Calculating Voltage of an Electrochemical Cell

By: Tyler J. and Kyle P.

Here is a 5 steps demonstration that will show you how to calculate the voltage of an electrochemical cell. We will use aluminum and lead as our examples through out this demonstration. Above, we have also uploaded a powerpoint that you may download that could be an easier example to follow.

We want to create an electrochemical cell using aluminum (Al|Al3+) and lead (Pb|Pb2+) half-cells. Our tasks:
  1. Determine the two half-reactions involved, and which reaction will undergo oxidation and which one will be reduced.
  2. Determine the voltage of the cell.
  3. Diagram the set-up of the electrochemical cell, including the following items:
    • the two half-cells, including the electrodes and electrolytic solutions
    • the external circuit, showing the direction of electron flow
    • the salt bridge with an electrolyte, including movement of ions
    • label the anode and the cathode
    • label the positive and negative posts

Step 1
Before you begin to create your diagram you need to determine what will be oxidized and what will be reduced. At the same time you can calculate the voltage of the cell. Locating the two half-reactions in the Table of Standard Electrode Potentials:



Reaction......................................................E°


reduction

Pb2+(aq) + 2e- → Pb(s) ..............................-0.13 V
reduction

Al3+(aq) + 3e- → Al(s)...................................-1.66 V


Step 2
Since lead has a larger electrode potential than does aluminum, lead will be reduced and aluminum will be oxidized. So we will reverse the aluminum equation and reverse its sign.
Also identify what is oxidized and what is reduced.



Reaction......................................................E°



reduction

Pb2+(aq) + 2e- → Pb(s)...............................-0.13 V


oxidation

Al(s) → Al3 + (aq) + 3e-...............................+1.66 V




Step 3
We can now calculate the cell voltage. A useful tip - you know if you've switched the correct equation when you get a positive voltage. All electrochemical cells will have a positive voltage!


Step 5
Now that we have all of the key information we can diagram the cell.
Step-by-step:
  1. Draw 2 beakers, each containing an electrode - the two metals used in our reactions. To each beaker add an electrolytic solution. The electrolyte will include the metal ion and another ion (NO3- is usually a good choice since it will not form a precipitate).
  2. Add an external circuit (the wire connecting the two electrodes) and a salt bridge with an electrolytic solution (KNO3 is often a good choice).
Aluminum - lead electrochemical cell
Aluminum - lead electrochemical cell

3. Indicate the following on your diagram:
  • Direction of electron flow (Electrons flow from A to C)
  • Ion movement from the salt bridge (Anions to Anode; Cations to Cathode)
  • Positive and negative posts (the anode - site of reduction - releases electrons so it is the negative post; the cathode - site of oxidation - uses up electrons so it is the positive post).
Aluminum - lead electrochemical cell (complete)
Aluminum - lead electrochemical cell (complete)

Source Of Example

Standard Electrode Potentials in Aqueous Solution
*This is a Table that is needed to calculate the voltage, it gives you the charges of a given element's half reaction.
Cathode (Reduction)
Half-Reaction
Standard Potential
E° (volts)
Li+(aq) + e- -> Li(s)..................................................
-3.04
K+(aq) + e- -> K(s)..................................................
-2.92
Ca2+(aq) + 2e- -> Ca(s)...........................................
-2.76
Na+(aq) + e- -> Na(s)...............................................
-2.71
Mg2+(aq) + 2e- -> Mg(s)..........................................
-2.38
Al3+(aq) + 3e- -> Al(s)..............................................
-1.66
2H2O(l) + 2e- -> H2(g) + 2OH-(aq).............................
-0.83
Zn2+(aq) + 2e- -> Zn(s)............................................
-0.76
Cr3+(aq) + 3e- -> Cr(s).............................................
-0.74
Fe2+(aq) + 2e- -> Fe(s)............................................
-0.41
Cd2+(aq) + 2e- -> Cd(s)............................................
-0.40
Ni2+(aq) + 2e- -> Ni(s)..............................................
-0.23
Sn2+(aq) + 2e- -> Sn(s)............................................
-0.14
Pb2+(aq) + 2e- -> Pb(s)............................................
-0.13
Fe3+(aq) + 3e- -> Fe(s)............................................
-0.04
2H+(aq) + 2e- -> H2(g).............................................
0.00
Sn4+(aq) + 2e- -> Sn2+(aq).......................................
0.15
Cu2+(aq) + e- -> Cu+(aq)..........................................
0.16
ClO4-(aq) + H2O(l) + 2e- -> ClO3-(aq) + 2OH-(aq)........
0.17
AgCl(s) + e- -> Ag(s) + Cl-(aq)..................................
0.22
Cu2+(aq) + 2e- -> Cu(s)............................................
0.34
ClO3-(aq) + H2O(l) + 2e- -> ClO2-(aq) + 2OH-(aq)........
0.35
IO-(aq) + H2O(l) + 2e- -> I-(aq) + 2OH-(aq).................
0.49
Cu+(aq) + e- -> Cu(s)...............................................
0.52
I2(s) + 2e- -> 2I-(aq).................................................
0.54
ClO2-(aq) + H2O(l) + 2e- -> ClO-(aq) + 2OH-(aq).........
0.59
Fe3+(aq) + e- -> Fe2+(aq).........................................
0.77
Hg22+(aq) + 2e- -> 2Hg(l).........................................
0.80
Ag+(aq) + e- -> Ag(s)...............................................
0.80
Hg2+(aq) + 2e- -> Hg(l)............................................
0.85
ClO-(aq) + H2O(l) + 2e- -> Cl-(aq) + 2OH-(aq).............
0.90
2Hg2+(aq) + 2e- -> Hg22+(aq)...................................
0.90
NO3-(aq) + 4H+(aq) + 3e- -> NO(g) + 2H2O(l).............
0.96
Br2(l) + 2e- -> 2Br-(aq).............................................
1.07
O2(g) + 4H+(aq) + 4e- -> 2H2O(l)..............................
1.23
Cr2O72-(aq) + 14H+(aq) + 6e- -> 2Cr3+(aq) + 7H2O(l)..
1.33
Cl2(g) + 2e- -> 2Cl-(aq)............................................
1.36
Ce4+(aq) + e- -> Ce3+(aq)........................................
1.44
MnO4-(aq) + 8H+(aq) + 5e- -> Mn2+(aq) + 4H2O(l).....
1.49
H2O2(aq) + 2H+(aq) + 2e- -> 2H2O(l).......................
1.78
Co3+(aq) + e- -> Co2+(aq).......................................
1.82
S2O82-(aq) + 2e- -> 2SO42-(aq)...............................
2.01
O3(g) + 2H+(aq) + 2e- -> O2(g) + H2O(l)...................
2.07
F2(g) + 2e- -> 2F-(aq).............................................
2.87
Source of Table