How to do electrolysis problems ….
AP Chem
Note: Redox and electrochemistry questions always appear in the MC and FR sections of the AP Chem
test (just like Acids and Bases).
By definition, all electrolysis reactions are not spontaneous (G > 0 ) and require the addition of energy to
make happen. This means that if you simply add up the cell potentials of an electrolysis reaction, the E o
would be negative.
In contrast, in a galvanic or voltaic cell the redox reaction is spontaneous. A convenient shorthand for
galvanic or voltaic cells is
anode anode solution cathode solution cathode
Where the single line represents a change of phase and the double lines the salt bridge.
Special Note: The oxidation of Na+ to form Na(l)/Na(s) does not happen in water. Imagine the fireworks!
Instead, the H2O will be oxidized to form O2 and H+ (Eored = 1.23)
Tables of cell potentials are generally Eored - meaning reduction potentials. Eooxid are just the negative of
Eored. It is very easy to get confused with the signs, so it is best to include the red or oxid subscripts. I like
to keep everything as Eored so that I don’t make a silly sign mistake.
Oxidation always occurs at the anode and reduction always occurs at the cathode.
When you have a combination of potential reactions in an electrolysis reaction:
 The oxidation that occurs first will have the lowest Eored (least positive or most negative). [This
means it will have an Eooxid that is least negative/most positive].
 The reduction that occurs first will have the highest Eored (least negative or most positive).
If these reactions are occurring in an aqueous solution, the reduction and oxidation of H2O must also be
included (sometimes one of them will be occur first!).
It is always a good idea to write down all of the possible half reactions and their potentials and then work
the logic.
Let’s look at some examples:
1)
2)
3)
3)
4)
5)
6)
F2(g) + 2e  2F
O2(g) + 4H+ + 4e-  2 H2O(l)
Br2(g) + 2e-  2BrAg+ + e-  Ag(s)
Cl2(g) + 2e-  2Cl2H2O(l) + 2e-  H2(g) + 2 OHNa+ + e-  Na(s)
-
-
Eored
2.87
1.23
1.07
0.8
1.36
-0.83
-2.71
Determine the oxidation and reduction reaction in each of these electrolysis scenarios. Ignore any
precipitation from AgCl2.
1.
Suppose you had a mixture of molten NaBr and AgF.
2.
Suppose you had a mixture of aqueous solutions of NaBr and AgF?
3.
Suppose you had a mixture of molten NaCl and AgF?
4.
Suppose you had a mixture of aqueous solutions of NaCl and AgF?
Answers:
1) Molten salts mean you have the elements present as ions.
Possible Reduction
Na+ + e-  Na(s)
Ag+ + e-  Ag(s)
Eored
-2.71
0.8
Possible Oxidation
2Br -  Br2(g)+ 2e2F -  F2(g) + 2e-
Eored
1.07
2.87
Eooxid
-1.07
-2.87
1st reduction is the most positive Eored which is Ag. 1st oxidation is the most negative Eored which is Br.
2) Now we need to also include the oxidation and reduction of H2O since the salts are aqueous.
Possible Reduction
Na+ + e-  Na(s)
Ag+ + e-  Ag(s)
2H2O(l) + 2e-  H2(g) + 2 OH-
Eored
-2.71
0.8
-0.83
Possible Oxidation
2Br -  Br2(g)+ 2e2F -  F2(g) + 2e2 H2O(l)  O2(g) + 4H+ + 4e-
Eored
1.07
2.87
1.23
Eooxid
-1.07
-2.87
-1.23
1st reduction is the most positive Eored which is Ag. 1st oxidation is the most negative Eored which is Br.
3)
Possible Reduction
Na+ + e-  Na(s)
Ag+ + e-  Ag(s)
Eored
-2.71
0.8
Possible Oxidation
2Cl-  Cl2(g) + 2e2F -  F2(g) + 2e-
Eored
1.36
2.87
Eooxid
-1.36
-2.87
1st reduction is the most positive Eored which is Ag. 1st oxidation is the most negative Eored which is Cl.
4) Same as 3) but aqueous.
Possible Reduction
Na+ + e-  Na(s)
Ag+ + e-  Ag(s)
2H2O(l) + 2e-  H2(g) + 2 OH-
Eored
-2.71
0.80
-0.83
Possible Oxidation
2Cl-  Cl2(g) + 2e2F -  F2(g) + 2e2 H2O(l)  O2(g) + 4H+ + 4e-
Eored
1.36
2.87
1.23
Eooxid
-1.36
-2.87
-1.23
1st reduction is the most positive Eored which is Ag. 1st oxidation is the most negative Eored which is H2O.