Topic 14 – Redox II
In order to develop their practical skills, students should be encouraged to carry out a range of practical experiments related to this topic. Possible
experiments include investigating redox systems (especially within transition metals), setting up a variety of electrochemical cells, investigating
disproportionation in copper(I) salts, performing redox titrations using potassium manganate(VII) or iodine-thiosulfate. Mathematical skills that
could be developed in this topic include calculating redox potentials, balancing redox equations from half cells, calculations involving redox
titrations. Within this topic, students can consider how ideas developed in different contexts within chemistry can be shown to be related to a major
explanatory principle. Here, cell emfs and equilibrium constants are shown to be related to the fundamental criterion for the feasibility of a chemical
reaction: the total entropy change. Students can also consider how chemists continue to search for alternative sources of energy, through the
development of fuel cells.
1/2. understand the terms ‘oxidation’ and ‘reduction’ in terms of electron transfer and in terms of changes
in oxidation number, applied to s-, p- and d-block elements
Give the definition for oxidation, in terms of electron transfer AND change in oxidation number.
Oxidation is loss of electrons and therefore an increase in oxidation number (as it’s becoming more ‘positive’).
Give the definition for reduction, in terms of electron transfer AND change in oxidation number.
Reduction is a gain of electrons and therefore a decrease in oxidation number (as it’s becoming more ‘negative’).
3. know what is meant by the term ‘standard electrode potential’, E o
Well, what is it?
Standard electrode potential is the voltage/potential measured when a half-cell is connected to the standard
hydrogen electrode, under standard conditions.
4. know that the standard electrode potential, E o, refers to certain conditions
What are the conditions for the standard electrode potential?
A temperature of 298K, a pressure of 100kPa, and a concentration of 1.0moldm -3.
5. know the features of the standard hydrogen electrode + understand why such a reference is necessary
Explain why using the SHE (see right) is necessary to measure the potential of a half-cell.
It isn’t possible to measure the potential of a single electrode/half-cell on its own.
Instead, we need to combine it with a half-cell of known potential and measure the
difference between the two. Combining two half-cells with a salt bridge is called an
electrochemical cell. The standard hydrogen electrode is defined with a potential of 0V,
allowing other cells to have a measured potential difference in relation to the SHE.
6. understand that different methods are used to measure standard electrode potentials of:
i metals or non-metals in contact with their ions in aqueous solution
, ii ions of the same element with different oxidation numbers
When are platinum electrodes used? When are they not?
Platinum electrodes are used if the reaction taking place involves a
gas, or if both sides of the reaction are in solution. Conversely, if the
ion is in solution on one side of the reaction, but forms a solid
product on the other, use that solid as the electrode.
7. be able to calculate a standard emf, Eo cell, by combining two standard
electrode potentials
Please use the data booklet for these questions! Remember, do forwards minus backwards for these.
Calculate the Eocell for the copper and silver cell. Write the feasible reaction and the cell diagram for the system.
fwd = 0.8V rev = 0.34V Eocell = 0.8 – 0.34 = +0.46V
2Ag+ + Cu ----> 2Ag + Cu2+ Cu(s) | Cu2+(aq) || Ag+(aq) | Ag(s)
Remember the reverse reaction is on the left of the cell diagram if the reaction is feasible.
8. be able to write cell diagrams using the conventional representation of half-cells
Write the cell diagrams for the following reactions: (remember LoHiHiLo (ox-state), more -ve/rev on LHS)
Fe(s) + Sn2+(aq) ----> Fe2+(aq) + Sn(s)
Fe(s) | Fe2+(aq) || Sn2+(aq) | Sn(s)
5Fe2+(aq) + MnO4-(aq) + 8H+(aq) ----> 5Fe3+(aq) + Mn2+(aq) + 4H2O(l)
Pt(s) | Fe2+, Fe3+ || [MnO4- + 8H+], [Mn2+ + 4H2O] | Pt(s)
Ag+(aq) + e- <---> Ag(s), E = 0.8V VO2+(aq) + 2H+(aq) <---> VO2+(aq) + H2O(l), E = 1.0V
Ag(s) | Ag+(aq) || [VO2+(aq) + 2H+(aq)], [VO2+(aq) + H2O(l)] | Pt(s)
9. understand the importance of the conditions when measuring the electrode potential, E
What conditions affect the electrode potential?
Temperature, pressure and solution concentration all affect the electrode potential. This is why measuring under
standard conditions is so important!
10. be able to predict the thermodynamic feasibility of a reaction using standard electrode potentials
Calculate whether the reduction of acidified dichromate(VI) ions to chromium(III) ions is feasible using iron(II)
ions. Write the chemical equation that takes place as a result.
fwd = +1.33V rev = +0.77V Eocell = 1.33 – 0.77 = +0.56V, so feasible.
Cr2O72- + 14H+ + 6Fe3+ -----> 2Cr3+ + 7H2O + 6Fe2+
11. understand that Eo cell is directly proportional to the total entropy change and to ln K for a reaction
Why does this make sense?
A more positive Ecell means a reaction is more likely to happen. A more positive ln K and ΔStotal mean the same.
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In order to develop their practical skills, students should be encouraged to carry out a range of practical experiments related to this topic. Possible
experiments include investigating redox systems (especially within transition metals), setting up a variety of electrochemical cells, investigating
disproportionation in copper(I) salts, performing redox titrations using potassium manganate(VII) or iodine-thiosulfate. Mathematical skills that
could be developed in this topic include calculating redox potentials, balancing redox equations from half cells, calculations involving redox
titrations. Within this topic, students can consider how ideas developed in different contexts within chemistry can be shown to be related to a major
explanatory principle. Here, cell emfs and equilibrium constants are shown to be related to the fundamental criterion for the feasibility of a chemical
reaction: the total entropy change. Students can also consider how chemists continue to search for alternative sources of energy, through the
development of fuel cells.
1/2. understand the terms ‘oxidation’ and ‘reduction’ in terms of electron transfer and in terms of changes
in oxidation number, applied to s-, p- and d-block elements
Give the definition for oxidation, in terms of electron transfer AND change in oxidation number.
Oxidation is loss of electrons and therefore an increase in oxidation number (as it’s becoming more ‘positive’).
Give the definition for reduction, in terms of electron transfer AND change in oxidation number.
Reduction is a gain of electrons and therefore a decrease in oxidation number (as it’s becoming more ‘negative’).
3. know what is meant by the term ‘standard electrode potential’, E o
Well, what is it?
Standard electrode potential is the voltage/potential measured when a half-cell is connected to the standard
hydrogen electrode, under standard conditions.
4. know that the standard electrode potential, E o, refers to certain conditions
What are the conditions for the standard electrode potential?
A temperature of 298K, a pressure of 100kPa, and a concentration of 1.0moldm -3.
5. know the features of the standard hydrogen electrode + understand why such a reference is necessary
Explain why using the SHE (see right) is necessary to measure the potential of a half-cell.
It isn’t possible to measure the potential of a single electrode/half-cell on its own.
Instead, we need to combine it with a half-cell of known potential and measure the
difference between the two. Combining two half-cells with a salt bridge is called an
electrochemical cell. The standard hydrogen electrode is defined with a potential of 0V,
allowing other cells to have a measured potential difference in relation to the SHE.
6. understand that different methods are used to measure standard electrode potentials of:
i metals or non-metals in contact with their ions in aqueous solution
, ii ions of the same element with different oxidation numbers
When are platinum electrodes used? When are they not?
Platinum electrodes are used if the reaction taking place involves a
gas, or if both sides of the reaction are in solution. Conversely, if the
ion is in solution on one side of the reaction, but forms a solid
product on the other, use that solid as the electrode.
7. be able to calculate a standard emf, Eo cell, by combining two standard
electrode potentials
Please use the data booklet for these questions! Remember, do forwards minus backwards for these.
Calculate the Eocell for the copper and silver cell. Write the feasible reaction and the cell diagram for the system.
fwd = 0.8V rev = 0.34V Eocell = 0.8 – 0.34 = +0.46V
2Ag+ + Cu ----> 2Ag + Cu2+ Cu(s) | Cu2+(aq) || Ag+(aq) | Ag(s)
Remember the reverse reaction is on the left of the cell diagram if the reaction is feasible.
8. be able to write cell diagrams using the conventional representation of half-cells
Write the cell diagrams for the following reactions: (remember LoHiHiLo (ox-state), more -ve/rev on LHS)
Fe(s) + Sn2+(aq) ----> Fe2+(aq) + Sn(s)
Fe(s) | Fe2+(aq) || Sn2+(aq) | Sn(s)
5Fe2+(aq) + MnO4-(aq) + 8H+(aq) ----> 5Fe3+(aq) + Mn2+(aq) + 4H2O(l)
Pt(s) | Fe2+, Fe3+ || [MnO4- + 8H+], [Mn2+ + 4H2O] | Pt(s)
Ag+(aq) + e- <---> Ag(s), E = 0.8V VO2+(aq) + 2H+(aq) <---> VO2+(aq) + H2O(l), E = 1.0V
Ag(s) | Ag+(aq) || [VO2+(aq) + 2H+(aq)], [VO2+(aq) + H2O(l)] | Pt(s)
9. understand the importance of the conditions when measuring the electrode potential, E
What conditions affect the electrode potential?
Temperature, pressure and solution concentration all affect the electrode potential. This is why measuring under
standard conditions is so important!
10. be able to predict the thermodynamic feasibility of a reaction using standard electrode potentials
Calculate whether the reduction of acidified dichromate(VI) ions to chromium(III) ions is feasible using iron(II)
ions. Write the chemical equation that takes place as a result.
fwd = +1.33V rev = +0.77V Eocell = 1.33 – 0.77 = +0.56V, so feasible.
Cr2O72- + 14H+ + 6Fe3+ -----> 2Cr3+ + 7H2O + 6Fe2+
11. understand that Eo cell is directly proportional to the total entropy change and to ln K for a reaction
Why does this make sense?
A more positive Ecell means a reaction is more likely to happen. A more positive ln K and ΔStotal mean the same.
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