-Galvanic Cells-
Table 1. Standard Reduction potentials at 25°C
Half-Reaction
Ag+(aq) + 1e- → Ag(s)
Cu2+(aq) + 2e- → Cu(s)
2H+(aq) + 2e- → H2(g)
Zn2+(aq) + 2e- → Zn(s)
E°
+0.80 V
+0.34 V
0.00 V
- 0.76 V
Before adding the half-cell equations to obtain the overall reaction, you must reverse one of them and multiply the stoichiometric coefficients by a number which will cause free electrons to cancel out upon addition of the half equations. Before adding the half-cell potentials to obtain the standard cell potential, you must also reverse the sign of the half-cell potential associated with the half-cell equation which was reversed. The final equation represents the spontaneous reaction only if E° is positive.
A calculation of E° for Cell A is:
anode Cu(s) → Cu2+(aq) + 2e- - 0.34 v
cathode 2 [ Ag+(aq) + e- → Ag(s) ] +0.80 v
overall 2Ag+(aq) + Cu(s) → 2 Ag(aq) + Cu2+(aq) +0.46 v