g})

A current of x A is passed through the solution for 14.0 minutes. $462 \mathrm{~cm}^{3}$ of hydrogen are produced at the cathode, measured under room conditions.

(a)

Calculate the number of hydrogen molecules produced during the electrolysis.

number of hydrogen molecules =

[ 2 ]

(b)

Calculate the total number of electrons transferred to produce this number of hydrogen molecules.
total number of electrons =

[ 1 ]

(c)

Calculate the quantity of charge, in coulombs, of the total number of electrons calculated in (b).
quantity of charge = C

[ 1 ]

(d)

Calculate the current, x, passed during this experiment.
x = A

[ 1 ]

(a)

Iron(III) chloride, $\mathrm{FeCl}_{3}$ , is used to dissolve unwanted copper from printed circuit boards (PCBs) by the following reaction.

2 \mathrm{FeCl}_{3}(\mathrm{aq})+\mathrm{Cu}(\mathrm{~s}) \rightarrow 2 \mathrm{FeCl}_{2}(\mathrm{aq})+\mathrm{CuCl}_{2}(\mathrm{aq})

A solution in which $\left[\mathrm{Fe}^{3+}(\mathrm{aq})\right]$ was originally equal to $1.50 \mathrm{~mol} \mathrm{dm}^{-3}$ was re-used several times to dissolve copper from the PCBs, and was then titrated as follows.

A $2.50 \mathrm{~cm}^{3}$ sample of the partially-used-up solution was acidified and titrated with $0.0200 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{KMnO}_{4}$ .
This oxidised any $\mathrm{FeCl}_{2}$ in the solution back to $\mathrm{FeCl}_{3}$ .
It was found that $15.0 \mathrm{~cm}^{3}$ of $\mathrm{KMnO}_{4}(\mathrm{aq})$ was required to reach the end point.

[ 6 ]

(i)

Calculate the number of moles of $\mathrm{MnO}_{4}^{-}$ used in the titration.

(ii)

Calculate the number of moles of $\mathrm{Fe}^{2+}$ in $2.50 \mathrm{~cm}^{3}$ of the partially-used-up solution.

(iii)

Calculate the $\left[\mathrm{Fe}^{2+}\right]$ in the partially-used-up solution.

[Maximum number: 4]

Potassium iodide, KI, is used as a reagent in both inorganic and organic chemistry.

(a)

A student electrolyses a solution of KI(aq) for 8 minutes using a direct current. The half-equation for the reaction that occurs at the anode is given.

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

[ 4 ]

(i)

Write a half-equation for the reaction that occurs at the cathode.

Include state symbols.

[ 1 ]

(ii)

After the electrolysis, the $I_{2}(a q)$ produced requires $21.35 \mathrm{~cm}^{3}$ of $0.100 \mathrm{moldm}^{-3} \mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq})$ to react completely.

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

Calculate the average current used in 8 minutes during the electrolysis.
current = A

[ 3 ]

[Maximum number: 1]

A sports medal has a total surface area of $150 \mathrm{~cm}^{2}$ . It was evenly coated with silver by electrolysis. Its mass increased by 0.216 g .

How many atoms of silver were deposited per $\mathrm{cm}^{2}$ on the surface of the medal?

A

$8.0 \times 10^{18}$

B

$1.8 \times 10^{19}$

C

$8.7 \times 10^{20}$

D

$1.2 \times 10^{21}$

[Maximum number: 1]

Aluminium is the most abundant metal in the Earth's crust. The extraction of aluminium is carried out by the electrolysis of aluminium oxide dissolved in molten cryolite.

Which material is used for each of the electrodes in this electrolysis?

anode

cathode

aluminium

carbon

steel

aluminium

[Maximum number: 1]

Aluminium is extracted by the electrolysis of a molten mixture containing aluminium oxide.
By a similar method, magnesium is extracted by the electrolysis of a molten mixture containing magnesium chloride.

Which statement about this electrolysis is correct?

A

Chloride ions travel to the anode and are oxidised to chlorine gas.

B

Chloride ions travel to the anode and are reduced to chlorine gas.

C

Chloride ions travel to the cathode and are oxidised to chlorine gas.

D

Chloride ions travel to the cathode and are reduced to chlorine gas.

[Maximum number: 1]

Impure copper is purified by electrolysis. The electrolyte used in this process is aqueous copper(II) sulfate.

Which reaction takes place at the anode?

A

$\mathrm{Cu}(\mathrm{s}) \rightarrow \mathrm{Cu}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-}$

B

$\mathrm{Cu}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-} \rightarrow \mathrm{Cu}(\mathrm{s})$

C

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

D

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