Electrolysis (CIE A Level Chemistry)

A student decided to determine the value of the Faraday constant by an electrolysis experiment. Fig. 2.1 is an incomplete diagram showing the apparatus that was used.

Fig. 2.1

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List the measurements the student would need to make in order to use the results to calculate a value for the Faraday constant.

Using an equation, state the relationship between the Faraday constant, F, the Avogadro constant, L, and the charge on the electron, e.

The value the student obtained was: 1 faraday = 9.65 × 10⁴ coulombs

Use this value and your equation in (c) to calculate the Avogadro constant (take the charge on the electron to be 1.60 × 10^–19 coulombs).

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

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	Eθ / V
Ag+ + e– Ag	+0.80
Cu2+ + 2e–  Cu	+0.34
Fe2+ + 2e–  Fe	-0.44
Ni2+ + 2e– Ni	-0.25
SO42– + 4H+ + 2e–  SO2 + 2H2O	+0.17
Zn2+ + 2e– Zn	-0.76

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

Suggest why the blue colour fades.

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]

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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.

This question is about electrolysis cells.

State the relationship between the Faraday constant, F, the charge on an electron, e, and the Avogadro constant, L.

If the charge on the electron, the A_r and the charge of copper ions are known, the Avogardo constant can be determined experimentally by passing a known current for a known time through a copper electrolysis cell and weighing the mass of copper deposited onto the cathode.

Draw a diagram of apparatus which is suitable for carrying out this practical.
Label the following:

• power supply with the + and - terminals identified
• anode
• cathode
• ammeter

State the composition of the electrolyte.

The following are the results obtained from one such experiment.

Use the experimental results to calculate a value of the Avogardo constant, L, to 3 significant figures.

[electronic charge, e = -1.60 x 10^-19 C]

Table 2.1 shows a list of standard electrode potentials at 298 K.

Electrode reaction	Eθ / V
Ag+ + e–    Ag	+0.80
Br2 + 2e–   2Br–	+1.07
F2 + 2e–    2F–	+2.87
Fe2+ + 2e–    Fe	–0.44
2H+ + 2e– H2	0.00
Mg2+ + 2e–    Mg	–2.38
O2 + 2H2O + 4e–  4OH–	+0.40
SO42– + 4H+ + 2e–    SO2 + 2H2O	+0.17

Use the information in Table 2.1 to determine the substances formed at the anode and cathode when the following substances are electrolysed.

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.

 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.

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

• overall equation ................................................................................

Table 1.1 lists electrode potentials for some electrode reactions.

Table 1.1

Electrode reaction	Eθ / 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

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

A student set up an electrochemical cell using a concentrated sodium chloride solution using a current of 6 A. 

State the half-equations occurring at the electrodes during the electrolysis of the concentrated aqueous solution of sodium chloride.

   Cathode ..................................................................................................
 

   Anode ....................................................................................................

Calculate the time, in minutes, to produce 2.00 dm³ of gas at the anode at standard temperature and pressure.

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

State your answer to 2 significant figures and show your working.

time = ..................... minutes

The student changed the electrolyte to a very dilute sodium chloride solution.

State what change would occur at the anode and give the half equation for the process.

In a different electrolysis experiment, copper(II) sulfate solution was electrolysed using graphite electrodes.

Table 2.1 lists electrode potentials for some electrode reactions.

Table 2.1

Electrode reaction	Eθ / 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
SO42– + 4H+ + 2e–SO2 + 2H2O	+ 0.17
S2O82– + 2e–2SO42–	+ 2.01

Explain how the products at the anode and cathode are produced. 

Table 2.1 lists electrode potentials for the Cr₂O₇^2– (aq) / Cr³⁺ (aq) and Br₂ (l) / Br^– (aq) half-cells.

Table 2.1

Electrode reaction	Eθ / V
½Br2 (l) + e−Br− (aq)	+ 1.09
Cr2O72– (aq) + 14H+ (aq) + 6e–  2Cr3+ (aq) + 7H2O (l)	+ 1.36

Deduce the full equation for the Cr₂O₇^2- (aq) / Cr³⁺ (aq) and Br₂ (l) / Br^– (aq) cell.

Using Table 2.1, calculate the E^θ_cell for the electrochemical cell outlined in part (a).

The electrochemical equation for standard free energy change is given.

ΔG^θ = -nFE^θ

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

Use your answer to parts (a) and (b) to determine whether the reaction of the electrochemical cell is feasible.

An electrochemical cell has a free energy change of -14.475 kJ mol^-1.

Use the information in Table 2.1 to determine the reactions taking place at each electrode of the electrochemical cell. 

Table 2.1

• reaction at anode ........................................................................................
   
• reaction at cathode ........................................................................................