A LevelChemistryCIETopic Questions24. Electrochemistry (AL)24.2 Standard Electrode Potentials E⦵, Standard Cell Potentials E⦵cell & the Nernst Equation

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Standard Electrode Potentials E⦵, Standard Cell Potentials E⦵cell & the Nernst Equation (CIE A Level Chemistry)

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1a3 marks

Standard electrode potentials are a measure of how easily a substance is reduced.

Give the standard conditions for the measurement of standard electrode potentials.

 

Temperature = .............................. 

Pressure = .................................... 

Ion concentration = ........................ 

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1b2 marks

Table 4.1 shows three standard electrode potentials.

 
Table 4.1 
 
  Equilibrium equation Standard electrode potential, Eθ / V
A Ag+ + e– ⇌ Ag + 0.80
B ClO– + H2O + 2e– ⇌ Cl– + 2OH–  + 0.89
C 2H2O + 2e– ⇌ H2 + 2OH– – 0.83
 

Identify the standard electrode potential with the equilibrium position that lies:

 
i)
Furthest left.
 
[1]
 
ii)
Furthest right.
 
[1]

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1c3 marks

The standard hydrogen electrode is used as the primary reference electrode for measuring standard electrode potential.

 
i)
Describe a standard hydrogen electrode.
 
[2]
 
ii)
State the standard electrode potential of the standard hydrogen electrode.
 
Eθ = .......... V
 
[1]

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2a2 marks

Electrochemical cells are made by connecting two half-cells with a salt bridge and a high-resistance voltmeter.

The standard hydrogen electrode is an example of a non-metal / non-metal ion half-cell which requires a platinum electrode.

 

Complete Table 5.1 to identify the two other types of half-cell and whether they require a platinum electrode.

 
Table 5.1 
 
Type of half-cell Require a platinum electrode?
non-metal / non-metal ion Yes
   
   
 

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2b1 mark

The direction of electron flow can be determined by comparing the Eθ values of the two half-cells in an electrochemical cell.

In terms of poles, state the flow of electrons.

             from .................................................. to ..................................................

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2c3 marks

Table 5.2 shows six standard electrode potentials.

 
Table 5.2 
 
Electrode reaction Eθ / 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
 
i)
Identify an electrode reaction that will ensure that the Cu+ / Cu half-cell undergoes oxidation.
[1]
 
ii)
Write the equation for the feasible reaction of this electrochemical cell.
 
[2]

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1a2 marks

Explain why Fig. 3.1 does not represent the standard hydrogen electrode.

9sq3ta~a_4

Fig. 3.1

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1b1 mark

The standard electrode potential for Zn2+ (aq) + 2e– → Zn (s) is –0.76 V.

State the meaning of the minus sign in the value of –0.76 V.

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1c2 marks

Zinc coating on metals serves as physical protection which prevents rust from affecting the underlying metal surface. This is achieved by electroplating as shown in Fig. 3.2.


 
afd94662-9a93-474a-8987-28dac1ec459f

Fig. 3.2

i)
Suggest a suitable solution to act as the electrolyte during zinc electroplating.
 
[1]
 
ii)
Complete the diagram by labelling the polarity of the power source by using a + and - sign. 
 
[1]

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1d
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Calculate the length of time, in hours, required to deposit 1.0 g of zinc on the item to be electroplated. Assume the current is a constant 0.1 A throughout this period.  

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

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



time = .................... hours

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2a4 marks

This question is about the Ag+(aq) / Ag(s) half-cell.

A student was asked to plan an experiment to measure the standard electrode potential of the Ag+ (aq) / Ag(s) half-cell.

i)
State the conditions of temperature and pressure under which standard electrode potentials are measured.
 
[1]
 
ii)
Fig. 1.1 shows the diagram drawn by the student.
 
q2aii-9cho-al-3-june-2019-qp-edexcel-a-level-chem
 
Fig. 1.1
  
Complete Table 1.1 to identify three mistakes in this diagram and the modifications that should be made to correct them.
 
Table 1.1
 
Mistake in diagram Modification needed to correct mistake
   
   
   
 
[3]

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2b
Sme Calculator3 marks

The standard electrode potential, Eθ, of the Ag+ (aq) / Ag (s) half-cell is +0.80 V.

The effect of changing the concentration of the ions on the value of the electrode potential, E, in this half-cell is calculated using the equation

E = Eθ + begin mathsize 14px style fraction numerator R T over denominator 96500 end fraction end styleln[Ag+ (aq)]

where T is the temperature in kelvin and R is the gas constant, 8.31 J K–1 mol–1.

The electrode potential of an Ag+ (aq) / Ag (s) half-cell was measured at 20 °C and found to be +0.72 V.

Calculate the concentration of silver ions, in mol dm-3, in this half-cell. Show your working.


[Ag+] = .................. mol dm-3

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3a1 mark

Table 3.1 lists electrode potentials for some electrode reactions.

Table 3.1 

Electrode reaction  Eθ / V
Br2 + 2e– ⇌ 2Br– + 1.07
Cl2 + 2e– ⇌ 2Cl –  + 1.36
[Co(H2O)6]3++ e– ⇌  [Co(H2O)6]2+ + 1.81
[Co(NH3)6]3+ + e– ⇌ [Co(NH3)6]2+ 
 + 0.11
Cu2+ + 2e– ⇌ Cu + 0.34
Fe2+ + 2e– ⇌ Fe  – 0.44
Fe3+ + e– ⇌ Fe2+ + 0.77
2H+ + 2e– ⇌ H2 0.00
I2 + 2e– ⇌ 2I– + 0.54
NO3– + 2H+ + e– ⇌ NO2 + H2O + 0.81
SO42– + 4H+ + 2e– ⇌ SO2 + 2H2O + 0.17
VO2+ + 2H+ + e– ⇌ VO2+ + H2O +1.00

Explain how Table 3.1 could be adapted to show an electrochemical series.

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3b1 mark

Use Table 3.1 to identify the halide ion that is the weakest reducing agent.

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3c1 mark

Use Table 3.1 to justify why sulfate ions should not be capable of oxidising iodide ions.

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3d
Sme Calculator3 marks
i)
Use Table 3.1 to identify an acid that will oxidise copper. Explain your answer.
 
[1]
 
ii)
Suggest a possible equation for the reaction.
 
[1]
 
iii)
Calculate the Eθcell for the same overall reaction.
 
[1]

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3e1 mark

Suggest why the two cobalt(III) complex ions in Table 3.1 have different electrode potentials.

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3f2 marks

Use Table 3.1 to explain why [Co(H2O)6]3+(aq) will undergo a redox reaction with [Fe(H2O)6]2+(aq).

Give an equation for this reaction.

 

explanation ................................................................................

 

equation ................................................................................

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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, Eθ, 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– rightwards harpoon over leftwards harpoon Fe Eθ = -0.44 V
Cu2+ + 2e– rightwards harpoon over leftwards harpoon Cu Eθ = +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

Eθ cell of this cell is +0.78 V.

State whether this reaction is feasible. Explain your answer.

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2a6 marks

This question is about electrochemical cells.

The diagram shown in Fig. 3.1 represents the standard hydrogen electrode.

standard-hydrogen-electrode

Fig 3.1

i)
Name the substance used as the electrode in Fig 3.1.

[1]

ii)
Suggest why this substance is used as an electrode.

[2]

iii)
Give the standard conditions used in a standard hydrogen electrode.

[3]

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2b6 marks

A student set up an electrochemical cell consisting of copper and zinc.

i)
Complete Fig. 3.2 to show the components and reagents, including their concentrations and label any apparatus required to complete the electrochemical cell.

[4]

zinc-half-cell

Fig. 3.2

ii)
Use the IUPAC convention to give the half-equations occurring at each electrode.

[2]

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2c3 marks

A student set up another electrochemical cell consisting of copper and silver as shown in the following cell representation where the half cell with the greatest negative standard electrode potential is written on the left: 

 

Cu / Cu2+ ; Ag+ / Ag

i)
Write a half-equation for the reaction that occurs at the positive electrode.

[1]

ii)
Write a half-equation for the reaction that occurs at the negative electrode.

[1]

iii)
Use the half-equations to deduce an overall equation for the cell. Include all state symbols.

[1]

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2d2 marks

A diagram of a cell is shown below in Fig. 3.3.

electrochem-ag-and-

Fig. 3.3

i)
Explain how the salt bridge, in Fig. 3.3, provides an electrical connection between the two solutions.

[1]

ii)
Suggest why potassium chloride would not be suitable for use in the salt bridge of this cell.

[1]

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3a2 marks

The apparatus shown was used to measure the standard electrode potential for the reduction of Cr2O72– ions to Cr3+ ions in acid solution:

Cr2O72– (aq) + 14H+ (aq) + 6e–→ 2Cr3+ (aq) + 7H2O (l)

q1-paper-5-oct-2021-edexcel-ial-chemistry

Which material should be used for each electrode?

Electrode 1 .............................................................

Electrode 2 .............................................................

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3b1 mark

Suggest a suitable solution that could be used as solution 1.

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3c
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Solution 2 contains 14.71g of K2Cr2O7 .

What mass of Cr2(SO4)3⋅18H2O should also be used?

[Mr values: K2Cr2O7 = 294.2 Cr2(SO4)3⋅18H2O = 716.3]

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3d2 marks

Solution 2 is best acidified with H2SO4 instead of HCl or HBr.

Suggest why.

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4a1 mark

An electrochemical cell is set up as shown in Fig. 2.1.

5-4-2a-m-iron-and-silver-cell


Fig. 2.1

Use the electrode potential list in Table 2.1 to calculate the value of Eθcell under standard conditions, stating which electrode is the negative one.


Table 2.1
Electrode reaction Eθ / V
Ag+ + e– rightwards harpoon over leftwards harpoon Ag +0.80
Fe2+ + 2e– rightwards harpoon over leftwards harpoon Fe -0.44
Fe3+ + e– rightwards harpoon over leftwards harpoonFe2+ +0.77
SO42– + 4H+ + 2e– rightwards harpoon over leftwards harpoon SO2 + 2H2O  +0.17

 Eθcell  = ..............................................

negative electrode = ..............................................

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4b2 marks

How would the actual Ecell of the above cell compare to the Eθcell under standard conditions?


Explain your answer.

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4c4 marks

How would the Ecell of the cell in Fig. 2.1 change, if at all, if a few cm3 of concentrated Na2SO4 (aq) were added to the following?

i)
The beaker containing Fe3+ (aq) and Fe2+ (aq).

[1]

ii)
The beaker containing Ag2SO4 (aq).

[1]

iii)
Explain any changes in Ecell you have stated in (i) and (ii).

[2]

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4d1 mark

Write an equation to show the reaction taking place in the electrochemical cell in Fig. 2.1.

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5a
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The standard electrode potentials for seven different redox systems are shown in Table 3.1.

Table 3.1

redox system equation Eθ / V
1 2H+ (aq) + 2e– rightwards harpoon over leftwards harpoon H2 (g)   0.00
2 Fe3+ (aq) + e– rightwards harpoon over leftwards harpoon Fe2+ (aq) +0.77
3 Cu2+ (aq) + 2e– rightwards harpoon over leftwards harpoon Cu (s) +0.34
4 Cl2 (aq) + 2e– rightwards harpoon over leftwards harpoon 2Cl– (aq) +1.36
5 O2 (g) + 4H+ (aq) + 4e– rightwards harpoon over leftwards harpoon 2H2O (l) +1.23
6 Al3+ (aq) + 3e– rightwards harpoon over leftwards harpoon Al (s) -1.66
7 I2 (aq) + 2e– rightwards harpoon over leftwards harpoon 2I– (aq) +0.54

An electrochemical cell can be made based on redox systems 2 and 3.

i)
Write the overall cell reaction.

[1]

ii)
Calculate the voltage of this cell.

[1]

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5b5 marks
i)
Using redox systems 5, 6 and 7 only in Table 3.1, write the overall equations for three reactions that might be feasible.

[3]

ii)
Give two reasons why these reactions might not take place, even if they are feasible.

[2]

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5c2 marks

Select from Table 3.1,

i)
an oxidising agent that oxidises Fe2+ (aq) to Fe3+ (aq),

[1]

ii)
a species that reduces Fe3+ (aq) to Fe2+ but does not reduce Cu2+ (aq) to Cu (s).

[1]

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5d
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Calculate the electrode potential at 298 K of redox system 3 if the concentration of Cu2+ (aq) ions was 0.0002 mol dm-3.

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6a2 marks

Table 4.1 below contains some standard electrode potential data which you will need to answer the following questions.

Table 4.1

electrode reaction  Eθ / V
Cu2+ (aq) + 2e− rightwards harpoon over leftwards harpoon Cu (s) +0.34
Ni2+ (aq) + 2e− rightwards harpoon over leftwards harpoon Ni (s) -0.25
Fe3+ (aq) + e− rightwards harpoon over leftwards harpoon Fe2+ (aq) +0.77
Sn2+ (aq) + 2e− rightwards harpoon over leftwards harpoon Sn (s) -0.14
Fe2+ (aq) + 2e− rightwards harpoon over leftwards harpoon Fe (s) -0.45
 

Deduce the species from Table 1 that is the weakest oxidising agent. Explain your choice.

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6b
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A cell is made by connecting two half-cells with a salt bridge.  


A student produced a cell by using nickel in a solution of nickel chloride solution and another consisted of copper in a solution of copper sulfate solution.


Calculate the standard cell potential of this cell using the values given in Table 4.1 in part (a).

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6c
Sme Calculator3 marks

Calculate the standard Gibbs free energy change, and give the units, for the electrochemical cell in part (b) and state whether the reaction is feasible.


Show your working.

[Faraday constant, F = 9.65 × 104 C mol–1]

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6d2 marks

Two half-cells, involving species in Table 4.1, are connected together to give a cell with a standard cell potential = +0.31 V.

i)
Determine which two half equations are connected using the data from Table 4.1.

[1]

ii)
Suggest the half-equation for the reaction that occurs at the cathode.

[1]

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7a3 marks

The standard electrode potentials in Table 5.1 can be used to predict redox reactions.

Table 5.1

redox system equation Eθ / V
1 Ag+ (aq) + e– rightwards harpoon over leftwards harpoon Ag (s) +0.80
2 Cr3+ (aq) + 3e– rightwards harpoon over leftwards harpoon Cr (s) -0.74
3 Mg2+ (aq) + 2e– rightwards harpoon over leftwards harpoon Mg (s) -2.37


Using the information in Table 5.1, write equations for the reactions that are feasible.

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7b
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A student sets up a standard cell, using half-cells based on redox systems 2 and 3 at 298 K.

i)
Calculate the standard cell potential.

[1]

ii)
State the sign of the electrode in redox system 2 of the cell.

[1]

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7c3 marks

The student diluted the solution in redox system 3 with distilled water.

Predict what would happen to the cell potential.
Explain your reasoning.

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7d
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The student set up the cell between redox systems 2 and 3 again. This time, they did not dilute the solution of redox system 3 but they used a solution of 0.01 mol dm-3 of Cr3+ ions for redox system 2. The temperature remained at 298 K.

Use the Nernst equation below to predict what the new electrode potential, E, would be for redox system 2.

E = Eθ + fraction numerator 0.059 over denominator z end fraction log subscript 10 fraction numerator open square brackets oxidised space species close square brackets over denominator open square brackets reduced space species close square brackets end fraction

where z = the number of electrons transferred in the reaction.

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