How a Concentration Cell Works
In a concentration cell, the half-reactions are the same but the
concentrations are different. Because of this, the nonstandard cell
potential, Ecell, is nonzero because it depends upon the ratio of ion
concentrations. In Fig 21.11A we see a system with 0.10 M Cu2+ in the
anode half-cell and 1.0 M Cu2+, a 10-fold higher concentration, in the
cathode half-cell.
Cu(s) Cu2+(aq,0.10 M) + 2 e- [anode; oxidation]
Cu2+(aq, 1.0 M) + 2 e- Cu(s) [cathode; reduction]
Cu2+(aq, 1.0 M) Cu2+(aq,0.10 M) Ecell = ?
Using the Nernst equation, and n = 2, we get:
Ecell = Eocell - log = 0 V - log
0.0592 V
2
[Cu2+]dil
[Cu2+]conc
0.0592 V
2
0.10 M
1.0 M
Ecell = 0 V - = + 0.0296 V
0.0592 V
2
(-1.00)
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