Principles of Electrochemistry (A Level Only) | CIE A Level Chemistry Structured Questions 2022

1a2 marks

This question is about electrolysis.

Explain what is meant by the term electrolysis.

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1b4 marks

Complete Table 1.1 to show the correct charge associated with each term.

Table 1.1

Term	Charge (positive / negative)
Anion
Anode
Cathode
Cation

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1c3 marks

i)

Explain what reduction and oxidation mean in terms of electrons.

[1]

ii)

Write a chemical equation for the reduction of copper(II) ions, Cu²⁺ to form copper metal.

[1]

iii)

Complete the chemical equation for the oxidation of chloride ions, Cl^– to form chlorine, Cl₂.

2Cl^– → .......... + ..........

[1]

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1a2 marks

Fig. 1.1 shows a lithium–iodine electrochemical cell. These cells are often used in pacemakers as they are reliable and have a life span in the region of 10 years.

5-3-1a-h-lithium-iodine-battery

Fig. 1.1

It consists of a lithium electrode and an inert electrode immersed in body fluids that are separated by a nickel mesh and collect charge from the anode. It has a high internal resistance which means that only a low current can be drawn.

Explain why the lithium-iodine electrochemical cell is a dry cell.

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1b1 mark

Write the overall equation for the reaction taking place at the electrodes of the lithium-iodine electrochemical cell when a current flows.

overall equation ................................................................................

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1c

1 mark

Table 1.1 lists electrode potentials for some electrode reactions.

Table 1.1

Electrode reaction	E^θ / V
I₂ + 2e^– $⇌$ 2I^–	+ 0.54
Li⁺ + e^– $⇌$ Li	– 3.04
Ni²⁺ + 2e^– $⇌$ Ni	– 0.25

Calculate the E^θ_cell for the lithium-iodine electrochemical cell.

E^θ_cell = .................... V

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1d

4 marks

A current of 1.5 × 10^–5 A is drawn from this cell.

Calculate the number of days for 0.08 g of the lithium electrode to be used up. Assume the current remains constant throughout this period. Show your working.

The Faraday constant, F = 9.65 × 10⁴ C mol^–1.

time = .............................. days

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1a2 marks

This question is about electrochemical cells.

Fig. 1.1 shows the apparatus used to measure the standard electrode potential, E^θ, of a cell composed of a Cu(II)/Cu half-cell and an Fe(II)/Fe half-cell.

5-3-1a-m-cu--fe-cell-a

Fig. 1.1

Finish the diagram by adding components to show the complete circuit. Label the components you add.

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1b3 marks

In the spaces below, identify or describe what the four letters A-D in Fig. 1.1 represent.

A	...........................................................................................
B	...........................................................................................
C	...........................................................................................
D	...........................................................................................

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1c1 mark

The standard electrode potentials of the two half-cells are as follows.

Fe²⁺+ 2e^–	$⇌$	Fe	E^θ= -0.44 V
Cu²⁺ + 2e^–	$⇌$	Cu	E^θ= +0.34 V

i)

Identify which electrode acts as the negative electrode.

[1]

ii)

Write the overall equation of the electrochemical cell.

[2]

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1d1 mark

E^θ_cellof this cell is +0.78 V.

State whether this reaction is feasible. Explain your answer.

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5a7 marks

The electrolytic purification of copper can be carried out in an apparatus similar to the one shown in Fig. 5.1.

5-3-5a-m-copper-purification-cell

Fig. 5.1

The impure copper anode contains small quantities of metallic nickel, zinc and silver, together with inert oxides and carbon resulting from the initial reduction of the copper ore with coke. The copper goes into solution at the anode, but the silver remains as the metal and falls to the bottom as part of the anode ‘sludge’. The zinc also dissolves.

Table 5.1 shows a list of standard electrode potentials at 298 K.

Table 5.1

Electrode reaction	E^θ / V
Ag⁺ + e^– $⇌$ Ag	+0.80
Cu²⁺ + 2e^– $⇌$ Cu	+0.34
Fe²⁺ + 2e^– $⇌$ Fe	-0.44
Ni²⁺ + 2e^– $⇌$ Ni	-0.25
SO₄^2– + 4H⁺ + 2e^– $⇌$ SO₂ + 2H₂O	+0.17
Zn²⁺ + 2e^– $⇌$ Zn	-0.76

i)

Write a half-equation including state symbols for the reaction of copper at the anode.

[1]

ii)

Use data from Table 5.1 to explain why silver remains as the metal.

[2]

iii)

Use data from Table 5.1 to predict what happens to the nickel at the anode.

[2]

iv)

Write a half-equation including state symbols for the main reaction at the cathode.

[1]

v)

Use data from Table 5.1 to explain why zinc is not deposited on the cathode.

[1]

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5b2 marks

As the electrolysis proceeds, the blue colour of the electrolyte slowly fades.

Suggest why the blue colour fades.

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5c

5 marks

Most of the current passed through the cell is used to dissolve the copper at the anode and precipitate pure copper onto the cathode. However, a small proportion of it is ‘wasted’ in dissolving the impurities at the anode which then remain in solution. When a current of 20.0 A was passed through the cell for 10.0 hours, it was found that 225 g of pure copper was deposited on the cathode.

[Faraday constant, F = 9.65 x 10⁴ C mol^-1]

i)

Calculate the number of moles of copper produced at the cathode.

[1]

ii)

Calculate the number of moles of electrons needed to produce this copper.

[1]

iii)

Calculate the number of moles of electrons that passed through the cell.

[2]

iv)

Hence calculate the percentage of the current through the cell that has been ‘wasted’ in dissolving the impurities at the anode.

[1]

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5d2 marks

Nickel often occurs in ores along with iron. After the initial reduction of the ore with coke, a nickel-iron alloy is formed.

Use data from Table 5.1 to explain why nickel can be purified by a similar electrolysis technique to that used for copper, using an impure nickel anode, a pure nickel cathode, and nickel sulfate as the electrolyte.

Explain what would happen to the iron during this process.

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Hard

Ionic compound	Product at anode	Product at cathode
NaH	Hydrogen	Sodium
CaCl₂
Pb₃O₄

Equation	Number of moles of electrons	Amount of charge / C
K⁺ + e^– → K	1	96 500
Cr³⁺ + 3e^– → Cr
S^2– → ........ + ........

	Equilibrium equation	Standard electrode potential, E^θ / V
A	Ag⁺ + e^– ⇌ Ag	+ 0.80
B	ClO^– + H₂O + 2e^– ⇌ Cl^– + 2OH^–	+ 0.89
C	2H₂O + 2e^– ⇌ H₂ + 2OH^–	– 0.83

Type of half-cell	Require a platinum electrode?
non-metal / non-metal ion	Yes

CIE A Level Chemistry

Topic Questions

5.3 Principles of Electrochemistry (A Level Only)

Electrode reaction	E^θ / V
Cu⁺ + e^– ⇌ Cu	+ 0.52
Cu²⁺ + 2e^– ⇌ Cu	+ 0.34
Cu²⁺ + e^– ⇌ Cu⁺	+ 0.15
Fe²⁺ + 2e^– ⇌ Fe	– 0.44
Fe³⁺ + 3e^– ⇌ Fe	– 0.04
Fe³⁺ + e^– ⇌ Fe²⁺	+ 0.77

Electrode reaction	E^θ / V
Cu⁺ + e^– $⇌$ Cu	+ 0.52
Cu²⁺ + 2e^– $⇌$ Cu	+ 0.34
Cu²⁺ + e^– $⇌$ Cu⁺	+ 0.15
2H₂O + 2e^– $⇌$ H₂ + 4OH^–	– 0.83
O₂ + 2H₂O + 4e^– $⇌$ 4OH^–	+ 0.40
½O₂ + 2H⁺ + 2e^– $⇌$ H₂O	+ 1.23
SO₄^2– + 4H⁺ + 2e^– $⇌$ SO₂ + 2H₂O	+ 0.17
S₂O₈^2– + 2e^– $⇌$ 2SO₄^2–	+ 2.01