GRADE 12
WORKSHEET - REDOX REACTIONS and ELECTROCHEMICAL CELLS
Question 1
A standard cell is set up by connecting an Al / Al3+ half-cell to a Cu / Cu2+ half-cell. Aluminium
sulphate and copper sulphate are used as electrolytes in the two half cells respectively.
1.1
Write down the equation of the oxidation half reaction from the table of standard reduction
potentials of this cell.
(2)
1.2
Mention two possible changes that can be observed in the Cu / Cu2+ half-cell while the cell
reaction proceeds.
(4)
1.3
What physical change can the aluminium electrode undergo as the reaction proceeds? (2)
1.4
Calculate the emf of this cell under standard conditions.
1.5
The standard electrode potential of a Cs / Cs+ half-cell is -2,92 V. How will the emf
calculated in 1.4 above be affected if the Al / Al3+ half-cell is replaced by a Cs / Cs+ halfcell? (Write only INCREASES, DECREASES or REMAINS THE SAME.)
(2)
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(4)
Question 2
A standard zinc / iron electrochemical cell is set up as shown below:
V
Fe
Fe(NO3)3(aq)
2.1
Zn
Zn(NO3)2(aq)
By making use of the standard reduction tables, write the appropriate iron
half reaction.
(2)
2.2
Show by calculation that the emf of the cell is 0,72 V.
(4)
2.3
Which electrode (ie. zinc or iron) in the cell is the cathode?
(2)
2.4
Write down the half reaction that occurs at the anode.
(2)
2.5
Write down the balanced, net ionic equation of the cell.
(4)
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Worksheet – Redox reactions & Electrochem.
2
Question 3
In a reaction you are required to bubble sulphur dioxide through an acidified solution of potassium
dichromate solution.
3.1
Write down the reduction half reaction from the table of standard reduction
potentials.
(2)
Write down the oxidation half reaction from the table of standard reduction
potentials.
(2)
3.3
Use the two half reactions to write a balanced, net ionic equation.
(4)
3.4
If sulphuric acid was used to acidify the solution, add the spectator ions and write down the
balanced molecular equation for the reaction.
(4)
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3.2
Question 4
When sodium metal (Na) is added to water in a test tube, an explosive gas forms, the water gets a
soapy feeling, and when bromothymol blue is added to the solution, it turns blue.
4.1
Write down the balanced equation for:
4.1.1
the oxidation half reaction.
(2)
4.1.2
the reduction half reaction.
(2)
4.1.3
the complete (overall) reaction that takes place in the test tube.
(4)
4.2
Explain briefly why the same observations are not made when zinc is used instead of
sodium.
(4)
[12]